Reversible Reaction

Reversible reaction definition

Some reactions only occur in one direction. The reactants react to form products, and that's it - the reaction is over. But in some cases, the products can react to form the reactants once again. We call these reactions reversible reactions.

You can look at a reversible reaction as being made up of two separate reactions:

• The reaction in which the reactants react to make the products is known as the forward reaction.
• The reaction in which the products react to make the reactants is down as the reverse reaction or the backward reaction.

Using our Lego analogy, combining the bricks together is like the forward reaction. We take the reactants, which are Lego bricks, and stack them up to build our product, the house. Taking the structure apart is like the backward reaction. We take the product, the house, and break it back down into the reactants, the Lego bricks. These two reactions combine to give one overall reversible reaction.

Reversible reaction symbol

So, we know that reversible reactions are made up of two separate reactions: the forward reaction and the backward reaction. Instead of writing both reactions out individually, we can combine them using two half-headed arrows to show a reversible reaction: ⇌

Here's an example. The reactants A and B react to form the product C. This is the forward reaction. C can then break down to give A and B again. This is the backward reaction. We can represent this reversible reaction using two separate equations, or we can combine them to give one overall equation:

Forward reaction: $\mathrm{A}\hspace{0.17em}+\mathrm{B}\to \mathrm{C}$

Backward reaction: $\mathrm{C}\to \mathrm{A}+\mathrm{B}$

Overall equation: $\mathrm{A}+\mathrm{B}\rightleftharpoons \mathrm{C}$

Reversible reaction example

Let's move on to look at some real-life examples of reversible reactions.

One visually interesting example is the hydration of cobalt(II) chloride. In its anhydrous form, it is blue. If you hydrate it, it turns pink. Evaporating the water off turns it back to its blue, anhydrous form once again. This makes cobalt(II) chloride a great test for water.

Forward reaction: ${\mathrm{CoCl}}_2+6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CoCl}}_2·6{\mathrm{H}}_2\mathrm{O}$

Backward reaction: ${\mathrm{CoCl}}_2·6{\mathrm{H}}_2\mathrm{O}\to {\mathrm{CoCl}}_2+6{\mathrm{H}}_2\mathrm{O}$

Overall equation: ${\mathrm{CoCl}}_2+6{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{CoCl}}_2·6{\mathrm{H}}_2\mathrm{O}$

Hydrating cobalt (II) chloride. Anna Brewer, StudySmarter Original

A biological example of a reversible reaction occurs in hemoglobin. Hemoglobin travels around in your blood, transporting oxygen from your lungs to your cells and waste carbon dioxide from your cells to your lungs. It does this by binding to the gas molecules with its four binding sites. You can think of the individual hemoglobin, oxygen and carbon dioxide molecules as the reactants and the bound hemoglobin molecule as the product. Once the bound hemoglobin molecule has reached its destination, it releases the gas molecules, breaking apart into the reactants once again.

Reversible reactions and equilibrium

If you leave the species involved in a reversible reaction in a closed system for a period of time, eventually, something particular will happen. If you start with high amounts of the reactants, initially you'll see lots of the forward reaction occurring. But as you produce more and more of the products, eventually the backward reaction starts to happen too. If you start with lots of the products, then the reverse happens - there'll initially be lots of the backward reaction, but as you produce more and more of the reactants, the forward reaction happens too. But no matter which side you start from, be it with lots of the reactants or lots of the products, eventually you'll reach a point of stability. Here, the rate of the forward reaction and the rate of the backward reaction is the same and the concentrations of reactants and products don't change. We call this a dynamic equilibrium.

Under certain conditions, a dynamic equilibrium always has a certain ratio of reactants to products. It doesn't matter whether you start with lots of reactants or lots of products - provided you keep variables like temperature and concentration the same, you'll end up with the same equilibrium. We express the ratio of reactants to products in a system at equilibrium using the equilibrium constant, K_eq.

Predicting the direction of a reversible reaction

Finally, we can use our knowledge of reversible reactions and equilibria to predict the direction of a reversible reaction and its effect on the ratio of reactants to products.

• If the rate of the forward reaction is greater than the rate of the backward reaction, then more of the reactants will be turned into products and there will be a net production of products.
• If the rate of the backward reaction is greater than the rate of the forward reaction, then more of the products will be turned into reactants and there will be a net production of reactants.
• If the rate of the forward reaction and the rate of the backward reaction are the same, then the system is at dynamic equilibrium and there is no net production of either products or reactants.

That's the end of this article. You should now feel confident defining reversible reactions and using the terms forward reaction and backward reaction. You should also be able to explain how a reversible reaction reaches equilibrium.