Oxidation States & Balancing Redox Equations (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about oxidation states: what oxidation and reduction are, the rules for working one out, and how a redox equation gets balanced.

Oxidation states are used to tell whether oxidation or reduction has taken place, to work out what has been oxidised and what has been reduced, and to balance redox equations, so one number runs through all three parts of this video.

The oxidation state of an atom is the charge that would exist on that atom if the bonding were completely ionic. The rules give you that number for any atom, and a change in that number is what oxidation and reduction actually are, so balancing the changes is what balances a redox equation.

We start with what oxidation and reduction are, and what an oxidation state is. Then oxidising and reducing agents, which are the substances doing the oxidising and the reducing. Then the rules for assigning an oxidation state to any atom. And last, using those states to balance a redox equation.

There are three definitions of oxidation and reduction, used in different branches of chemistry. Oxidation is the gain of oxygen, the loss of hydrogen, or the loss of electrons. Reduction is the loss of oxygen, the gain of hydrogen, or the gain of electrons. Redox reactions are reactions in which oxidation and reduction take place at the same time, so while one species is being oxidised, another is being reduced in the same reaction. The oxidation state of an atom is the charge that would exist on an individual atom if the bonding were completely ionic, and it is what tells you which of these processes has happened.

An oxidising agent is a substance that oxidises another atom or ion by causing it to lose electrons. It gains those electrons itself, so an oxidising agent is reduced, and its own oxidation state decreases. A reducing agent works the other way round: it reduces another atom or ion by causing it to gain electrons, it donates those electrons itself, so it is oxidised and its own oxidation state increases. For a reaction to be a redox reaction, there has to be both an oxidising agent and a reducing agent present. Some substances can act as either, depending on what they are reacting with and the reaction conditions. The oxidation state is what tells you which role a substance is playing.

A few rules let you work out the oxidation state of any single atom. An uncombined element is zero. Some elements have the same oxidation state in their compounds: Group 1 is always plus one, Group 2 is always plus two, aluminium is always plus three, and fluorine is always minus one. Hydrogen, oxygen and chlorine have usual values as well, but each of those has exceptions. In a compound, all the oxidation states add up to zero, and in an ion they add up to the charge on the ion, so a monoatomic ion has an oxidation state equal to its charge. Where you have to decide which element takes the negative value, it is the more electronegative one. By following these rules, the oxidation state of any atom in a compound or ion can be deduced.

The most powerful way to understand redox is to track the oxidation states of the elements involved, and this works for all redox reactions. A redox reaction is one where the oxidation states of elements change. If an element's oxidation state increases, becoming more positive, it has been oxidised, and that is a loss of electrons. If it decreases, becoming more negative, it has been reduced, and that is a gain of electrons. Balancing a redox equation means balancing those changes for the species being oxidised and the species being reduced. A disproportionation reaction is one in which the same species is both oxidised and reduced. In the example on the slide, chlorine is the species that is both oxidised and reduced. The oxidation states are what make the balancing possible.

A good mnemonic for this is OIL RIG: Oxidation Is Loss of electrons, and Reduction Is Gain of electrons. An oxidation state increasing is the same thing as losing electrons, and an oxidation state decreasing is the same thing as gaining them.

Oxidation is the loss of electrons, reduction is the gain of electrons, and the two happen together in the same reaction. The rules let you assign an oxidation state to any atom in a compound or ion. A change in that state is what a redox reaction is: an increase means oxidised, a decrease means reduced. Balancing those increases and decreases against each other is how a redox equation gets balanced. One number, the oxidation state, does all three jobs.