Thermodynamics and Thermochemistry 2 Question 13
13. The following is/are endothermic reaction(s)
(1999, 3M)
(a) Combustion of methane
(b) Decomposition of water
(c) Dehydrogenation of ethane to ethylene
(d) Conversion of graphite to diamond
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Solution:
- $\mathrm{H}{2} \mathrm{O} \longrightarrow \mathrm{H}{2}+\frac{1}{2} \mathrm{O}_{2}, \Delta H>0$
It is reverse of combustion of $\mathrm{H}_{2}(g)$, hence endothermic.
$$ \mathrm{C}{2} \mathrm{H}{6} \longrightarrow \mathrm{C}{2} \mathrm{H}{4}+\mathrm{H}_{2} ; \quad \Delta H>0 $$
Here, more stable (saturated) hydrocarbon is being transformed to less stable (unsaturated) hydrocarbon, hence endothermic.
$$ \mathrm{C}{(\mathrm{gr})} \longrightarrow \mathrm{C}{(\mathrm{d})}, \quad \Delta H>0 $$
More stable allotrope is being converted to less stable allotrope.
