Chemistry

Enthalpy Change Example Problem

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
Answer

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

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