Chemistry 3012 Foundational Chemistry Laboratory Manual

Transcription

1 Chemistry 3012 Foundational Chemistry Laboratory Manual

2 Table of Contents Page Experiment 1. Experiment 2. Experiment 3. Experiment 4. Experiment 5. Experiment 6. Experiment 7. Experiment 8. Determining the Density of an Unknown Liquid Development of the Periodic Table: Part I Development of the Periodic Table: Part II Calorimetry Atomic Spectroscopy Synthesis of Soap Molecular Geometry Water Purification

3 Experiment 4. Calorimetry Part 1 A common method for determining the energy content of food is through a process known as calorimetry. In this process, food is combusted, releasing heat energy which is then absorbed by another substance, usually water, resulting in an increase in the water s temperature. The amount of heat necessary to induce a temperature change in water is given by the equation: q = m w c w T (1) where q is the quantity of heat energy released, m w is the mass of water that is heated, T is the change in the temperature of water (T final - T initial ), usually given in o C or K, and c w is the specific heat of water and given in units of J/g- o C or J/g-K. The amount of heat absorbed by the water (heat intake) must have originated from the combustion of the food (heat output). Barring any measurable heat loss or input from other sources, the magnitude of both quantities must be equal to one another; however the signs will be opposite reflecting that the direction of heat flow was out in one case and in for another. q out = -q in (2) In this lab, you will determine the amount of energy stored in a peanut through Calorimetry. To facilitate this measurement, you must first build an inexpensive calorimeter, a device that is used to measure the transfer of heat energy during a chemical or physical change. The calorimeter will be fashioned from a soda can, to which you will add water. Water has a well-known specific heat of 4.18 J/g-K; aluminum has a specific heat of 0.90 J/g-K. You will use this information to deduce the amount of energy transferred to the can by recording the temperature rise for the specific volume of water used. Assuming that all the energy absorbed by the calorimeter originated from the peanut, you will calculate the amount of energy released per gram of peanut consumed (not all of the peanut will be combusted). In this determination, we must include in our calculations that the peanut heated up the soda can as well as the water. Assuming that the soda can is made of aluminum, the equation Q = m w c w T + m c c c T (3) is used to find the total amount of heat energy absorbed by the can, where m c and c c are the mass and the specific heat of the can, respectively. CAUTION: Be sure the room is well-ventilated. Use gloves or forceps to handle hot equipment and burned peanut. If you have peanut allergies, alert your instructor and an alternative nut will be used. Part 2 The law of the conservation of energy states that energy can never be created or destroyed - it can only change form. We utilized this law here in Part 1 in that the chemical energy stored within the peanut was transformed during combustion into heat energy, which was then transferred to the calorimeter. We are assuming that the all of the chemical energy released by the peanut is transformed into heat energy, but conversions such as this one are never 100%

4 efficient. Heat radiates in all directions, so not all of it was absorbed by the calorimeter. Energy may have also been lost through other mechanisms, such as sound. In addition, incomplete chemical combustion may have occurred which would carry unburned portions of the peanut away in the form of soot. A more efficient method of increasing the temperature of water, one that we are more familiar with, is through electrical heating. In this portion of the experiment, you will determine the amount of electrical energy that is required to obtain the same temperature increase that you observed from Part 1. The device that we are using will deliver power (Watts = J/second) to the heating filament. To obtain the energy that was needed to heat the filament, the duration (in seconds) of how long that power was used needs to be recorded. The apparatus you will use is shown below. To Interface Power Supply Stirrer Heater Temp sensor MATERIALS 600 ml beaker 12-oz. soda can Celsius thermometer compass or scissors heat resistant gloves, forceps or thongs. shelled peanut or other high-fat food such as a potato chip Alternative power supply/calorimeter setup PROCEDURE Part 1 1. Punch four holes evenly around a soda can, 2 to 3 cm from the top (use a compass or scissors). Place two straightened paper clips (or pieces of a wire hanger) into opposing holes. Hang the can in the wire ring on the ring stand, and adjust the height accordingly. 2. Weigh the can (m c ).

5 Part 2 3. Add 200 ml of water to the soda can, and reweigh the can plus water (m c + m w ). This apparatus is your calorimeter. It is important to use the same volume of water for all experiments. 4. Place the thermometer into the mouth of the can. Remember to suspend the thermometer in the water when taking a temperature reading. Before going any further, check the apparatus to make sure that everything is secured. 5. Fashion a stand for the peanut out of a paperclip, and place this stand and a peanut in a metal weighing dish. Measure the initial mass of the weighing dish, stand and peanut. 6. Let the water sit in the can for a couple of minutes, and then take an initial temperature reading of the water (T initial ) in the can. We will assume that the water and the can have come to an equilibrium temperature that is the same for both. 7. Use a match or Bunsen burner to set the peanut on fire. This may take several tries. Closely observe the nut as it burns. If the peanut falls off the stand, start over immediately. 8. As the peanut is burning, take water temperature readings every 15 seconds. After the peanut stops burning, continue to record the temperature every 15 seconds until the temperature no longer continues to increase. The highest temperature that you record is your final temperature (T final ). Assuming that both the can and the water share the same T final and T initial, calculate T. 9. As soon as the weighing dish cools, use forceps to lift the dish containing the burned remains onto the balance. Take a final mass, and record in the data table. 10. Calculate the amount of heat transferred to the calorimeter and water from the peanut using Equation Repeat the experiment, and average your results from Step 10. You should create your own data table to record your answers in a logical fashion. 1. Weigh the inner canister, and then add 200 ml of cold tap water to the small canister and again weigh the canister with the water in it. Determine the mass of the canister (m c ) and the mass of water (m w ) in g. 2. Place the small canister inside the empty larger one. Position the end of the temperature sensor in the middle of the small canister. 3. Record the initial temperature of the water in your notebook. Turn on the power supply and set the voltage to 10.0 V. 4. Record the voltage (V) and current (I) from the power supply. These values should not change during the course of the measurement. Continue to stir the water while the power supply is on.

6 5. When you have reached the desired temperature (T initial + T), click Stop. Record how long it took, t, to reach the final temperature (T f ). Calculate the change in temperature of the water T = T f T i. 6. The electrical power is the current (I) in amperes times the voltage (V), in volts. The electrical energy dissipated during time t is: E = IV t (Joules) (4) Calculate how much energy was produced by the power supply to heat the water to the desired temperature. 7. The heat input to the water during that time is: Q = m w c w T + m c c c T (Joules) (5) where the specific heats are c w = J/kg C and c c = 900 J/kg C. This takes into account the energy needed to heat the water plus the energy needed to heat the canister. With your data, calculate how much energy was used to heat the water and canister by T.