# Question 1: Thermal Energy and Heat Transfer a. Three people are gathered around a campfire. One has his hands cupped around a ceramic mug of hot…

Question 1: Thermal Energy and Heat Transfer

a.Three people are gathered around a campfire. One has his hands cupped around a ceramic mug of hot chocolate to warm them. Another is toasting a marshmallow above the fire. The third is roasting a hot dog above the glowing coals. Identify the primary source of heat transfer each person is enjoying.

i.Person 1, the hand-warmer

ii.Person 2, the marshmallow-toaster

iii.Person 3, the hot-dog-roaster

b.A fourth person at the campfire doesn’t care about cold hands. He only cares that his hot chocolate stays warm. So he pours his hot chocolate into a Styrofoam cup instead of a ceramic mug. Why?

Question 2: Phase Changes

a.Rewrite each of the following equations for phase changes, to include the heat required for the phase change. Use values from latent heat and specific heat constant tables when necessary. Indicate whether each phase change is endothermic or exothermic.

i.H2O(l)  H2O(s)

ii.H2O(l)  H2O(g)

b.Calculate the total energy required to bring 2 kg ice from -5°C to 115°C. Use values from latent heat and specific heat constant tables when necessary. Show your work.

c.Why doesn’t the temperature of water keep changing at the melting point or boiling point as heat continues to be added?

Question 3: Heat Capacity and Latent Heats

a.Revolutionary War soldiers made their own musket balls by melting lead (Pb). How much energy did it take to heat enough lead for one musket ball from 25°C to 327.5°C (the melting point of lead)? Assume a musket ball contained 21 g of lead.

b.If the melted lead got hot enough to vaporize, how much energy would it have taken to vaporize 21 g of lead (Pb)?

Question 4: Enthalpy of Reaction

a.For a chemistry lab final exam, a high school chemistry student was given a 1-mole sample of CaCl2 and a 1-mole sample of MgCl2 but was not told which sample was which. He was to identify the powders.

He looked up the enthalpies of formation for both of the chemicals and calculated the ΔHreaction for dissolving each powder: CaCl2(s)  Ca2+(aq) + 2Cl-(aq), and MgCl2(s)  Mg2(aq) + 2Cl-(aq). He then put each powder in a coffee-cup calorimeter and added water.

When sample A dissolved, the temperature increased by 0.74°C. When sample B dissolved, the temperature increased by 0.39°C. Which chemical was A, and which was B? Use the table of enthalpies of formation to help you. Explain your reasoning.

b.Use Hess’s law and the following equations to calculate the ΔHreaction for the reaction C3H8(g) + 5O2(g)  3CO2(g) + 4H2O(l). Show your work.

• 3C(s) + 4H2(g)  C3H8(g)       ΔH = -103.85 kJ/mol
• C(s) + O2(g)  CO2(g)       ΔH = -395.5 kJ/mol
• 2H2(g) + O2(g)  2H2O(l)       ΔH = -571.66 kJ/mol

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