Question

The difference between ΔH and ΔU (ΔH – ΔU), when the combustion of one mole of heptane (1) is carried out at a temperature T, is equal to:
1. - 4RT
2. - 3RT
3. 4RT
4. 3RT

Answer 1

Correct Answer - Option 1 : - 4RT

Concept:

The combustion reaction is given below:

C₇ H₁₆ (l) + 11O₂ (g) ⟶ 7CO₂ (g) + 8H₂O (l)

The change in enthalpy and change in energy is given by the equation:

ΔH = ΔU + Δn_g RT

⇒ ΔH – ΔU = Δn_g RT

Where,

Δn_g = change in the number of moles of gas molecules 

Calculation:

Δn_g = n_p - n_r = 7 – 11 = -4

Now,

⇒ ΔH – ΔU = (-4)RT

∴ ΔH – ΔU = -4RT