How to Rank Reactions by Enthalpy Changes

Instructor: Justin Wiens

Justin teaches college chemistry and has Bachelor and Doctorate degrees in chemistry.

This lesson describes the difference between exothermic and endothermic chemical reactions. We then show to rank enthalpy changes for chemical reactions, from least to most exothermic or endothermic, depending on the sign of the enthalpy change.

Introduction to Enthalpy Changes

When we observe chemical reactions, we are often interested not only in the transformation of one type of matter to another, but also the energy associated with the change.

Often, a chemical reaction either gives off or absorbs heat as the reaction proceeds. When the experiment is done under constant pressure conditions, such as the benchtop in a laboratory, the heat is called enthalpy. Technically, since we measure the flow of heat from one object to another, the heat is actually the enthalpy change, or ΔH.

Exothermic Vs Endothermic Reactions

The amount of enthalpy associated with a reaction is simply the amount of heat given off or absorbed by the chemical reactants. We can compare these amounts between chemical reactions to determine which ones are most exothermic (give off the most heat) or endothermic (absorb the most heat).

The heat has to go somewhere, though. When we feel a beaker with chemicals in it getting colder, this actually means that the enthalpy change for the reaction is positive because the heat leaves the beaker and goes into the chemicals. If the beaker gets warmer, then heat is leaving the chemicals as new chemical bonds are formed, and going into the beaker.

Quantifying Enthalpy Changes

We quantify the amount of heat in units of energy per mole-equivalent in a chemical reaction. It's easiest to understand what this means by looking at an example. Let's look at the combustion of methane, natural gas. The balanced chemical equation for this reaction is the following:

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2 O (g) ΔH = -802.3 kJ

The -802.3 kJ means that the reaction as written will give off that much heat to the environment; this reaction is exothermic, so ΔH is negative. It's like a chemical recipe: one mole of methane plus two moles of oxygen yields one mole of carbon dioxide and two moles of water, as well as 802.3 kJ heat.

If we double the recipe, that's the same thing as doubling the coefficients in the chemical equation: we would then produce 1604.6 kJ of heat.

If we reverse the chemical reaction, we would get the following:

CO2 (g) + 2H2 O (g) → CH4 (g) + 2O2 (g) ΔH = +802.3 kJ

Now, ΔH is positive, which tells us the reaction as written is endothermic. This tells us that we must put heat into the carbon dioxide and water to transform them back into methane and oxygen.

Ranking Enthalpy Changes

Let's compare several chemical reactions to determine which ones are most exothermic or endothermic.

Exothermic Reactions

Look at the following three chemical reactions:

C3 H8 (g) + 5O2 (g) → 3CO2 (g) + 4H2 O (g) ΔH = -2219.9 kJ

2H2 (g) + O2 (g) → 2H2 O (g) ΔH = -571.6 kJ

3H2 (g) + 2N2 (g) → 2NH3 (g) ΔH = -92.2 kJ

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