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Question 1: Thermal Energy and Heat Transfer (5 points) a. Cooking relies on heat transfer. What is the main type of heat transfer occurring in the

Question 1: Thermal Energy and Heat Transfer (5 points)

a. Cooking relies on heat transfer. What is the main type of heat transfer occurring in the following cooking methods? (3 points)

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      i. Boiling noodles in a pot of water on the stove (1 point)

      ii. Reheating leftovers in the microwave oven (1 point)

      iii. Frying an egg in a hot skillet (1 point)

b. There are some ovens called convection ovens, which cook food faster and more evenly. What might a convection oven have that a regular oven doesn’t? (1 point)

c. Why do people use potholders in the kitchen? (Give a scientific answer.) (1 point)

Question 2: Phase Changes (14 points)

a. Label each of the following phase changes as either exothermic or endothermic. (2 points)

      i. H2O(s) + heat  H2O(l) (1 point)

      ii. H2O(g)  H2O(l) + heat (1 point)

b. Describe how the temperature changes within a phase and during a phase change as heat is added to a water sample. (2 points)

c. How much energy is required to heat 3 kg of water through the following changes? Use values from latent heat and specific heat constant tables when necessary. Show your work. (10 points)

      i. Heat 3 kg ice from -10°C to 0°C (2 points)

      ii. Melt 3 kg ice at 0°C (2 points)

      iii. Heat 3 kg water from 0°C to 100°C (2 points)

      iv. Vaporize 3 kg water at 100°C (2 points)

      v. Heat 3 kg steam from 100°C to 110°C (2 points)

Question 3: Heat Capacity and Latent Heats (6 points)

Use the specific heat capacity and latent heat tables as needed to answer the following questions.  

a. A chef (who was good at chemistry) was in a tremendous hurry to make a meal. He had a copper-bottom pan as well as an aluminum-bottom pan of the same size. After looking up the specific heat capacity of each metal in his chemistry book (which, of course, he kept next to his cookbooks), which pan did he use? Why? (2 points)

b. The chef put a 523 g aluminum cookie sheet into an oven to heat. The cookie sheet started out at 25°C (room temperature) and was heated to 177°C. How much energy did the oven supply to heat the cookie sheet? (2 points)

c. The chef was careless and put the aluminum cookie sheet directly on the hot stove, which melted 0.05 kg of the aluminum. How much energy did the stove supply to melt the cookie sheet? (2 points)

Question 4: Enthalpy of Reaction (11 points)

a. Use enthalpies of formation to determine the ΔHreaction for the reaction MgCl2(s)  MgCl2(aq). (2 points)

b. Use Hess’s law and the following equations to calculate the ΔHreaction for the reaction CO(g) + 3H2(g) CH4(g) + H2O(g). (Show your work.) (4 points)

  • C(s) + O2(g)  CO(g)       ΔH = -110.5 kJ 
  • C(s) + 2H2(g)  CH4(g)       ΔH = -74.85 kJ 
  • H2(g) + O2(g)  H2O(g)       ΔH = -241.83 kJ

c. Use the equation C(s) + 2H2(g)  CH4(g), ΔH = -74.85, for the following questions. (5 points)

      i. Is the formation of methane (CH4) an exothermic or endothermic reaction? (1 point)

      ii. Draw a potential energy diagram for the reaction. (2 points)

      iii. Label the graph with the following: “Reactants,” “Products,” “ΔHreaction,” and “Ea.” (2 points)

Question 5: Entropy (4 points)

a. Predict if entropy would increase or decrease during the following phase changes. (2 points)

      i. H2O(g)  H2O(l) (1 point)

      ii. H2O(s)  H2O(l) (1 point)

b. Predict whether the entropy would increase or decrease in the reaction KCl(s)  K+(aq) + Cl-(aq). Explain your answer. (2 points)

Question 6: Gibbs Free Energy (10 points)

a. What is the Gibbs free energy for this reaction at 298 K?

2NH3(g)  N2(g) + 3H2(g)

ΔH = 91.8 kJ

ΔS = 0.1987 kJ/K

Is the reaction spontaneous at 298 K (room temperature)? (3 points)

b. How is it possible for an endothermic reaction to be spontaneous? (1 point)

c. What predictions can you make about ΔH, ΔS, and ΔG for the general reaction A(s) + 2B(g)  3C(g) + D(g) + heat? What conclusions can you make about the spontaneity of the reaction? Explain your reasoning. (4 points)

d. The reaction 2NO2(g)  N2(g) + 2O2(g) is spontaneous at all temperatures, yet it hardly happens under normal conditions. How can this be? (2 points)

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