Q18 / 90 · JEE Main 2019 (10 Apr Shift 2) · Chemistry

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

Correct answer
A
$- 4 R T$
B
$3 R T$
C
$4 R T$
D
$- 3 R T$

Solution

Combustion of heptane

C_{7} H_{16} (l) + 11 O_{2} (g) ⟶ 7 C O_{2} (g) + 8 H_{2} O (l)

∆ H = ∆ U + ∆ n g R T

∆ H - ∆ U = ∆ n g R T

∴ ∆ n g = 7 - 11 = - 4

= - 4 \times R T

Hence, the correct answer is option: 'A'.

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