AP Problem Set 5: Due New Topics: Heat Transfer, Enthalpy, Hess s Law, Specific Heat, Calorimetry Directions: Complete all assigned problems. Show your work for all problems, write in complete sentences using specific vocabulary, don t forget units or sig figs. 1. The experimental apparatus represented above is used to demonstrate the rates at which gases diffuse. When the cotton balls are placed in the ends of a tube at the same time, the gases diffuse from each end and meet somewhere in between, where they react to form a white solid. Which of the following combinations will produce a solid closest to the center of the tube? Show work and explain your answer. (A) HCl and CH 3NH 2 (B) HCl and NH 3 (C) HBr and CH 3NH 2 (D) HBr and NH 3 2. For 2a-c refer to the diagram above and explain your answers. a. In which flask do the molecules have the greatest average speed? b. Which flask contains the smallest number of moles of gas? c. Which flask contains the sample with the greatest density?
3. Consider the data in the table below Container A B C Gas CO2 O2 NO2 Molar Mass 44 32 46 (g/mol) Volume 2.0 L 2.0 L 3.0 L Temperature 30 C 30 C 25 C a. Which gas has the greatest kinetic energy? Explain. b. If we define gas A as the system and gas A and C are mixed, is this process endothermic or exothermic? Explain and draw a diagram to support your reasoning. c. Initially, the pressure of all gases is the same. Then, the pressure of each gas is doubled. Which gas would now have the least kinetic energy? Explain. 4. You and your friend both grab a cup of hot chocolate, however when you grab the cup it feels hot and when your friend grabs the cup it feels cold to them. Explain how this is possible. Draw a diagram to support your reasoning. 5. Ozone, O3 (g) is a form of elemental oxygen that is important in the absorption of ultraviolet radiation in the stratosphere. It decomposes to form O2 (g) at room temperature and pressure according to the following reaction: 2O3 (g) 3 O2(g) H = -284.6 kj a. What is the enthalpy change for this reaction per mole of O3 (g)?
b. Which has the higher enthalpy under these conditions, 2O3 (g) or 3O2(g)? Explain. 6. Consider the following reaction: 2Mg(s) + O2 (g) 2MgO(s) H = -1204 kj a. Is this reverse reaction endothermic or exothermic? Explain. b. What is the enthalpy of formation of MgO (s)? Explain/show your work 7. What is the enthalpy change when 12.8g H2(g) reacts with excess Cl2(g) to form HCl(g)? H2(g) + Cl2(g) 2HCl(g) H = -184.6 kj 8. During discharge of a lead-acid storage battery, the following chemical reaction takes place: Pb + PbO2 + 2H2SO4 ---> 2PbSO4 + 2H2O Using the following two reactions: (1) Pb + PbO2 + 2SO3 ---> 2PbSO4 ΔH = -775 kj (2) SO3 + H2O ---> H2SO4 ΔH = -113 kj Determine the enthalpy of reaction for the discharge reaction above.
7. Given the following data, find the ΔH of the following reaction: C(s, gr) + O2(g) ---> CO2(g) SrO(s) + CO2(g) ---> SrCO3(s) ΔH = -234 kj 2SrO(s) ---> 2Sr(s) + O2(g) 2SrCO3(s) ---> 2Sr(s) + 2C(s, gr) + 3O2(g) ΔH = +1184 kj ΔH = +2440 kj 8. When a 10.0 g sample of Butane (C4H10) undergoes a combustion reaction, 80.5 kj of heat are released. Based on this information a. Write a balanced chemical equation for this process b. Calculate the molar heat of combustion of butane in kj/mol c. Calculate the standard heat of formation, H of butane in kj/mol 9. A sample of aluminum absorbed 9.86 J of heat, and its temperature increased from 23.2 C to 30.5 C. What was the mass of the aluminum sample? (The specific heat of aluminum is 0.90 J/g-K.)
10. Your friend gives you a 20 g block of cement (Cp = 1.54 J/g C) and a 20 g piece of clay (Cp = 0.92 J/g C). a. If you put both on a heat source for the same amount of time, which substance will be hotter an hour later? Explain. b. Imagine that 500 J of heat is added to each sample. Then, after the sample have been heated, the clay and cement are put in contact so that heat can directly transfer between them. What will the temperature of the samples be when they reach thermal equilibrium. 11. When a 4.25 g sample of solid ammonium nitrate dissolves in 60.0 g of water in a coffee-cup calorimeter, the temperature drops from 22.0 C to 16.9 C. Calculate H (in kj/mol NH4NO3) for the solution process. Draw a picture of the experimental set up as well. Assume the specific heat of the solution is the same as that of pure water NH4NO3(s) NH4 + (aq) + NO3 - (aq)