Homogeneous vs. Heterogeneous Equilibrium Reactions

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

There are a lot of scary words in chemistry. This lesson will unpack some of those words, defining homogenous and heterogeneous equilibrium reactions and then explaining how to write out the equilibrium constant for both types of reactions.

What Are Homogenous and Heterogenous Equilibrium Reactions?

There's a whole bunch of words that have the prefixes hetero and homo. For example, there's heterophony, heterosexual, and heterodyne. Or there's homology, homosexual, and homophone. While these words all mean something different, the prefixes give you hints about what the words mean. Hetero comes from the Greek word for different and homo comes from the Greek word for the same. For example, heterosexual means a person is attracted to someone of a different sex, or homophone means two words are pronounced the same.

In keeping with the hetero and homo prefix theme, it should be no surprise that a homogenous equilibrium reaction is a reaction where all of the products and reactants are in the same phase. For example, all are solids or all are liquids, and so on. A heterogeneous equilibrium reaction, in contrast, is a reaction where the products and reactants are in different phases. For example, one compound or molecule might be a solid, while another is a liquid, and another is a gas. You can determine the phase of the molecules and compounds in a reaction by the small letters that are in parenthesis. A gas, for example, is (g), a liquid is (l), a solid is (s), and aqueous is (aq).

Equilibrium just refers to a reaction that can go in both the forward and reverse directions, and the concentrations of the products and the reactants don't change. You'll be able to determine that a reaction is at equilibrium by the double arrow.

Two reactions that are at equilibrium. The top reaction is a homogenous equilibrium reaction and the bottom is a heterogeneous equilibrium reaction.
Homo and hetero

The Equilibrium Constant

Before we delve into how to calculate the equilibrium constant, let's take a moment to discuss what the equilibrium constant means. The equilibrium constant is represented by a K or a Kc and is an expression that relates the concentrations of products and reactants after the reaction has gone to equilibrium. If the temperature remains the same, the equilibrium constant should remain the same for the reaction. Just a reminder, the reactants are on the left side of the arrows and the products are on the right side.

In order to understand how we write out the equilibrium constant, let's keep things simple and represent the molecule and compounds with A, B, C and D. And let's represent the coefficients, or the numbers in front of the molecules and compounds, with a, b, c, and d.

Simplified reaction
Example

In order to write the equilibrium constant, place C and D on the top and A and B on the bottom. The coefficients will become exponents.

Writing out the equilibrium constant
Equilibrium constant

Homogenous vs. Heterogeneous Equilibrium Constants

Now that you know the formula, let's see how you would set up a couple of problems, starting with homogenous equilibrium reactions.

Homogenous Example

Let's use the following homogenous equilibrium reaction where sulfur dioxide combines with oxygen to form sulfur trioxide.

Homogenous equilibrium example 1
Homo example 1

For homogenous equilibrium reactions, place the products and reactants in the proper place in the formula. And remember, when there are numbers in front of the products or reactants, they become exponents. The subscripts, or the small numbers next to the compounds and molecules, don't do anything and just go into the formula with the products and reactants.

Writing out the equilibrium constant for a homogenous equilibrium reaction
Homo2

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