Enthalpy Change: Definition & Calculation

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we will learn how much energy is in a reaction and what can drive that reaction forward. We will also learn how to calculate the efficiency of that reaction.

What is Enthalpy?

Imagine that you had two cars and both needed 30 gallons of gasoline put into them to run, yet one would go 100 miles on that tank, while the other would go 600 miles. You'd probably choose to take the 600-mile tank for a long trip, right? Although both cars require a lot of energy, one is much better at using that energy to operate.

This same idea is what drives some reactions. Often a lot of energy is needed to be put into a reaction to make it start. But for favorable reactions, the ending energy level is often lower than the starting energy. The amount of energy in a reaction or in a compound is called enthalpy.

Change in Enthalpy

Enthalpy is often described as the amount of heat in a system. When it comes to measuring the energy associated with enthalpy, heat and energy can be used synonymously. Enthalpy is often measured by burning the compound and seeing how much heat it gives off. Really what is happening when burning the compound is a reaction that breaks down the compound. So the change in energy (from the original compound to the broken-down compound) is being measured to determine how much energy that compound is capable of giving off.

One way for a reaction to be driven forward is by having lower energy in the products than the reactants. This means that the change in enthalpy is negative. A reaction can still occur if the enthalpy change is positive, there just needs to be other things driving that reaction forward. Enthalpy change is the difference in enthalpy (energy) at the beginning of the reaction to the end of the reaction.

Calculating Enthalpy Change

In order to calculate enthalpy change, you first need to know the beginning and ending enthalpy. One of the most common ways for this to be done is by burning the compound (as mentioned above). This is done in a calorimeter, which is simply a container which burns the compound and measures the amount of heat given off. Tables called heat of formation tables will list common compounds and the enthalpy as it has already been computed.

Once you know the enthalpy of each of the starting and ending materials you can determine the change in enthalpy. Using a balanced chemical equation, you multiply each compound's enthalpy by the number of molecules used in the balanced equation. Then you find the difference between the total enthalpy of all compounds on the reactants side from the total enthalpy of all compounds on the products side.

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