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# Enthalpy Change Example Problem

## Enthalpy Change of a Reaction

Enthalpy is a measure of thermal energy.

PM Images, Getty Images

Practice calculating the enthalpy change of a chemical reaction:

Review

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

H2O2(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

ΔH = +196.4 kJ for the reaction: 2 H2O(l) + O2(g) → 2 H2O2(l)

Anne Marie Helmenstine, Ph.D.