Question

What is the change in enthalpy for the following reaction?

Answer 1

Answer : The reaction will be $2H_{2}O_{2} ----\ \textgreater \ 2H_{2}O + O_{2}$.

Now as the given enthalpy of $H_{2}O$ is -242 KJ
and the enthalpy of $H_{2}O_{2}$ is -609 KJ.

According to the formula of Hess's summation, ΔHreaction= (ΣΔHproducts)-(ΣΔHreactants)

Therefore, ΔHreaction = [$H_{2}O$ + $O_{2}$] - $H_{2}O_{2}$

on substituting the values and omitting the value for oxygen as it is in gaseous state we get,
ΔHreaction = [0 + (2 X -242)] - [2 X -609] 

on solving we get, ΔHreaction = 734 KJ.

Answer 2

The change in enthalpy is determined by the formula:

$\Delta H_{reaction} = \Sigma \Delta H_{products} - \Sigma \Delta H_{reactants}$

The reaction is:

$2H_2O_2(aq)\rightarrow 2H_2O(l)+O_2(g)$

Given:

$\Delta H_{H_2O} = -242 kJ$

$\Delta H_{H_2O_2} = -609 kJ$

The change in enthalpy for the reaction is calculated as:

$\Delta H_{reaction} = (2H_{H_2O}+H_{O_2}) - 2H_{H_2O_2}$

$H_{O_2} = 0 kJ$ (as it is in its standard state)

Substituting the values:

$\Delta H_{reaction} = (2\times (-242)+0) - 2\times (-609)$

$\Delta H_{reaction} = -484 kJ + 1218 kJ = 734 kJ$

Hence, change in enthalpy for the reaction is $734 kJ$.