Equilibrium — Definition & Overview

What Is Equilibrium?

Equilibrium is when something has reached a state of balance so that there are no net changes occurring. However, this doesn't mean the system is static.

In chemical reactions, a reaction is at equilibrium when the forward reaction is occurring at the same rate as the reverse reaction. Chemical equilibrium is often described as a dynamic, steady state.

In biology, equilibrium is used in different contexts, but the concept of equilibrium is the same. It can refer to organisms maintaining homeostasis, transporting materials across the cell membrane, genetic equilibrium (as in Hardy-Weinberg equilibrium), and chemical equilibrium as discussed here.

A Tug-of-War

Imagine you are playing tug of war. Even as you’re pulling with all your might, the rope isn’t going anywhere because the people on the other side are pulling just as hard. Though the rope isn’t moving at all, it would also be fair to say that it is moving in both directions at the exact same speed resulting in no net change.

Similarly, when a chemical system is not changing, it doesn’t necessarily mean there is no reaction in progress. Oftentimes, when a reaction can go both forwards and backwards, it ends up moving in both directions at the same pace, causing the reactants and products to stay the same. This process is called equilibrium. It's easy to remember because the rates of the forward and backward reaction are equal.

Now, imagine that your tug of war opponents suddenly get an extra burst of energy. As their “reaction” rate gets faster than yours, the rope moves towards their side. Of course, pulling extra hard takes strength, and the more they pull, the more tired they’ll get. They will gradually slow down until both sides are pulling at the same rate again. But since they’ve already pulled the rope further towards their side, the new equilibrium position, or place where the rope stays as both teams pull at the same rate, has shifted closer to the opposing team.

In the same way, when the rate of one side of a reaction in chemical equilibrium is increased by changing its circumstances, the balance of the forward and backwards products changes. The balance now favors more of the products created by the reaction whose rate was increased. This is called Le Chatelier's Principle.

How Does Equilibrium Change?

To understand this, let’s take a look at the following equilibrium reaction:

$2H_{2} + O_{2} \leftrightharpoons 2H_{2}O + heat$

This reaction describes the formation of water. $H_{2}O$ is the product of the forward reaction and $2H_{2}$ and $O_{2}$ are the products of the backwards reaction.

The rate of a chemical reaction can be increased in a few ways:

When more reactant is added, that reaction becomes faster. What this means is that if more $H_{2}$ and/or $O_{2}$ are added to this reaction at equilibrium, $H_{2}O$ will be created more quickly, causing the equilibrium to shift towards a state that contains more $H_{2}O$ than before.

When heat is added to an endothermic reaction, it becomes faster. Similarly, when heat is taken from an exothermic reaction, it becomes faster. Since an exothermic reaction is one that releases heat as a product, and an endothermic reaction is one that uses heat as a reactant, if one direction of the reaction is exothermic, the other will always be endothermic. For example, in the reaction above, heat is released as a product of the forwards reaction, meaning it is exothermic. The backwards reaction is therefore endothermic. So when heat is added to this reaction at equilibrium, the endothermic reaction will get faster, causing the equilibrium to shift to contain more $H_{2}$ and $O_{2}$.

When the space that a reaction is occurring in gets smaller, the atoms get closer together, causing the reaction to move towards the state that takes up less space. In this reaction, there are three molecules of backwards products (two molecules of $H_{2}$ and one molecule of $O_{2}$) for each two molecules of forward reactants (two molecules of $H_{2}O$). These $H_{2}O$ molecules will take up less space than the $H_{2}$ and $O_{2}$ molecules will. Therefore, if we put this equilibrium reaction into a smaller space, the forward reaction will get faster and the equilibrium will shift towards more $H_{2}O$.

(Video) Equilibrium: Crash Course Chemistry #28

by CrashCourse

Dynamic equilibrium is simply when two opposing forces or influences are balanced. In the natural world, it can get pretty complex.

The video above explains equilibrium in the biological and chemical context.