Question

$△G_f^0$ (standard free energy of formation ) of HF(g) = -278.20 KJ mol ${}^{-1}$

$HI\left(g\right)$ = +1.72 KJ mol ${}^{-1}$

Based on the above data, select which reaction is most favourable in each case?

(a) $H_2+I_2\to 2HI$, or 2HI $\to H_2+I_2$

(b) $H_2+F_2\to 2HF$, or $2HF\to H_2+F_2$

• A. (a) $H_2+I_2\to 2HI$
  (b) $H_2+F_2\to 2HF$
• B. (a) $H_2+I_2\to 2HI$
  (b) $2HF\to H_2+F_2$
• C. (a) $2HI\to H_2+I_2$
  (b) $H_2+F_2\to 2HF$
• D. (a) $2HI\to H_2+I_2$
  (b) $2HF\to H_2+F_2$
• E. None of the above

Answer 1

The correct option is C (a) $2HI\to H_2+I_2$

(b) $H_2+F_2\to 2HF$
$2HI\to H_2+I_2$ is favourable because it is kinetically controlled rather than thermodynamically controlled.

$△G_r^0$= Negative for the reaction

$H_2+F_2\to 2HF$.

Higher value of Gibbs free energy for the reaction make it highly favourable.