Question

Which of the following reaction has the positive value of standard enthalpy of reaction?

• A. C$_2$H$_5$OH(l) + 3O$_2$(g) $\rightarrow$ 2CO$_2$(g) + 3H$_2$O(l)
• B. 2SO$_2$(g) + O$_2$(g) $\rightarrow$ 2SO$_3$(g)
• C. 2NH$_3$(g) $\rightarrow$ N$_2$(g) + 3H$_2$(g)
• D. C(s) + O$_2$(g) $\rightarrow$ CO$_2$(g)
• E. CO(g) + $\frac12$O$_2$(g) $\rightarrow$ CO$_2$(g)

Hint:

Chemistry Tip: Most combustion reactions are strongly exothermic, so their $\Delta H$ is negative.

Answer 1

The Correct Option is C

Concept:
Positive standard enthalpy means: $$\Delta H^\circ>0$$ The reaction absorbs heat (endothermic).
Step 1: Check combustion reactions.
(A), (D), and (E) are combustion/oxidation reactions. These release heat, so: $$\Delta H^\circ<0$$
Step 2: Check formation of SO$_3$.
(B) Oxidation of SO$_2$ to SO$_3$ is also exothermic. So: $$\Delta H^\circ<0$$
Step 3: Check decomposition of ammonia.
(C) $$2NH_3(g)\rightarrow N_2(g)+3H_2(g)$$ Breaking ammonia into elements requires energy overall, hence endothermic. So: $$\Delta H^\circ>0$$
Step 4: Final answer.
Therefore correct option is (C). :contentReference[oaicite:1]{index=1}