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Hess's law
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Concepts tested here
- Combining thermochemical equations 2
All Questions
2011 AIPMT-MAINS 1 question
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Consider the following processes:
$\frac{1}{2}A \rightarrow B$; $\Delta H = +150$ kJ/mol
$3B \rightarrow 2C + D$; $\Delta H = -125$ kJ/mol
$E + A \rightarrow 2D$; $\Delta H = +350$ kJ/mol
For $B + D \rightarrow E + 2C$, $\Delta H$ will be(i) $\frac{1}{2}A \rightarrow B$; $\Delta H = +150$
(ii) $3B \rightarrow 2C + D$; $\Delta H = -125$
(iii) $E + A \rightarrow 2D$; $\Delta H = +350$
Multiply (i) by 2: $A \rightarrow 2B$; $\Delta H = +300$
Reverse (iii): $2D \rightarrow E + A$; $\Delta H = -350$
Add 2 × (i), (ii) and reversed (iii): $A + 3B + 2D \rightarrow 2B + 2C + D + E + A$
Cancelling common species: $B + D \rightarrow E + 2C$
$\Delta H = 2(150) + (-125) - 350 = 300 - 125 - 350$
$\Delta H = -175$ kJ/mol
2010 AIPMT-MAINS 1 question
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The following two reactions are known:
$Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g)$; $\Delta H = -26.8$ kJ
$FeO(s) + CO(g) \rightarrow Fe(s) + CO_2(g)$; $\Delta H = -16.5$ kJ
The value of $\Delta H$ for the following reaction $Fe_2O_3(s) + CO(g) \rightarrow 2FeO(s) + CO_2(g)$ is(1) $Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$; $\Delta H_1 = -26.8$ kJ
(2) $FeO + CO \rightarrow Fe + CO_2$; $\Delta H_2 = -16.5$ kJ
Reverse (2) and multiply by 2: $2Fe + 2CO_2 \rightarrow 2FeO + 2CO$; $\Delta H = +33.0$ kJ
Add this to (1): $Fe_2O_3 + 3CO + 2Fe + 2CO_2 \rightarrow 2Fe + 3CO_2 + 2FeO + 2CO$
Cancelling common species: $Fe_2O_3 + CO \rightarrow 2FeO + CO_2$
$\Delta H = \Delta H_1 - 2\Delta H_2 = -26.8 - 2(-16.5) = -26.8 + 33.0$
$\Delta H = +6.2$ kJ