2001 AIPMT ThermodynamicsEnthalpy of Reaction from Enthalpies of Formation Easy Question 77 of 82 in Thermodynamics

Change in enthalpy for reaction $2H_2O_2(l) \rightarrow 2H_2O(l) + O_2(g)$, if heat of formation of $H_2O_2(l)$ and $H_2O(l)$ are –188 and –286 kJ/mol respectively : -

A –196 kJ/mol
B +196 kJ/mol
C +948 kJ/mol
D –948 kJ/mol

The answer is A.

Solution

$\Delta H_r = \sum\Delta H_f(\text{products}) - \sum\Delta H_f(\text{reactants})$
$\Delta H_f(O_2) = 0$ (element in its standard state)
$\Delta H_r = [2(-286) + 0] - [2(-188)] = -572 + 376$
$\Delta H_r = -196$ kJ/mol

Concept Hess's law with heats of formation

Thermodynamics in past papers

82 questions from this chapter have appeared across 26 exam years.