Prelab attached (p 8-9) (g)! MgO (s) + heat (1)

Transcription

1 CHEM 151 ENTHALPY OF FORMATION OF MgO FALL 2008 Fill-in Prelab attached (p 8-9) Stamp Here Name Partner Lecture instructor Date INTRODUCTION Chemical reactions either produce heat as they proceed (exothermic) or require heat to proceed (endothermic). Very sophisticated and expensive pieces of equipment, called calorimeters, can accurately measure the amount of heat energy associated with a reaction by isolating the reaction in a well insulated container and measuring either the rise or fall in temperature. This temperature change, when multiplied by the heat capacity of the system, provides a measure of the energy change associated with converting the reactants to products. An expensive calorimeter can measure temperature changes of less than 1 C, a precision required for some reactions. However, many reactions produce sufficient amounts of heat to be measured using a much simpler system. In the experiment today, you will use a very simple and inexpensive calorimeter to indirectly measure the heat of formation of MgO. Discussion By using various thermochemical measurements and applying Hess's Law, one can derive basic information about a chemical reaction in more than one way. It is possible, for instance, to directly measure the heat of formation ( H f ) of MgO simply by burning magnesium in oxygen and measuring the heat evolved in the reaction. Mg (s) O 2 (g)! MgO (s) + heat (1) This simple reaction, however, requires a rather elaborate calorimeter to perform the measurements. We therefore need to find a simpler method that will provide the same information. Hess's Law states that for any process that can be defined as the sum of several stepwise processes, the value of H for the whole process must be equal to the sum of the H values for the individual steps. Therefore, if you dissolve Mg in acid and then dissolve MgO in the same acid, the difference between these two heats of reaction will be the heat of formation of MgO from its elements, providing you account for all side reactions that occur. The only side reaction involves the formation of water. If you can find (try your textbook!) the H f for the reaction H 2 (g) + ½ O 2 (g) H 2 O (l) then Hess s Law can be used to determine the heat of formation ( H f ) for MgO, reaction (1). You will experimentally determine the heat of reaction for the following reactions A and B. ΔH C a) Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) H A b) MgO (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 O (l) H B c) H 2 (g) + ½ O 2 (g) H 2 O (l) H C (text appendix) #7 H f of MgO Rev F08AEM Fall 2008 Page 1 of 9

2 In this experiment you will determine experimentally the heats of reaction for Mg with HCl and MgO with HCl. Then, by appropriately manipulating the data gathered, you will determine the heat of formation of MgO. The calorimeter you will be using is a thermally insulated container (two Styrofoam cups) with a cardboard cover in which a reaction can be conducted, accompanied by a thermometer that can measure the temperature change of the system (see Figure 1). Figure 1: A Styrofoam Cup Calorimeter You will determine if the reactions run in lab are exothermic or endothermic. Since both reactions are run in dilute water solutions of HCl, the heat produced by the reaction with the HCl (the reaction, or the system ) will be transferred to the water (the surroundings ). By knowing the heat capacity of water, (assuming that the heat capacity of water and that of dilute HCl are the same, which is reasonable) the heat of reaction can be calculated. Some heat will also be transferred to the calorimeter whose heat capacity is unknown. However, using the double cup technique, this heat transfer to the cups is assumed to be minimal and therefore, should not affect the final determination of your determination of ΔH. We know that when a reaction occurs, if the heat is lost by the system (the reaction) then the heat will be gained by the surroundings (in this case, the water). Conversely, if the heat is gained by the system (the reaction), then the heat will be lost by the surroundings (the water). Therefore, the q values will be numerically equal but they will be opposite in sign. heat system (rxn) = -heat surroundings (H2O) m 1 C 1 ΔT 1 = -(m 2 C 2 ΔT 2 ) m = mass; C = specific heat; T = temperature change = (T f T i ) when a reaction is run under a constant pressure, then q = ΔH #7 H f of MgO Rev F08AEM Fall 2008 Page 2 of 9

3 EXPERIMENTAL PROCEDURE All chemicals and supplies can be obtained from the reagents bench. In all parts, try to keep the total volume of solution at 100 ml. C Determination of ΔH values: J o g C H 2 O =. 1 calorie = Joules In order to calculate the heats of reaction for Mg and MgO with HCl, we must measure the heat transferred to the water. We know: Energy released or absorbed = m ic H2 i!t (3) O H 2 O This formula would tell us how much heat was produced by some amount of Mg or MgO reacting with HCl. Specific heats are typically given in J/g C; we, however, are interested in knowing the heat of reaction in terms of kj/mole Mg or MgO. We will reconcile units at the end of our calculations. Assume that the specific heat of your solution is the same as that for water (it is very close). A. Procedure for Determining Heat of Reaction of Mg with HCl First rinse and dry your calorimeter setup (two cups, nestled together referred to as CAL), and weigh them together to the nearest 0.01g; record this mass in the table on the next page. You must use these two cups for both parts of the experiment. Then place about 100 ml of 2M HCl into the inner cup. Weigh, the CAL and HCl solution, to the nearest 0.01 g and record this mass in the table. Subtract CAL weight to determine mass of the HCl used and record this in the table. Cover the CAL and insert the thermometer. While this is coming to thermal equilibrium, obtain about 0.50 g of Mg turnings and weigh them accurately to the nearest 0.01 g and record this mass in the table. Use the Fume Hood for this reaction!! Now read the temperature of your CAL + HCl. Record this initial temperature of the HCl solution to the tenths place and record in the table on the next page. Drop in the weighed magnesium, replace the cover and read the temperature at half minute intervals (stirring constantly) until three successive readings show no longer show a change in temperature. All of the Mg should react. From this reaction, the temperature either went up or it went down. In the table, record the highest (or lowest) temperature reached record this T final. Place the spent reaction mixture in the waste container in the hood. NOTE: record all masses to the hundredths place and all temperature readings to the tenths place in the table on the next page. #7 H f of MgO Rev F08AEM Fall 2008 Page 3 of 9

4 DATA TABLE: ALL MASSES SHOULD BE TO THE HUNDREDTHS PLACE, ALL TEMPERATURE READINGS SHOULD BE TO THE TENTHS PLACE!! Mass of CAL Mass of CAL + HCl solution Mass of HCl solution Mass of Mg Initial Temp of HCl solution Final Temp of HCl + Mg T Calculations Because the HCl is such a dilute solution: assume that the mass of HCl solution = mass of H 2 O Energy = q = m H2 O ic H 2 O i!t Calculate the amount of heat (q) in Joules. Show your work and put units on your answer. q= Remember that the energy calculated, q, is also for the mass of Mg that you used. Use conversions to calculate the value in kj/mol Mg. Show your work and units in your answer. q = Is the reaction exothermic or endothermic? Therefore, what is H for the reaction? Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) H A = Remember, use the correct sign for H, that is consistent with your answer to the previous question! Instructor initials for value of H A : (note: you must get initials before moving on) #7 H f of MgO Rev F08AEM Fall 2008 Page 4 of 9

5 B. Procedure for Determining Heat of Reaction of MgO with HCl You will be following the same procedure for this experiment, EXCEPT you will use MgO. Place 100 ml of 2M HCl (measured from the same stock used in the above experiment) into the clean, dry CAL. Weigh as before to determine mass of HCl and proceed with MgO. Calculate and weigh out accurately an amount of MgO that is equimolar (equal in moles) to the amount of magnesium used above. Show this calculation in your work and weigh the MgO on weighing paper to ±0.01 g. Record the temperature of the 2M HCl in the CAL. Add the MgO to the acid (stirring constantly) and record the highest/lowest temperature reached. This could take up to 20 minutes as the temperature changes more slowly in this reaction than in the reaction of Mg with HCl. When you are finished, place the spent reaction materials in the waste container in the hood. Calculation for determining amount of MgO needed: Mass MgO = g Instructor initials for MgO amount DATA TABLE: REMEMBER TO RECORD YOUR MASSES TO THE HUNDREDTHS PLACE AND YOUR TEMPERATURE TO THE TENTHS PLACE! Mass of CAL Mass of CAL + HCl solution Mass of HCl solution Mass of MgO Initial Temp of HCl solution Final Temp of HCl + MgO T #7 H f of MgO Rev F08AEM Fall 2008 Page 5 of 9

6 Calculations Again, because the HCl is a dilute solution, assume that the mass of HCl solution = mass of H 2 O Energy = q = m H2 O ic H 2 O i!t Calculate the amount of heat (q) in Joules. Show your work and put units on your answer. q= Remember that the energy calculated, q, is also for the mass of MgO that you used. Use conversions to calculate the value in kj/mol MgO. Show your work and put units on your answer. q = Is the reaction exothermic or endothermic? Therefore, what is H for the reaction? MgO (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 O (l) H B = Remember, use the correct sign for H, that is consistent with your answer to the previous question! Instructor initials for value of H B : (note: you must get initials before moving on) #7 H f of MgO Rev F08AEM Fall 2008 Page 6 of 9

7 HESS LAW CALCULATION Using the information determined above and Hess s Law, the heat of formation ( H f ) for MgO can be obtained. Show how you can calculate the heat of formation of MgO using the equations A, B, and C from the first page. Hint: set-up reactions so that the sum is the formation equation for MgO. Perform the calculation to determine H f MgO. PAY ATTENTION TO SIG FIGS! A. Mg (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 (g) H A = B. MgO (s) + 2 HCl (aq) MgCl 2 (aq) + H 2 O (l) H B = C. H 2 (g) + ½ O 2 (g) H 2 O (l) H C = Rearrange the equations above to determine ΔH for: Mg (s) + ½ O 2 (g) MgO (s): show your work! Your value for H f MgO (pay attention to sig figs!) = Look up the theoretical (textbook) value for the heat of formation of MgO and calculate your percent difference. Textbook value for H = kj/mol % difference = List at least two sources of error (other than human error) that could lead to a % difference between your calculated and the actual value. Also explain how they would lead to a higher or lower value for H f MgO. a. b. #7 H f of MgO Rev F08AEM Fall 2008 Page 7 of 9

8 Stamp: Prelab Exercises 1. Define exothermic and endothermic: Exothermic: Endothermic: 2. When we look at the ΔH for a reaction, how can we identify (simply!) if the reaction is exo or endo thermic? 3. The amount of heat, q, obtained in a reaction of 0.10 mole HCl (aq) with excess NaOH (aq) is 96 calories. a. Calculate the heat in terms of the number of Joules (note: 1 calorie = Joules) b. Calculate q in kj/mol HCl. q = q = c. If the reaction gives off the heat, is the reaction endothermic or exothermic? d. Therefore, what is H for the reaction in kj/mol? kj/mol Remember, use the correct sign for H, that is consistent with your answer to the previous question! #7 H f of MgO Rev F08AEM Fall 2008 Page 8 of 9

9 4. On page 2 of the lab, there is an explanation about how q rxn will be determined. Read more about this and answer the following questions about the reactions you will be performing in lab: Circle the appropriate answer: a. If the temperature of the water increases, did the water absorb heat or release heat? Absorbs heat Releases heat b. Therefore, will q H2O be a +q or q? +q -q c. If the temperature of the water increases, then did the reaction absorb heat of release heat? Absorbs heat Releases heat d. How are q H2O and q rxn related to one another? - They are exactly the same - They are numerically the same with opposite charges - They have nothing to do with one another e. Therefore, will the q rxn be +q or q? +q -q 5. Given the following thermochemical equations, using Hess law, calculate H for the decomposition of one mole of acetylene (C 2 H 2 gas) into its elements in their stable/natural state at 25 C and 1 atm pressure.(hint: write the balanced chemical reaction for the decomposition of 1 mole of acetylene into its elements!) C 2 H 2 (g) + 5/2 O 2 (g) 2 CO 2 (g) + H 2 O (l) C (s) + O 2 (g) CO 2 (g) H 2 (g) + ½ O 2 (g) H 2 O (l) H 1 = kj H 2 = kj H 3 = kj Balanced equation: H rxn = #7 H f of MgO Rev F08AEM Fall 2008 Page 9 of 9