Chem 210 – Summer 2009 – Problem Set #10
1. Ethanol {C2H5OH(l)} reacts with oxygen gas to create carbon dioxide and water.
a. Calculate ΔH°, ΔS°, and ΔG° for this reaction. Is this reaction spontaneous?
b. How much energy is transferred between the system and the surroundings when 10.83g of ethanol is burned in excess oxygen? Is energy transferred from the system to the surroundings, or from the surroundings to the system?
2. Ideally, the hydrogen gas used in fuel cells would come from water.
a. Calculate ΔH°, ΔS°, and ΔG° for the decomposition of water to hydrogen gas and oxygen gas. Is this reaction spontaneous?
b. How much energy is transferred between the system and the surroundings when 6.28g of hydrogen gas is produced by this reaction? Is energy transferred from the system to the surroundings, or from the surroundings to the system?
3. In the previous problems, did you use liquid water or gaseous water? Why? How will changing from liquid-to-gas or gas-to-liquid change your answers? On a molecular level, explain the differences in ΔH°f, S°, and ΔG°f for liquid and gaseous water using the numbers found in thermodynamics tables. Why is gaseous water higher or lower than liquid water for each thermodynamic quantity?
4. If burning ethanol is a step in a reaction mechanism, the reaction may have to be multiplied by some integer.
a. Double all the coefficients in the balanced equation from #1 and calculate ΔG° for the “new” reaction. How is this value of ΔG° related to the value calculated in #1?
b. How much energy is transferred between the system and the surroundings when 10.83g of ethanol is burned in excess oxygen? Compare your answer to the value you found in #1b.
5. If 50.00g of ethanol is burned and all of the water produced by the reaction is decomposed to form hydrogen gas, what is the change in Gibb’s free energy for the whole process?
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