Thermodynamics And Thermochemistry Result Question 2
2. Given :
(i) C (graphite) $+\mathrm{O}_2(g) \rightarrow \mathrm{CO}2(g) ; \quad \Delta{\mathrm{r}} H^{\ominus}=x \mathrm{~kJ} \mathrm{~mol}^{-1}$
(ii) C (graphite) $+\frac{1}{2} \mathrm{O}2(g) \longrightarrow \mathrm{CO}(g)$;$ \quad\Delta{\mathrm{r}} H^{\ominus}=y \mathrm{~kJ} \mathrm{~mol}^{-1}$
(iii) $\mathrm{CO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{CO}2(\mathrm{~g}) ;\quad \Delta{\mathrm{r}} H^{\ominus}= -z \mathrm{~kJ} \mathrm{~mol}^{-1}$
Based on the above thermochemical equations, find out which one of the following algebraic relationships is correct?
(2019 Main, 12 Jan II)
(a) $y=2 z-x$
(b) $x=y-z$
(c) $z=x+y$
(d) $x=y+z$
Show Answer
Answer:
Correct Answer: 2. ( d )
Solution:
- Second equation given in this question is wrong. Hence, no answer is correct.
If the corrected second equation is given,
i.e. C $\text { (graphite) }+\frac{1}{2} \mathrm{O}_2(g) \longrightarrow \mathrm{CO}(g)$
and if we take the above reaction into consideration, then $x = y + z$ will be the answer as:
(ii) C (graphite) $+\frac{1}{2} \mathrm{O}_2(g) \longrightarrow \mathrm{CO}(g), \quad \Delta_r H^{\circ}=y \mathrm{~kJ} / \mathrm{mol}$
(iii) $\mathrm{CO}(\mathrm{g})+\frac{1}{2} \mathrm{O}_2(g) \longrightarrow \mathrm{CO}_2(g), \quad \Delta_r H^{\circ}=z \mathrm{~kJ} / \mathrm{mol}$
Summing up both equations, you will get equation (i):
$\mathrm{C} \text { (graphite) }+\mathrm{O}_2(\mathrm{~g}) \longrightarrow \mathrm{CO}_2(\mathrm{~g}), \quad \Delta_r H^{\circ}=x \mathrm{~kJ} / \mathrm{mol}$
Hence, $x, y$ and $z$ are related as:
$x=y+z$
BETA
