Question

For the hypothetical reaction

A2(g) + B2(g) $\rightleftharpoons$2AB(g)

If $\Delta$rG⁰ and $\Delta$rS⁰ are 20 kJ/mol and –20 JK–1mol–1 respectively at 200 K.

$\Delta$rCp is 20 JK–1mol–1 then $\Delta$rHŗ at 400 K is :

• A. 20 kJ/mol
• B. 7.98 kJ/mol
• C. 28 kJ/mol
• D. None of these

Answer 1

Answer: (A)

20 kJ/mol

Answer 2

$\Delta G_{200}^{0}$ = $\Delta H_{200}^{0}$ – $T\Delta S_{200}^{0}$

$\Delta H_{200}^{0}$ = 20 – 4 = 16 kJ/mol

$\Delta H_{T_{2}}^{0}$ = $\Delta H_{T_{1}}^{0}$+ $\Delta$Cp[T2 – T1]

$\Delta H_{400}^{0}$ = $\Delta H_{200}^{0}$+ $\frac{20 \times 200}{1000}$kJ/ mol = 16 + 4 = 20 kJ/mol.