Oxidation, reduction and redox reactions

​​In a nutshell

Redox reactions involve the transfer of electrons where both reduction and oxidation occurs. Metals typically behave as reducing agents.

What is a redox reaction?

A loss of electrons is known as oxidation and a gain of electrons is known as reduction. This can be remembered by the mnemonic OIL RIG, oxidation is loss and reduction is gain. These reactions occur simultaneously, hence the term redox.

An oxidising agent is reduced (gains electrons), and a reducing agent is oxidised (electrons lost). 

Oxidation numbers and electrons

For each electron lost, the oxidation number will go up by one. For each electron gained, the oxidation number goes down by one. 

To determine whether an element has been oxidised or reduced, the oxidation numbers before and after the reaction have to be compared. If the oxidation number has increased the element has been oxidised, as electrons have been lost. If the oxidation number has decreased the compound has been reduced, as electrons have been gained. 

When metals form compounds, they generally form positive ions. They donate electrons and behave as reducing agents. When non-metals form compounds, they generally form negative ions. They receive electrons and behave as oxidising agents.  

Example

Identify the reducing and oxidising agents in this reaction. 

 $$Ca\ +\ Cl_2\ \rightarrow\ CaCl_2$$

$$Ca$$ went from an oxidation number of $$0$$​ to an oxidation number of $$+2$$​. Calcium lost two electrons. This means that the calcium atom has been oxidised. Therefore, calcium is a reducing agent. 

$$Cl$$ went from an oxidation number of $$0$$​ to an oxidation number of $$-1$$​. Each chlorine atom has gained an electron. This means that the chlorine atoms have been reduced. Therefore, chlorine is an oxidising agent. 

Acids and metals

When acids react with metals, they produce a salt and hydrogen gas. The metal is oxidised (forms positive ions) and hydrogen is reduced (gains electrons).

Example 

$$2HCl\ + \ Mg\ \rightarrow \ MgCl_2\ +\ H_2$$

Hydrogen went from an oxidation number of $$+1$$​ to $$0$$​. It has been reduced.

Magnesium went from an oxidation number of $$0$$​ to an oxidation number of $$+2$$​. The magnesium atom was oxidised.