Chemical Equilibrium Basics

Click through each step to understand the basics.

What Is Dynamic Equilibrium?
Many chemical reactions don't go to completion in one direction — instead, the forward reaction and reverse reaction happen simultaneously and settle into a state called equilibrium.
Le Chatelier's Principle
Le Chatelier's principle states that when a change in concentration, temperature, or pressure is applied to a system at equilibrium, the equilibrium shifts in the direction that counteracts that change. For example, adding more reactant shifts the equilibrium toward consuming that reactant — that is, toward the forward reaction — to establish a new equilibrium.
How Temperature Affects Equilibrium
For an exothermic reaction that releases heat, raising the temperature shifts the equilibrium toward the reverse reaction, which absorbs heat, reducing the amount of product; lowering the temperature shifts it toward the forward reaction. This is because the equilibrium shifts in the direction that offsets the change in heat.
How Concentration Affects Equilibrium
Increasing the concentration of a reactant shifts the equilibrium toward consuming that reactant to produce more product, while decreasing the concentration of a product (by continuously removing it) keeps driving the reaction forward. This is why, industrially, reactants are often added in excess or products are continuously removed to maximize the yield of a desired product.
How Pressure Affects Equilibrium
In a reaction involving gases, increasing pressure shifts the equilibrium toward the side with fewer gas molecules, reducing volume, while decreasing pressure shifts it toward the side with more gas molecules. This is another example of Le Chatelier's principle, where equilibrium shifts to counteract an external change in pressure.
What the Equilibrium Constant (K) Means
The equilibrium constant K represents the ratio of product to reactant concentrations at equilibrium, at a given temperature, showing how far a reaction tends to proceed toward products. A very large K means products dominate at equilibrium, while a very small K means reactants remain largely unreacted.
Everyday Examples of Chemical Equilibrium
When you keep the cap on a bottle of soda, an equilibrium is maintained between the carbon dioxide gas in the bottle and the carbon dioxide dissolved in the liquid; opening the cap lowers the pressure and breaks that equilibrium, releasing gas as bubbles. The way hemoglobin binds and releases oxygen in our bodies is also a biological process that follows the principle of chemical equilibrium.

Why Chemical Equilibrium Is Worth Understanding

Many chemical reactions settle into a state of equilibrium rather than running to completion in one direction, with both the forward and reverse reactions happening at once. Understanding the principle of chemical equilibrium lets you understand everything from the fizz in a soda bottle to how industrial processes maximize the yield of a desired product.

Concentration Calculations That Affect Equilibrium Are Worth Knowing Too

The concentration that affects equilibrium is precisely expressed as molar concentration — the basic calculation methods for moles and molar concentration are a natural companion topic worth exploring separately.

Frequently Asked Questions

Does the reaction completely stop once equilibrium is reached?

No, the reaction keeps happening. It's just that the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products no longer appear to change — this is why it's called dynamic equilibrium.

Does adding a catalyst shift the position of equilibrium?

No — a catalyst speeds up the forward and reverse reactions by the same proportion, so it only helps the system reach equilibrium faster; it doesn't change the ratio of concentrations at equilibrium (the equilibrium constant).