Up to this point, we’ve largely looked at chemical reactions as processes that have a starting point – reactants are combined – and an ending point – products are produced. This is not the case with all chemical reactions though. Some chemical reactions can occur in both directions at the same time.

When a reaction like this occurs, it will eventually reach a state called chemical equilibrium. Chemical equilibrium occurs when the rate of forward reaction is the same as the rate of reverse reaction. The result of this is that the concentrations of reactants and products are constant over time.

One critical condition to put on this is that this reaction must be taking place in a closed system. Neither energy nor matter can be added to or taken away from the system, otherwise that will impact the point of equilibrium.

An example of this type of reaction would be in a saturated solution, such as water, where more salt has been added than can be dissolved. While this might look like a static state to the naked eye, it is actually a very dynamic situation. Those salt crystals at the bottom of the solution are constantly dissolving back into the water, and as that occurs, new salt crystals precipitate out of the solution. The reaction is proceeding in both directions, at equal rates. Chemical equilibrium.

For these types of reactions – reactions that proceed in both directions – we use a different type of arrow in the chemical equation. This bidirectional arrow indicates that the reaction goes in both directions. In a chemical equation, it would look like this.

So let’s look at this example we have here. This is a process developed by a man named Fritz Haber – and while Haber himself did some pretty bad things with chemistry – developing this reaction allowed for the manufacture of ammonia from nitrogen and hydrogen. That allowed us to make fertilizer, which would prove to be critical for producing enough food to feed the world.

In this reaction, 1 mole of nitrogen gas reacts with 3 moles of hydrogen gas to produce 2 moles of ammonia.

Now, because this reaction proceeds in both directions, we also have ammonia breaking apart to produce nitrogen and hydrogen, and we know that it will eventually reach an equilibrium state, where the forward and reverse reaction occur at the same rate. The equilibrium position – that is to say the concentration of the reactants and products – will depend on the temperature and pressure that the reaction is occurring at.

So how do we know whether the equilibrium mixture will have mostly products or mostly reactants, and what the ratio between them will be? Well, we use a number called the equilibrium constant, K. This is a constant for a given reaction at a given temperature and pressure.

Using this reaction as an example, let’s show how we find the equilibrium constant. Now, when I put a chemical symbol in brackets, that represents that substance’s concentration in moles per liter, which you might recall, is called the molarity.

So this K-value is going to be a ratio. We’re going to put our products on top. We only have one product, NH3, and since that has a coefficient of 2 in the chemical equation, we are going to square that value. In the denominator, we’ll put our reactants. We have nitrogen, which has no coefficient in the equation, and hydrogen, which has a coefficient of 3, so we’ll cube that molarity value.

You can see from the structure of this formula, that when the k-value is very high, the equilibrium state will be mostly products. If it is very low, then it is mostly reactants. The equilibrium constant for this specific reaction, when it occurs at standard temperature and pressure, is well into the millions, so it’s mostly ammonia.

And that’s it, that’s how you find K. This is very useful! Often, though, you’ll be given a k-value for a reaction, and that will allow you to calculate how much of a certain reactant you need to produce a given amount of the product