Question:

The difference between ΔH and ΔU (ΔH - ΔU), when the combustion of one mole of heptane (l) is carried out at a temperature T, is equal to:

3RT

-3RT

-4RT

4RT

Correct option is C. -4RT
C7H16(l) + 11O2(g) → 7CO2(g) + 8H2O
Now, Δng = np - nr = 7 + 8 - 11 = 4
ΔH = ΔU + ΔngRT
∴ ΔH - ΔU = 4RT.