Chemical Equilibrium

Imagine a coffee shop, where customers enter and leave to get their drinks. If the rate at which people enter the shop equals the rate at which people leave the shop, then the number of customers in the shop is constant. This situation is known as "equilibrium", where the rate at which people exit and enter the shop are equal.

This same principle can be applied to the concentrations of species in chemical reactions. Recall that the molar concentration of a species is the number of moles of that species in a given volume. This means that the rate at which products form and reactants form are equal. If that sentence is one of the most baffling things you've heard, then don't worry, because that is often expected.

It is a misconception that every chemical reaction must go from left to right, reactants to products, and for a process to complete, all the reactants must turn into products. Well, reactions known as reversible reactions can also proceed from right to left. A system is at equilibrium if the rate at which products become reactants in the reverse reaction equal the rate at which reactants become products in the forward reaction. When this happens, the concentrations of the species in the reaction become constant. Note that at equilibrium, the number of reactants and products need not be equal. There could be more reactants than products at equilibrium and vice versa.

The way to denote a reversible reaction is with a bidirectional arrow(⇆).

Now, I just gave you a handful of properties on chemical equilibrium but let's provide context using the equation below:

N₂O₄(g) ⇆ 2NO₂(g)

This reaction is reversible as denoted by the bidirectional arrow above.

As you can see above with the concentration vs time graph, when equilibrium is reached, the concentrations of the involved species hit a plateau. Their rates also become equal at this time as seen from the rate vs time graph at the bottom.