CHEMISTRY 123 Problem Set #3 First Law of Thermodynamics
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?
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
(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