Question

Which of the following reaction will proceed in forward direction on decreasing the temperature?

• A. $H_2\left(g\right)+I_2\left(g\right)\rightleftharpoons 2HI\left(g\right){\Delta }_{r}H=+3000\text{cal}$
• B. $N_2\left(g\right)+O_2\left(g\right)\rightleftharpoons 2NO\left(g\right){\Delta }_{r}H=-43200\text{cal}$
• C. $2N{H}_3\left(g\right)\rightleftharpoons N_2\left(g\right)+3H_2\left(g\right){\Delta }_{r}H=+22400\text{cal}$
• D. $C{O}_2\left(g\right)\rightleftharpoons C\left(s\right)+O_2\left(g\right){\Delta }_{r}H=+94300\text{cal}$

Answer 1

The correct option is B $N_2\left(g\right)+O_2\left(g\right)\rightleftharpoons 2NO\left(g\right){\Delta }_{r}H=-43200\text{cal}$

If $\Delta H>0$ then endothermic
If $\Delta H<0$ then exothermic
In exothermic reaction, heat is liberated in products side. There is an increase in temperature as we go in the forward direction. Similarly for an endothermic reaction, there is a decrease in temperature in the forward direction.
On decreasing temperature, forward reaction will be favoured if there is an increase in temperature for that side. It happens for an exothermic reaction.
Only option B is an exothermic reaction because it has $\Delta H<0$.