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Problem Set: Thermodynamics

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CHEM 123
Angie O' Neill

CHEMISTRY 123 Problem Set #3 First Law of Thermodynamics Concept questions 1. A system expands and lifts a weight in its surroundings. According to our convention, is the work associated with this change in state a positive or negative quantity? How does this work affect the energy of the system? 2. You have your hands wrapped around a system. The system changes state and produces a flux of heat at its boundaries. This flux, qP, is positive. How does this feel to you? Is the change in enthalpy, ∆H, associated with this change in state positive or negative? 3. Let us say that the apparatus of question 2 above consists of plastic cup containing water with solutes undergoing a chemical reaction. The solutes constitute the system. The water solvent forms part of the surroundings (as do the walls of the cup and the universe beyond.) A thermometer in the solution shows the temperature dropping. What is the sign of ∆H for the water? What is the sign of ∆H for the solutes? Problems 1. One mole of an ideal gas at 298 K undergoes an isothermal expansion in a single step from an initial volume of 20 L to a final volume of 40 L against a constant opposing pressure of 0.5 atm. Calculate ∆E for this change in state. How does this result for ∆E compare with ∆H for the same change in state? 2. Given the enthalpy changes at a certain temperature for reactions (a) and (b), calculate the enthalpy change for reaction (c). Is reaction (c) endothermic or exothermic at this temperature? −1 (a) 3H 2g) + N 2g) → 2NH (g)3 ∆H = −92.4 kJ mol (b) 2H (g) + O (g) → 2H O(g) ∆H = −483 kJ mol −1 2 2 2 (c) 4NH (3) + 3O (g2 → 2N (g) 2 6H O 2 3. Given the following data, determine which of the numbered values is the correct one for t
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