CHEM108 Lab Manual. Includes Lab Reports and Pre-Lab Assignments. Required for all Saddleback CHEM108 Classes

Transcription

1 CHEM108 Lab Manual Includes Lab Reports and Pre-Lab Assignments Required for all Saddleback CHEM108 Classes

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3 NAME: Pre-lab #1: Introduction to Lab Techniques Introduction to Measurements There are numerous aspects to chemistry, but a common thread between them all is the process of collecting data and observations through field studies or in a laboratory. It is critical to understand how to make and read measurements, and we will focus on the basic types in this experiment: mass, length, volume and temperature. All quantitative measurements (involving a numerical value) are meaningless without an associated unit. It is necessary to always include a unit with a number, and that unit is determined by the measuring device. Since there are many different units for mass, volume, length, etc., we will look at the relation between some common units from the English system and the metric system during lab. The other major goal is to learn how to use, read and interpolate the basic measuring devices found in the laboratory, such as the balances, graduated cylinders, rulers and thermometers. The concept of significant figures will be used to aid in understanding the limitations inherent in any measurement and later calculations involving that value. Points to keep in Mind: There are many basic concepts that will be carried throughout the entire semester in lab which are briefly covered below. More details will be given as the semester progresses. 1) Safety: All safety rules must be followed at all times, with specific ones covered in the prelab lecture for each class. Make sure you are on time! Safety goggles are required to be worn at all times while doing experimental work in the lab. No flip-flops or open-toe shoes are to be worn in the lab. Shoes must cover the tops of your feet. 2) Pre-lab Assignments: These are due at the beginning of lab, no exceptions. You may not perform the lab if the prelab is not complete. You can download these from the course website and have an entire week to ask questions if you are unsure of anything. Be familiar with the proper instructions and calculations for the lab before you try anything! 4) Problem Solving and Calculations: All work must be shown for mathematical calculations, and all unit conversions should be set up as unit line equations for clarity. Be neat! Always include units for any number. For example, to calculate the density (D) of a liquid with a measured volume (V) of 9.83 ml and a mass (M) of g, show the formula, insert the primary data with units, show the unrounded answer, and then box the rounded answer to the appropriate number of significant figures with the correct units. D = M = g = g > V 9.83 ml ml 1.31 g/ml If this value was to be converted into units of grams per Liter (L), use the unrounded answer in the Unit Line Equation with the conversion factor to change the units of ml to L: g 1000 ml = g > or 1.31 x 10 3 g/l ml L L 1310 g/l in scientific notation

4 Making Measurements: Each measuring device has its own limitations and techniques. For digital readings, 50 there is no estimation involved and all digits shown are recorded except for any zero preceding a non zero number (i.e.: 3.20 g instead of g). For non digital instruments, the first step is to determine the value represented by the increment, or the space between the smallest divisions (lines) on the measuring device (represented by dashed lines in the figure to the right). This is most easily done by dividing the difference between two labeled lines by the number of lines between them. For the example shown to the right (assume it is a graduated cylinder), the difference between the labeled lines is: 50 ml 40 ml = 10 ml. There are 10 divisions between 40 these labeled lines, and so the value of the increment is 10 ml 10 = 1 ml. WHEN USING NON-DIGITAL MEASURING DEVICES, REPORT MEASURED VALUES ONE DEMICAL PLACE MORE PRECISE THAN THE INCREMENT. Metric Rulers: A portion of a metric ruler is shown on the right. The smallest division (the increment) is a tenth (0.1) of a centimeter (cm). You must estimate the reading between these lines to hundredth (0.01) of a cm in order to report the measured value one decimal place more precise than the increment. The position marked by the dashed line is read as 5.38 cm (or 5.39 cm). 5 6 Thermometers: All temperature measurements are made in degrees Celsius ( o C) 40 for the thermometers used in this lab. You do not need to shake the thermometer before use, and do not hold the bulb in your hand when making any measurements. If the red liquid in the thermometer is segmented or the thermometer broken, turn it in to your instructor to be fixed and get a new one. A portion of a thermometer is shown to the right in units of o C. Note the (increment) is one degree, and the measurement is estimated to a tenth of a degree (10 mental divisions), which is one decimal place more precise than the increment. The position marked by the dashed line is read as 33.5 o C. 30 Graduated Cylinders: There are two main factors to watch out for when reading a graduated cylinder: reading from the meniscus and avoiding parallax error. Since the liquid surface is not level (concave or convex), then a standard 3 must by agreed upon for consistency. This point is at the bottom of the curve, called the meniscus. Line up a burette reading card behind the cylinder so that the upper edge of the black line almost touches the bottom of the meniscus. This is shown on the figure to the right. Parallax error involves orienting your line of sight even with the meniscus. For very large cylinders, leave them on the counter and adjust your height to even your line of sight. For smaller cylinders, you can pick them up and hold them like a plumb bob between your fingers in line with your eyes. 2 There are some tricks involved with determining whether you are oriented properly which will be covered in lab as a demonstration. For the example on the right, the increment is 0.1 ml (3 ml 2 ml = 1 ml divided by 10 divisions). Measurements in this cylinder is are estimated to the nearest hundredth of a ml (± 0.01 ml), (one decimal place more precise than the increment). Since the meniscus falls exactly on the line, the value would be 2.60 ml. (You must include the last zero since this cylinder is estimated to 2 decimal places).

5 Balances: We use top loader digital balances in this lab, and there are some important rules to follow so as to not damage them. Unless instructed otherwise, report all of the numbers displayed in the digital readout. a) Never move a balance or turn them off. b) Never put chemicals directly on a balance (use weighing paper or a container). c) Clean up any spilled chemicals immediately! Practice Problems: For the following scales (no units), determine the size of the increment, and then estimate the readings at the lines given to the appropriate number of digits. Answers to the Practice problems: 1) Increment: _1 2) Increment: _0.1 Reading at X: 67.4 Reading at X: 7.74 Reading at Y: 61.0 Reading at Y: 7.10 Density Introduction: Matter is anything that has a mass associated with it, as well as encompassing a particular volume. The more matter there is in a certain volume (i.e.: the more mass within that volume), the more dense that matter is. So the density of matter, D, is the mass of that matter, M, divided by its volume, V. D = M V Solids normally have densities recorded in units of g/cm 3 whereas liquids are g/ml. Gases are much less dense than liquids or solids and so typically have densities given in units of g/l. Density is constant for a particular species, unless the temperature changes. Under thermal expansion, the volume of a species increases when it is heated or contracts when cooled. If the mass remains constant while the volume changes upon heating, then the density changes based on the equation above. In this experiment, the densities of various solids and liquids will be measured with the particular techniques used explained in each individual section (i.e.: regularly shaped vs. irregularly shaped solids vs. liquids). You will also be given an unknown liquid and are to determine its density as accurately as possible. The Density of a Liquid: In determining the density of any liquid, the volume is easily measured directly from a graduated cylinder or burette. The mass, though, must be measured indirectly through a method called measuring by difference. Using this method, the mass of a dry graduated cylinder is subtracted from the mass of that cylinder filled with a measured volume of liquid. This gives the liquid s mass, and when divided by its volume, its density.

6 The Density of a Regularly Shaped Solid: If a solid is square or rectangular, the volume can be directly measured using a ruler or other similar measuring device. Knowing that the volume of a square or rectangle is length (l) x width (w) x height (h), and with the mass being directly measured on a balance, then the density is easily obtained. Other solid volumes can be determined if their equation for volume is known (i.e.: Cones, cylinders, spheres). The Density of an Irregularly Shaped Solid: It can be very difficult or impossible to directly measure the volume of an irregularly shaped object (like an egg or tooth). A common method used is to measure the volume by water displacement. In this method, the solid object of known mass is placed in a graduated cylinder or other volume measuring device with a prerecorded volume of water in it. The increase in volume due to addition of the solid is the volume of the solid. Prelab Questions: These are due at the beginning of lab and must be completed to start the lab. 1) For the following scales (no units), determine the size of the increment, and then estimate the readings at the lines given to the appropriate number of digits. 100

7 2) If a brick has a length of cm, a width of 8.50 cm, and a height of 5.12 cm: a) What is the volume of the brick? b) If the brick has a mass of g, what is its density? 3) If 8.15 ml of water is placed in a graduated cylinder (dry mass = g), and the combined mass of both the cylinder + water is g: a) What is the mass of the water: b) What is the density of the water: 4) If a non-polar organic liquid with a density of 0.88 g/ml were mixed with the water from question #3, would the water or the organic liquid float on top?

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9 Chemistry 108 Lab #1! Name Lab #1: Introduction to Lab Techniques INTRODUCTION Our goals in this experiment are (1) to make some measurements using a metric ruler, (2) to learn how to determine volumes with a graduated cylinder, and (3) to determine the density of an unknown liquid and an unknown solid. PROCEDURE A. The Metric Ruler i) On the image of the metric ruler below, draw a small arrow pointing to 2.00 cm and write A, do the same and write B" at cm, "C" at 3.50 cm, "D" at 8.55 cm, and E" at cm. NOTE: The numbers that are displayed are cm, the increment is 0.1 cm. ii) Use a metric ruler to determine the length and width of this piece of paper. Metric rulers are on the counter in the front of the lab. Use the correct number of decimal places base on the ruler increments (as you did in the pre-lab). Increment on metric ruler = cm Length of the page = cm Do calculations to convert the length of the page to mm and meter (show your calculations for full credit) = mm = m. Width of the page = cm Do calculations to convert the width of the page to mm and meter (show your calculations for full credit) = mm = m.! 1!

10 Chemistry 108 Lab #1! B. The Graduated Cylinder There are 3 graduated cylinders set up in the lab. Each line on the 1000mL graduated cylinder represents ten milliliters (note that 1000 ml = 1 L). Each line on the 100mL graduated cylinder represents one milliliter. Each line on the 10 ml graduated cylinder represents 0.1 milliliters. Observe the top of the liquid in the 100 ml cylinder. Note that the liquid surface is curved, not level. The curved surface is called the meniscus. The volume is always read at the lowest point of the meniscus. Hold the graduated cylinder so the meniscus is exactly at eye level. Now raise and lower the graduated cylinder and observe that the volume reading changes as the cylinder is raised and lowered. Only when your eye is at exactly the same level as the bottom of the meniscus can you obtain an accurate volume reading. (The error introduced if your eye is high or low is called parallax.) Using the correct number of decimal places (as you did in the pre-lab), determine the volume of liquid in each of the 3 cylinders and record the data below. Graduate Cylinder 1000 ml (= 1L) Increment of Graduated Cylinder Volume of Liquid in Cylinder 100 ml 10 ml Remember to use units whenever you write a number.! 2!

11 Chemistry 108 Lab #1! C. Measuring the Density of a Solid and of a Liquid i) Density of a metal cylinder 1) Each student will be given a metal cylinder, record the unknown number in the table below. 2) Weigh the metal cylinder, record the mass (in table below) to at least three places past the decimal. 3) Determine the increment of the 100 ml graduated cylinder and record it in the table below. 4) Place about 30 ml of water in your 100 ml graduated cylinder, record the volume to one decimal place more precise than the increment (as done in pre-lab) in the table below. 5) While holding the graduated cylinder at an angle, carefully slide your metal slug into the graduated cylinder. The metal cylinder must be completely submerged. Record the new volume to one decimal place more precise than the increment in the table below. 6) Calculate the volume of your metal cylinder. 7) Go to page 5 and calculate the density of your metal cylinder. Show your calculations, using the correct number of significant figurers in the appropriate box on page 5. Metal Cylinder: Unknown number:.... Mass of metal... g Increment on a 100 ml graduated cylinder.. ml (Figure it out just as you did on the prelab or ruler) Volume before submersion of metal..... ml (report volume to one decimal place more precise than the increment) Volume after submersion of metal.... ml (report volume to one decimal place more precise than the increment) Calculate the volume of the metal (Use the data above and the volume by displacement method as discussed in the powerpoint prelab introduction to calculate the volume of the metal) = ml! 3!

12 Chemistry 108 Lab #1! ii) Density of a liquid 1) Obtain an unknown liquid from the instructor and record the number of the unknown on the data table below. 2) Determine the increment of the 10 ml graduated cylinder and record it in the table below. Note that the increment is different for the 10ml graduated cylinder than it was for the 100 ml graduated cylinder that was used for the metal slug. 3) Measure and record, in the table below, the mass of a clean, dry 10 ml graduated cylinder using a balance. Record the mass to at least three places past the decimal. 4) Pour between 5 and 10 ml of the unknown liquid in the 10 ml graduated cylinder. Re-weigh and record the mass, to at least three places past the decimal, in the table below. 5) Next, record the volume of the unknown liquid in the 10 ml cylinder to one decimal place more precise than the increment. If you need to redo the density of your unknown liquid, the cylinder must be completely dry before you weigh it. The graduated cylinder can be dried using a rolled-up paper towel. 6) Calculate the mass of your unknown liquid. 7) Go to page 5 and calculate the density of your unknown liquid. Show your calculations, using the correct number of significant figurers in the appropriate box on page 5. Unknown liquid: Unknown number:. Increment on a 10 ml graduated cylinder.. ml (Figure it out just as you did on the prelab or ruler) Mass of 10 ml graduated cylinder.. g Mass of 10 ml graduated cylinder + liquid g Volume of liquid. ml (Report volume to one decimal place more precise than the increment.) Calculate the mass of the liquid (Calculated using weight by difference method) = g! 4!

13 Chemistry 108 Lab #1! DENSITY CALCULATIONS In the area below, calculate the densities of the metal cylinder and the unknown liquid. Remember: Every number in a measurement must have a number and a unit) i) Calculation of the unknown solid s density Unknown solid number Density of your unknown solid Did you use the correct number of significant figures? ii) Calculation of the unknown liquid s density Unknown liquid number Density of your unknown liquid Did you use the correct number of significant figures?! 5!

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15 Chemistry 108 prelab Name Prelab #2 Coffee Cup Calorimetry Heat is a form of energy, sometimes called thermal energy that will pass spontaneously from an object at a high temperature to an object at a lower temperature. If the two objects are in contact they will, given sufficient time, both reach the same temperature. Heat flow is ordinarily measured in a device called a calorimeter. A calorimeter is simply a container with insulating walls, made so that essentially no heat is exchanged between the contents of the calorimeter and the surroundings. A coffee cup or thermos is an example of containers designed to prevent heat from escaping its contents. Within the calorimeter chemical reactions may occur or heat may pass from one part of the contents to another, but no heat flows into or out of the calorimeter from or to the surroundings. A. Specific Heat. When heat energy flows into a substance, the temperature of that substance will increase. The quantity of heat energy (Q) required to cause a temperature change in any substance is equal to the specific heat (S) of that particular substance times the mass (m) of the substance times temperature change ( T), as shown in Equation 1. Q = S x m x ( T) Equation (1) The specific heat can be considered to be the amount of heat required to raise the temperature of one gram of the substance by one degree Celsius. Amounts of heat energy are measured in either joules or calories. To raise the temperature of 1 gram of water by 1 degree Celsius, joules of heat must be added to the water. The specific heat of water is therefore joules/g o C. Since joules equals one calorie, we can also say that the specific heat of water is 1 calorie/g o C. That is not a coincidence, this is how the calorie was originally defined- the amount of energy needed to raise 1g of water by 1 o C! B. Calorimetry The specific heat of a metal can readily be measured in a calorimeter. A calorimeter is a container with a known heat capacity. When a heating or cooling process in a calorimeter occurs, the amount of energy (heat) gained or lost in the process is absorbed by the calorimeter. This energy can be calculated from equation (1) if the calorimeter s change in temperature is measured, since its heat capacity (S) and mass (m) are known. Very often, a calorimeter is composed of pure water in an insulating container. If one assumes that a negligible amount of heat escapes the insulated container and there is a negligible change in temperature of the container during the process, then all the energy released during the process of interest goes in to the water. Since we know the heat capacity and the mass of the water used in calorimeter and can measure the temperature change of the water, then we can use equation (1) to calculate the energy gained or lost by the water. The key point is that the energy gained (or lost) by the water is equal to the energy lost (or gained) in the process (such as cooling of the metal) that occurs in the calorimeter.

16 Chemistry 108 Heat Capacity Prelab A pre-weighed amount of metal is heated to some known temperature and is then quickly transferred into a calorimeter that contains a measured amount of water at a known temperature. Energy (heat) flows from the metal to the water, and the two eventually equilibrate (come to the same temperature) at some temperature between the initial temperatures of the water and metal. Assuming that no heat is lost from the calorimeter to the surroundings, and that a negligible amount of energy is absorbed by the calorimeter walls, the amount of energy that flows from the metal as it cools is equal to the amount of energy absorbed by the water. In other words, the energy that the metal loses is equal to the energy that the water gains. Since the metal loses energy (T final is less than T initial ; therefore T is negative) Q metal is negative. The water in the calorimeter gains energy (T final is greater than T initial ; therefore T is positive) and Q water is positive. Since the total energy is always conserved (cannot disappear), we can write equation (2): Q metal + Q water = 0 equation (2) Rearranging equation (2) gives (note the negative sign): Q metal = - Q water equation (3) In this experiment we measure the mass of water in the calorimeter and mass of the unknown metal and their initial and final temperatures. Using equation (1), the heat energy gained by the water and lost by the metal can be written: Q water = S water x m water x T water equation (4) Q metal = S metal x m metal x T metal equation (5) (Note that T metal < 0 and T water > 0, since T = T final - T initial ) We can calculate Q water from the experimental data; we measured m water, T water, and we know S water = J/g o C. We know that the heat energy lost by the metal is equal (but opposite sign) as the heat energy gained by the water. To determine the specific heat of your unknown metal (S metal ), substitute Q metal (in equation (5)) with Q water : -Q water = S metal x m metal x T metal equation (6) In this equation, we know 3 of the 4 variables: we calculated Q water from experimental data; we measured m metal and T metal. We can now solve equation (6) for S metal. Equation (6) can be rearranged to give S metal as a function of the known variables (Q water, m metal, and T metal ):!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!"#$% =!!!"#$%!!"#$%!!!"#$% equation (7) You will use this procedure to obtain the specific heat of an unknown metal. You will then compare the specific heat of your unknown metal to a table containing values of specific heats for several metals in order to determine the identity of your metal. 2

17 Chemistry 108 Heat Capacity Prelab Prelab Questions: 1) Why can we substitute Q metal (in equation (5)) with Q water? 2) Why is T metal < 0? 3) Why is T water > 0? 4) Should Q metal be positive or negative? Why? 5) Should Q water be positive or negative? Why? 6) A metal sample weighing 45.2 g and at a temperature of C was placed in 38.6 g of water in a calorimeter at 25.2 C. At equilibrium the temperature of the water and metal was 32.4 o C. a. What was T for the water? ( T = T final - T initial ) C b. What was T for the metal? o C c. Using the specific heat of water (4.184 J/g o C), calculate how much heat flowed into the water? joules d. Calculate the specific heat of the metal, using equation (7). joules/g o C 3

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19 Chemistry 108 lab Lab #2: Coffee Cup Calorimetry Name INTRODUCTION In this experiment, you will determine the specific heat for an unknown metal. The metal sample will be heated to a high temperature (100 o C) then placed into a coffee cup calorimeter containing a known amount of water. If you can find out how much heat was gained by the water in the calorimeter then you will know how much heat was lost by the metal. You can then calculate and compare the specific heat of your unknown metal to known values of metal specific heats and identify your metal. EXPERIMENTAL 1) Set up a calorimeter. The instructor will give each student a sample of an unknown metal. Each student must do their own unknown. Record your unknown number in the DATA section below. (The thermometer is very expensive, so be careful when handling it.) The calorimeter consists of two polystyrene coffee cups fitted with a styrofoam cover (placed in a 400 ml beaker for balance). There are two holes in the cover for a thermometer and a glass stirring rod. Do not fill the calorimeter with water yet. Assemble the experimental setup as shown in the figure below. 400 ml beaker 2) One of the two lab partners do the following: Fill a 600 ml beaker two-thirds full of water, add a couple of boiling chips, and begin heating it to boiling on a hotplate. Hotplates are located in the common drawers. This boiling water will be shared by both lab partners. 3) Each person will weigh an empty, extra large test tube and stopper. Record the mass of the empty, stoppered tube your data table (use all of the digits displayed on the balance).

20 Chemistry 108 Heat Capacity Lab 4) Transfer your unknown metal to the extra large test tube that you just weighed and replace the stopper. (the boiling water bath would remove the labels from the tubes your metals came in). Weigh your sample of unknown metal in the extra large, stoppered test tube. Record the mass of the stoppered tube and metal in your data table. Record all of the units displayed on the balance read-out. You will return your metal back to the original tube at the end of the experiment, so keep track of your own tube and do not mix it up with your partner s. 5) Place the loosely stoppered tube with the metal into the boiling water in the beaker. Do not put water in the tube with the metal! Do not put the stopper tightly in to the tube or the tube may explode as the air in the tube is heated. The water level in the beaker should be high enough so that the top of the metal is below the water surface. Continue heating the metal in the water for at least 15 minutes after the water begins to boil to ensure that the metal attains the temperature of the boiling water (100.0 o C). Add water to the beaker as necessary to maintain the water level. 6) While the water is boiling, weigh the empty calorimeter (both styrofoam cups and cover only; no water, no stirring rod, no thermometer, no 400ml beaker). Record the mass of the empty calorimeter in your data table, record this mass using three numbers after the decimal point. 7) Place about 30 ml of tap water in the calorimeter and weigh it again. Record the mass of the calorimeter and water in your data table. (styrofoam cups, water, and cover only; no stirring rod, no thermometer, no 400ml beaker) Do not be concerned that the last digit on the balance is not stable, you are seeing the water evaporate! Because of evaporation occurring, record this mass using three numbers after the decimal point. 8) Measure the initial temperature of the water contained in calorimeter. Note that you will need to hold the calorimeter at an angle so that the thermometer bulb is completely under the water. Record, to 0.1 C (one place to the right of the decimal), the temperature of the water (T initial water) in the calorimeter. 9) Insert the stirrer and thermometer into the calorimeter through the 2 holes in the cover. 10) Take the test tube out of the beaker of boiling water, remove the stopper, and pour the metal into the water in the calorimeter. Replace the calorimeter cover and stir the water/metal mixture as best you can with the glass stirrer. Record, to 0.l o C, the maximum temperature reached by the water in the calorimeter (this is the T final of the water and the metal). 11) OPTIONAL If you have 40 more minutes before the end of lab time, if you wish, you can repeat the experiment a second time (trial 2). Be sure to dry your metal before reusing it; this can be done using several paper towels. Be sure to dry the metal completely or you will introduce error to your measurements. 2

21 Chemistry 108 Heat Capacity Lab WHEN FINISHED: The metal used in this experiment is to be dried with paper towels and returned to the front counter in the test tube in which you obtained it. DATA Your unknown number Trial 1 Trial 2 Mass of empty test tube and stopper g g Mass of stoppered test tube plus metal g g Mass of empty calorimeter g g Mass of calorimeter and water g g Initial temperature of water in calorimeter (T initial water) C C Initial temperature of metal (assume C) (T initial metal) C C Equilibrium temperature of metal and water in the calorimeter (T final water = T final metal) C C CALCULATIONS Be sure to use the correct number of significant figures and to use units with every number you write!!!! 1) Calculate T metal Trial 1 Trial 2 2) Calculate T water Trial 1 Trial 2 3

22 Chemistry 108 Heat Capacity Lab 3) Calculate the mass of the water in the calorimeter (m water ). (Think about how you get this from the data table values.you have the mass of the calorimeter with the water in it and the mass of the empty calorimeter ) Trial 1 Trial 2 (OPTINAL) 4) Calculate the mass of the metal (m metal ). (Think about how you get this from the data table values.you have the mass of the metal in the tube and the mass of the empty tube ) Trial 1 Trial 2 5) Calculate the heat energy gained by the water (Q water ). Trial 1 Trial 2 Did you use the correct number of sig. figs.? 6) Knowing that the heat gained by the water is equal and opposite the heat lost by the metal, use equation (6) or (7) in the prelab to calculate the specific heat of your metal. Trial 1 Trial 2 Did you use the correct number of sig. figs.? 7) If you did two trials, take the average of the specific heats calculated in the two trials (above). 4

23 Chemistry 108 Heat Capacity Lab Average specific heat of metal (if you did 2 trials only).. (Did you use the correct number of significant figures and the correct units?) CONCLUSION Re-write your unknown number here: Your unknown metal is one of the metals in the table below. Use this table of specific heats to determine the identity of your metal. Choose the metal with the specific heat that is closest to your experimental value. Metal Specific Heat (J/g o C) Aluminum Bismuth Tin Nickel My metal is (circle one): Aluminum Bismuth Tin Nickel 5

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25 Chemistry 108 Lab #3 Name Prelab #3 Gases: Percent Yield of Hydrogen Gas from Magnesium and Hydrochloric Acid Introduction For chemical reactions involving gases, gas volume measurements provide a convenient means of determining stoichiometric relationships. A gaseous product is collected in a long, thin graduated glass tube, called a eudiometer, by displacement of a liquid, usually water. Magnesium reacts with hydrochloric acid, producing hydrogen gas: Mg (s) + 2HCl (aq) MgCl 2 (aq) + H2 (g) Note: for every mole of Mg (s) that is reacted, one mole of H 2 (g) is produced. If we know the mass of Mg(s) we can convert to moles of Mg(s). Then, since we get 1 mole of H 2 (g) for every mole of Mg(s), we can predict how many moles of H 2 (g) could be made (theoretical yield). We use an excess of HCl so that we would react all the Mg(s) before we used all of the HCl. Eudiometer Tube Set-up Reaction Set-up (after inverting eudiometer tube) 1

26 Chemistry 108 Lab #3 When the magnesium reacts with the acid, the evolved hydrogen gas is collected by water displacement and its volume is measured. The temperature of the gas is taken to be the same as the temperature of the water it is in contact with because, given a sufficient amount of time, the two will reach thermal equilibrium. The level of water in the eudiometer is adjusted so that it is equal to the level of water outside the eudiometer. This insures that the pressure in the eudiometer is equal to the prevailing atmospheric pressure. Because the hydrogen gas was collected above water, and water has a significant vapor pressure, to get the pressure of pure hydrogen (dry hydrogen), we must subtract the vapor pressure of water. The pressure of the dry hydrogen gas (PH 2 ) is calculated from Dalton's Law of Partial Pressures: Ptotal = PH 2 + PH 2 O so PH 2 = Ptotal - PH 2 O where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH 2 O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. We will get the vapor pressure of water from the table below of vapor pressure vs. temperature. The number of moles of hydrogen gas collected can then be calculated from the ideal gas law: (n = # moles H 2 ) n = PV (Use PH 2 here, not P total ) This will give you the experimental # moles of hydrogen gas collected. The theoretical # of moles of H 2 (g) can be calculated by converting the mass of Mg to moles Mg, and understanding that we get 1 mole of H 2 from every mole of Mg(s). From the theoretical yield and the experimental yield, one can calculate the percent yield: RT Percent Yield = experimental # moles H 2 100% theoretical #mole H 2 2

27 Chemistry 108 Lab #3 PRELAB QUESTIONS: 1. Suppose you did this experiment and obtained the following data. The initial mass of magnesium metal was g. The volume of the gas produced in the eudiometer was ml, the atmospheric pressure is torr, and the water temperature is 24.1 C. As if you carried out this experiment and obtained the following data, fill in the data table and do the calculations below. DATA TABLE: CALCULATIONS: Mass of Magnesium Metal. g Volume of Gas. ml Temperature of Gas (assumed to be the same temp. as the water). C Atmospheric Pressure. torr Water Vapor Pressure (at the above temperature, see table on last page). torr 1. Theoretical (calculated) yield of H 2 gas (# moles H 2 ). a) convert mass Mg to #moles Mg b) Convert # moles Mg reacted to moles of H 2 that could be produced. (1 mole H 2 is produced for every 1 mole Mg reacted- this comes from the balanced chemical equation): Mg (s) + 2HCl (aq) MgCl 2 (aq) + H2 (g) # moles H 2 = (theoretical yield of H 2 ) 2. Experimental yield of H 2 gas (# moles H 2 ). a) Determine pressure of dry H 2 (PH 2 ) by subtracting the vapor pressure of water from the total pressure, Ptotal = PH 2 + PH 2 O so PH 2 = Ptotal - PH 2 O where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH 2 O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. Use the table provided on the previous page to find the vapor pressure of water as a function of temperature. PH 2 = torr b) Convert this pressure from torr to atm (760.0 torr = 1atm) PH 2 = atm 3

28 Chemistry 108 Lab #3 c) Use the Ideal Gas Equation to calculate the number of moles (n) of H 2 that you produced in your experiment (experimental yield). Make sure to use the correct units so that they match the units in the gas constant (R). 3. Calculate the Percent Yield. % Yield = experimental # moles H 2 100% theoretical #mole H 2 % Yield = OTHER PRE-LAB QUESTIONS 1. When the volume of gas is measured in a eudiometer, the water levels are the same on the inside and outside of the eudiometer. Why do we need to do this? 2. If a gas is collected by water displacement in a eudiometer, the atmospheric pressure is known, and the water vapor pressure is known, what equation will allow you to calculate the pressure of the dry gas? 4

29 Chemistry 108 Lab #3 Name Lab # 3: Gases Percent Yield of Hydrogen Gas from Magnesium and Hydrochloric Acid Introduction For chemical reactions involving gases, gas volume measurements provide a convenient means of determining stoichiometric relationships. A gaseous product is collected in a long, thin graduated glass tube, called a eudiometer, by displacement of a liquid, usually water. Magnesium reacts with hydrochloric acid, producing hydrogen gas: Mg (s) + 2HCl (aq) MgCl 2 (aq) + H 2 (g) Note: for every mole of Mg (s) that is reacted, one mole of H 2 (g) is produced. If we know the mass of Mg(s) we can convert to moles of Mg(s). Then, since we get 1 mole of H 2 (g) for every mole of Mg(s), we can predict how many moles of H 2 (g) could be made (theoretical yield). We use an excess of HCl so that we would react all the Mg(s) before we used all of the HCl. When the magnesium reacts with the acid, the evolved hydrogen gas is collected by water displacement and its volume is measured. The temperature of the gas is taken to be the same as the temperature of the water it is in contact with because, given a sufficient amount of time, the two will reach thermal equilibrium. The level of water in the eudiometer is adjusted so that it is equal to the level of water outside the eudiometer. This insures that the pressure in the eudiometer is equal to the prevailing atmospheric pressure. Because the hydrogen gas was collected above water, and water has a significant vapor pressure, to get the pressure of pure hydrogen (dry hydrogen), we must subtract the vapor pressure of water. The pressure of the dry hydrogen gas (PH 2 ) is calculated from Dalton's Law of Partial Pressures: Ptotal = PH 2 + PH 2 O so PH 2 = Ptotal - PH 2 O where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH 2 O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. We will get the vapor pressure of water from the table below of vapor pressure vs. temperature. 1

30 Chemistry 108 Lab #3 The number of moles of hydrogen gas collected can then be calculated from the ideal gas law: (n= # moles H 2 ) n = PV (Use PH 2 here, not P total ) This will give you the experimental # moles of hydrogen gas collected. The theoretical # of moles of H 2 (g) can be calculated by converting the mass of Mg to moles Mg, and understanding that we get 1 mole of H 2 from every mole of Mg(s). From the theoretical yield and the experimental yield, one can calculate the percent yield: RT PROCEDURE: Percent Yield = experimental # moles H 2 100% theoretical #mole H 2 1.Fill the largest beaker in your drawer (400 ml or a 600mL beaker) about 2/3 full of water and allow it to sit on the base of a ring stand so that the temperature of the water may adjust to room temperature. Place a double buret clamp on the ring stand well above the beaker. 2.Obtain a 4-5 cm length of magnesium ribbon from the back counter of the lab room. With sand paper, sand the magnesium ribbon until it is shiny. Determine its mass on the side-loading balance, and record its mass in your data table. Your magnesium should have a mass no larger than g. Roll the magnesium ribbon into a loose coil. Obtain a piece of thread 25 cm in length, and tie it to one end of the magnesium ribbon in such a way that all the loops of coil are tied together (see Figure 1). Figure 1 3.Obtain a eudiometer from the stockroom. Always carry a eudiometer in a vertical position. The eudiometer will contain water to keep dust out. Empty out the water into your sink and temporarily attach it to the buret clamp, open end up. 4.Measure out 10 ml of hydrochloric acid in a graduated cylinder and pour it into your eudiometer. Remove the eudiometer from the buret clamp, hold it on a slight slant, and add enough water to the eudiometer to fill it completely. Try to mix the water and the acid as little as possible. Reattach the eudiometer to the buret clamp, open end up (see Figure 2). 5.Obtain a one-hole rubber stopper from the back counter. Take your magnesium coil and lower it into the water of the eudiometer to a depth of about 5 cm. Have the thread attached to the coil hang over the lip and out of the eudiometer. Insert the onehole rubber stopper into the eudiometer so the thread is held firmly against the edge, and when water squirts out of the hole in the stopper, cover the hole firmly with your thumb (see Figure 1). Figure 2 2

31 Chemistry 108 Lab #3 6. PUT ON RUBBER GLOVES. Taking care that no air enters, remove the eudiometer from the buret clamp, invert it, and place its open end underwater in the beaker. Re-clamp the eudiometer to the buret clamp so that the bottom of the eudiometer is about 1 cm below the surface of the water in the beaker. The acid will flow down the eudiometer and react with the magnesium. 7.When the magnesium has disappeared entirely and the reaction has stopped, tap the tube with your finger to dislodge any bubbles you see attached to the side of the eudiometer. Measure the temperature of the water in your beaker; this will be the temperature of the hydrogen gas in the eudiometer. Record this value, to the nearest 0.1 o C, in your data table. Because your thermometer reads to a tenth of a degree Celsius, add when converting to Kelvin. 8.Place your finger over the hole in the stopper and remove the eudiometer from the beaker. Lower the eudiometer into the leveling tank and remove your finger. Raise or lower the eudiometer until the water level inside the eudiometer is the same as the water level in the leveling tank. Read the volume of gas in the eudiometer and record it in your data table. Record the volume to the nearest 0.01 ml. Figure 3. Inverted eudiometer illustrating gas collection and water displacement. 9.Your instructor will write the barometer for today s atmospheric pressure on the chalk board. Record this value in your data table. The water vapor pressure can be found in the table on page 2. Record this value for the vapor pressure of water in your data table. 10.When finished with the experiment, clean the eudiometer with water, dry the outside, fill it with deionized water, and return it to the stockroom. 3

32 Chemistry 108 Lab #3 DATA TABLE: Mass of Magnesium Metal = g (must be less than g) Volume of Gas = ml Temperature of Gas = Temperature of the water = = C Atmospheric Pressure = torr Water Vapor Pressure (at the above temperature, see table on last page) = torr CALCULATIONS: 1. Theoretical (calculated) yield of H 2 gas (# moles H 2 ). a) Convert mass Mg to #moles Mg b) Convert # moles Mg reacted to moles of H 2 that could be produced (1 mole H 2 is produced for every 1 mole Mg reacted- this comes from the balanced chemical equation): Mg (s) + 2HCl (aq) MgCl 2 (aq) + H2 (g) # moles H 2 = (theoretical yield of H 2 ) 2. Experimental yield of H 2 gas (# moles H 2 ). a) Determine pressure of dry H 2 (PH 2 ) by subtracting the vapor pressure of water from the total pressure, Ptotal = PH 2 + PH 2 O so PH 2 = Ptotal - PH 2 O where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH 2 O (the water vapor pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. Use the table provided on the last page to find the vapor pressure of water as a function of temperature. PH 2 = torr b) Convert this pressure from torr to atm (760.0 torr = 1atm) PH 2 = atm 4

33 Chemistry 108 Lab #3 c) Use the Ideal Gas Equation to calculate the number of moles (n) of H 2 that you produced in your experiment (experimental yield). Make sure to use the correct units so that they match the units in the gas constant (R). 3. Calculate the Percent Yield. % Yield = experimental # moles H 2 100% theoretical #mole H 2 % Yield = 4. What are the possible sources of error in this experiment? 5

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35 Chemistry 108 Chemical Reactions Prelab 4 Pre-Lab #4: Chemical Reactions Many chemical reactions can be placed into one of two categories: oxidation-reduction reactions and double-replacement reactions. Oxidation-reduction reactions are ones in which electrons are transferred from one species to another. There are four types of oxidation-reduction reactions that we will investigate: synthesis, decomposition, single-replacement, and combustion. A) SYNTHESIS REACTIONS A synthesis reaction is one in which a single compound is formed from two or more substances: A + B AB An example of a synthesis reaction (one type of oxidation-reduction reaction) is the reaction that occurs between sodium metal and oxygen gas (O 2 ): Na(s) + O 2 (g) Na 2 O(s) The equation above is not balanced. Balance this equation: Na(s) + O 2 (g) Na 2 O(s) As stated earlier, oxidation-reduction reactions are ones in which electrons are transferred from one species to another. Let s see how this transfer of electrons occurred in our reaction of sodium with oxygen. First, we must remember that the total charge of any pure element or compound is always ZERO! This fact will help you to determine the charge of each atom in a compound. Na(s) and O 2 (g) are pure elements (not combined with other elements) therefore the charge of each of the atoms in a piece of pure sodium metal and a sample of pure oxygen gas is equal to ZERO. Next, let s consider the charges on the Na and O in the Na 2 O(s) product. We once again must understand that the total charge of any pure element or compound is always ZERO! Since sodium oxide (Na 2 O(s)) is a compound, it has a total charge = ZERO. We also know that sodium oxide is an ionic compound, and that although the total charge is ZERO, the sodium cations and oxygen anions are charged particles. What is the charge of an oxide ion? What is the charge of a sodium ion? Note that ions combine in a ratio such that the total charge of a compound = ZERO, that is why sodium oxide has the formula Na 2 O. This discussion of charge can be found in detail in chapter 3, it was repeated here as a review. Now, back to our discussion as to why this synthesis reaction is classified as an oxidation-reduction reaction. Let us consider how electrons are removed from one reactant and transferred to the other. We will do so by considering what happens to the charge of sodium and oxygen as they are converted from reactants to products. 1

36 Chemistry 108 Chemical Reactions Prelab 4 Let s consider Na first. In the reactant, Na exists as Na(s) and has a charge of ZERO (charges are shown below as superscripts). In the product, Na has a charge of 1+. Na 0 (sodium has 0 charge in Na 2 O) Na + (sodium has a +1 charge in Na 2 O) In this chemical reaction, sodium went from ZERO charge to 1+ charge. THIS IS ONLY DONE BY LOSING AN ELECTRON! When a species loses electron(s), we call that oxidation. Na 0 Na + + e- Where does the electron go? It must get transferred to the oxygen! Let s consider the charge on oxygen as it is converted from reactant to product. In the reactant, oxygen exists as O 2 and has a charge of ZERO (charges are shown below as superscripts). In the product, O has a charge of 2-.. O 0 (oxygen has 0 charge in O 2 ) O 2- (oxygen has a 2- charge in Na 2 O) We see in this chemical reaction, oxygen went from ZERO charge to 2-. THIS IS ONLY DONE BY GAINING ELECTRONS! When a species gains electron(s), we call that reduction. O 0 + 2e- O 2- The fact that oxygen gains 2 electrons and sodium only loses 1 electron is accounted for in the balanced chemical equation, note that 4 sodium atoms react with 2 oxygen atoms (O 2 ).making the electron donation ration equal 2:1, two sodium atoms are required to reduce each oxygen atom. Practice Problems: 1) Predict the product and balance the following synthesis reaction: Li(s) + O 2 (g) What is the charge of the lithium in the reactant Li(s)? What is the charge of the lithium ion in the product? Did lithium gain or lose electron(s) in this reaction: if so, how many? Was lithium oxidized or reduced? What is the charge of the oxygen in the reactant O 2 (g)? What is the charge of the oxide ions in the product? Did oxygen gain or lose electron(s) in this reaction: if so, how many? Was oxygen oxidized or reduced? 2

37 Chemistry 108 Chemical Reactions Prelab 4 2) Predict the product and balance the following synthesis reaction: Mg(s) + Cl 2 (g) What is the charge of the magnesium in the reactant Mg(s)? What is the charge of the magnesium ion in the product? Did magnesium gain or lose electron(s) in this reaction: if so, how many? Was magnesium oxidized or reduced? What is the charge of the chlorine in the reactant Cl 2 (g)? What is the charge of the chloride ion in the product? Did chlorine gain or lose electron(s) in this reaction: if so, how many? Was chlorine oxidized or reduced? B) DECOMPOSITON REACTIONS A decomposition reaction is an oxidation-reduction reaction in which a single reactant breaks down into two or more substances: AB A + B An example of a decomposition reaction is the thermal (heat induced) decomposition of mercury(ii) oxide: HgO (s) Hg (l) + O 2 (g) (not balanced, you will balance this reaction in the problem below) Note that the key to identifying a decomposition reaction is that one reactant species is converted to two or more products species. In our example, we start the reaction with just one compound present, HgO (s), after the reaction there are two different chemical species present, Hg (l) and O 2 (g). 3

38 Chemistry 108 Chemical Reactions Prelab 4 Practice Problems: 3) Balance our example decomposition reaction: HgO (s) Hg (l) + O 2 (g) What is the charge of the mercury ion in the reactant (HgO (s))? What is the charge of the mercury in the product (Hg (l))? Did mercury gain or lose electron(s) in this reaction: if so, how many? Was mercury oxidized or reduced? What is the charge of the oxygen ion in the reactant (HgO (s))? What is the charge of the oxygen in the product (O 2 (g))? Did oxygen gain or lose electron(s) in this reaction: if so, how many? Was oxygen oxidized or reduced? C) SINGLE-REPLACEMENT REACTIONS A single-replacement reaction is an oxidation-reduction reaction in which an element replaces another element from a compound: A + BX AX + B For example, copper metal (Cu(s)) will replace the silver ion (Ag) in silver nitrate: Practice Problems: Cu(s) + AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + Ag (s) (not balanced, you will balance this reaction in the problem below) 4) Balance our example single-replacement reaction: Cu(s) + AgNO 3 (aq) Cu(NO 3 ) 2 (aq) + Ag(s) What is the charge of the copper in the reactant Cu(s)? What is the charge of the copper in the product (Cu(NO 3 ) 2 )? Did copper gain or lose electron(s) in this reaction: if so, how many? 4

39 Chemistry 108 Chemical Reactions Prelab 4 Was copper oxidized or reduced? What is the charge of the silver in the reactant (AgNO 3 )? What is the charge of the silver in the product (Ag(s))? Did silver gain or lose electron(s) in this reaction: if so, how many? Was silver oxidized or reduced? D) COMBUSTION REACTIONS A combustion reaction is an oxidation reduction reaction in which a compound reacts with oxygen. In the case of organic compounds, complete combustion will always result in the formation of CO 2 (g) and H 2 O (g). Incomplete combustion of organic molecules often results in the formation of C(s), often in the form of small particles of pure carbon called soot and other small organic molecules like carbon monoxide. Soot can be thought of as tiny pieces of charcoal. In chemistry 108, we only focus on complete combustion of organic molecules to form the products CO 2 (g) and H 2 O (g). As an example, let us consider the combustion of pentane: Write in the products and balance the reaction for the complete combustion of pentane: C 5 H 12 (l) + O 2 (g) + In part A of the prelab, we learned how to identify an oxidation reduction reaction by understanding how electrons are transferred from one species to another. We were able to see which species was oxidized and which species was reduced by keeping track of the charges of atoms in reactants and products. Seeing how the charges of atoms changed in an inorganic chemical reaction allows us to determine how electrons were transferred, which species gained electrons, and which species lost electrons. In the case of the oxidation and reduction of organic compounds, that process is not as easily applied, so we learned a couple of rules (in chapter 6) to determine if a species is oxidized or reduced: Oxidation and Reduction of Organic Molecules Method for determining oxidation or reduction in organic compounds: An atom in a molecule is oxidized if it: gains oxygen (is attached to more oxygen atoms in the product than in the reactant) OR loses hydrogen (is attached to fewer hydrogen atoms in the product than in the reactant) An atom in a molecule is reduced if it: loses oxygen (is attached to fewer oxygen atoms) OR gains hydrogen (is attached to more hydrogen atoms) Let s apply these rules to our example of the combustion of pentane by asking: is carbon oxidized or is it reduced in this reaction? 5