# Enthalpy Change for a Specific Amount of Reactant

Here is how to determine the change in enthalpy of a chemical reaction with a given amount of reactant

You may wish to review the laws of thermochemistry and endothermic and exothermic reactions​ before you begin.

## Problem:

For the decomposition of hydrogen peroxide, it is known that:
H2O2(l) → H2O(l) + 1/2 O2(g); ΔH = -98.2 kJ
Using this information, determine ΔH for the reaction:
2 H2O(l) + O2(g) → 2 H2O2(l)

## Solution:

When looking at the second equation, we see it is double the first reaction and in the opposite direction.

First, change the direction of the first equation. When the direction of the reaction is changed, the sign on ΔH changes for the reaction

H 2O2(l) → H2O(l) + 1/2 O2(g); ΔH = -98.2 kJ

becomes

H2O(l) + 1/2 O2(g) → H2O2(l); ΔH = +98.2 kJ

Second, multiply this reaction by 2. When multiplying a reaction by a constant, the ΔH is multiplied by the same constant.
2 H2O(l) + O2(g) → 2 H2O2(l); ΔH = +196.4 kJ