Chemistry » Chemical Equilibrium » What Is Chemical Equilibrium?

# Reversible Reactions

## Reversible Reactions

Some reactions can take place in two directions. In one direction the reactants combine to form the products. This is called the $$\color{orange}{\text{forward reaction}}$$. In the other direction, the products react to form the reactants again. This is called the $$\color{purple}{\text{reverse reaction}}$$. A special double-headed arrow ( $$\leftrightharpoons$$) is used to show this type of $$\color{blue}{\text{reversible reaction}}$$:

$$\text{XY} + \text{Z}$$ $$\leftrightharpoons$$ $$\text{X} + \text{YZ}$$

### Definition: A reversible reaction

A reversible reaction is a chemical reaction that can proceed in both the forward and reverse directions. In other words, the reactants and products of the reaction may reverse roles.

#### Fact:

Remember that $$\to$$ implies that A goes to B and cannot become A again. $$\rightleftharpoons$$ implies that the reaction is reversible. A can go to B and B can go to A.

So, in the following reversible reaction:

$\overset{{\color{orange}{\textbf{forward}\longrightarrow}}}{\text{H}_{2}\text{(g)} + \text{I}_{2}\text{(g)}} {\color{blue}{\leftrightharpoons}} \underset{\color{purple}{\longleftarrow\textbf{reverse}}}{2\text{HI(g)}}$

$$\color{orange}{{\text{H}}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}} \to {\text{2HI(g)}}}$$ is the $$\color{orange}{\text{forward}}$$ reaction.

The forward reaction is always taken from the given equation and written: left to right.

$$\color{purple}{{\text{2HI(g)}} \to {\text{H}}_{2}{\text{(g)}} + {\text{I}}_{2}{\text{(g)}}}$$ is the $$\color{purple}{\text{reverse}}$$ reaction.

The reverse reaction is always taken from the given equation and written: right to left.