What is a chemical reaction and why do they occur?

What may be a simple question can have chemists discussing its definition in detail for days, and the reactions driving force can add even more complexity to it. But why do they occur?

In basic terms, a chemical reaction is an event where one or more molecules break and reform chemical bonds to end up in a different form than they started in.

However, to delve deeper into this idea we must know what causes chemical reactions to happen.

A chemical reaction favours one thing above all others, stability. Stability is expressed in chemistry through a value of the overall “randomness” of the product. The higher the value of different configurations that energy can be spread out in the product, the more stable a product is.

This sounds all well and good in terms of theory, but may be a difficult concept to visualize in reality. Let us look at a very standard chemical reaction in the melting of ice. Whilst ice may seem physically more stable than water, the latter is true. Its rigid structure of strong hydrogen bonds does not allow for much in the way of special distributions of energy in the structure. In comparison, the liquid nature of water allows kinetic energy and flow. This ability for liquid to have and hold kinetic energy (eg: mixing the water) allows many more distributions of the layout of individual water molecules; thus giving more overall randomness to the solution than the solid-state of ice.

For this reason, ice will always tend to melt unless otherwise acted on by an outside source of energy to reassemble the liquid molecules back into a solid form.

This concept is known to chemists by a different name. That being entropy, which pairs nicely with the ending to this blog. Entropy usually cannot go without its thermodynamic cousin, enthalpy which we will discuss later in our series of blogs.