Q4. For N2(g) + 3H2(g) → 2NH3(g) at 298 K, find ΔH − ΔU.
Δng = 2 − 4 = −2. ΔH − ΔU = ΔngRT = −2 × 8.314 × 298, about −4.96 × 103 J, or −4.96 kJ.
Q5. Is ΔH equal to ΔU for H2(g) + Cl2(g) → 2HCl(g)? Explain.
Yes (for ideal gases): there are 2 mol of gas on each side, so Δng = 0 and ΔH = ΔU + 0.
Q6. How much heat is needed to warm 100 g of water from 25 °C to 75 °C? Take c = 4.18 J g−1 K−1.
q = mcΔT = 100 × 4.18 × 50 = 20 900 J, about 20.9 kJ.
Q7. A gas expands from 5.0 L to 10.0 L against a constant external pressure of 2.0 atm. Find the work done, in joules (1 L atm = 101.3 J).
w = −pexΔV = −2.0 × 5.0 = −10 L atm = −10 × 101.3, about −1.01 × 103 J. Negative: the gas does work on the surroundings.
Q8. For 2.0 mol of an ideal gas, what is Cp − CV (as total heat capacities, not molar)?
For one mole the difference is R; heat capacity is extensive, so for 2.0 mol it is 2R = 2 × 8.314, about 16.6 J K−1.