Enthalpy Change of a Reaction Example Problem

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Helmenstine, Anne Marie, Ph.D. "Enthalpy Change of a Reaction Example Problem." ThoughtCo, Feb. 15, 2017, thoughtco.com/example-enthalpy-change-problem-609555. Helmenstine, Anne Marie, Ph.D. (2017, February 15). Enthalpy Change of a Reaction Example Problem. Retrieved from https://www.thoughtco.com/example-enthalpy-change-problem-609555 Helmenstine, Anne Marie, Ph.D. "Enthalpy Change of a Reaction Example Problem." ThoughtCo. https://www.thoughtco.com/example-enthalpy-change-problem-609555 (accessed September 24, 2017).
Enthalpy is a measure of thermal energy.
Enthalpy is a measure of thermal energy. PM Images, Getty Images

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

Enthalpy 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

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

Answer:

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

Format
mla apa chicago
Your Citation
Helmenstine, Anne Marie, Ph.D. "Enthalpy Change of a Reaction Example Problem." ThoughtCo, Feb. 15, 2017, thoughtco.com/example-enthalpy-change-problem-609555. Helmenstine, Anne Marie, Ph.D. (2017, February 15). Enthalpy Change of a Reaction Example Problem. Retrieved from https://www.thoughtco.com/example-enthalpy-change-problem-609555 Helmenstine, Anne Marie, Ph.D. "Enthalpy Change of a Reaction Example Problem." ThoughtCo. https://www.thoughtco.com/example-enthalpy-change-problem-609555 (accessed September 24, 2017).