question_answer

In which of the following reactions, standard reaction entropy changes \[(\Delta {{S}^{o}})\] is positive and standard Gibb's energy change \[(\Delta {{S}^{o}})\] decreases    sharply   with    increasing temperature?     [AIPMT (S) 2012]

A)  \[C\,(\text{graphite})+\frac{1}{2}{{O}_{2}}(g)\xrightarrow[{}]{{}}CO(g)\]

B)  \[CO(g)+\frac{1}{2}{{O}_{2}}(g)\xrightarrow[{}]{{}}C{{O}_{2}}(g)\]

C)  \[Mg(s)+\frac{1}{2}{{O}_{2}}(g)\xrightarrow[{}]{{}}MgO(s)\]

D)  \[\frac{1}{2}C(\text{graphite})+\frac{1}{2}{{O}_{2}}(g)\xrightarrow[{}]{{}}\frac{1}{2}C{{O}_{2}}(g)\]

Correct Answer: A

Solution :

Among the given reactions only in the case of C (graphite)entropy increases because randomness (disorder) increases. Thus, standard entropy change  is positive.

Moreover, it is a combustion reaction and all the combustion reactions are generally exothermic, i.e.,

We know that

Thus, as the temperature increases, the value of  decreases.