Reduction and oxidation (redox)
\(\require{mhchem}\) Reduction can be:
Oxidation can be:
(I've made the iron a liquid here, because at the high temperatures needed for this reaction, molten iron is produced.)
Oxygen has been removed from the iron III oxide to leave pure iron, so the iron III oxide has been reduced. Oxygen has been added to the carbon monoxide to make carbon dioxide, so that has been oxidised. We have redox.
Test yourself 1: when oxygen reacts with methane \(\ce{CH4}\) to make water and carbon dioxide, is that oxidation or reduction of the methane molecules? What about the oxygen molecules?
At GCSE level, the most used definition is:
By remembering OILRIG you will be able to work out whether a substance has been oxidised or reduced.
Another technique for understanding redox is to use oxidation states. Very useful introduction here.
- removal of oxygen;
- addition of hydrogen;
- gain of electrons.
We often want to reduce metal ores to get the useful metals from them.
- addition of oxygen;
- removal of hydrogen;
- loss of electrons.
A substance that reduces another substance is a reducing agent. And guess what? A substance that oxidises another substance is an oxidising agent.
Reduction and oxidation always go hand in hand. If a reducing agent like \(\ce{H2}\) acts on a substance (let's say oxygen \(\ce{O2}\)), oxygen is reduced, but in doing so hydrogen is oxidised.
$$\ce{hydrogen + oxygen -> water}$$ $$\begin{equation} \ce{2H2(g) + O2(g) -> 2H2O(g)} \end{equation}\label{water}$$
(I've made the water a gas here because when you burn hydrogen in oxygen, things get hot, and the water is produced as vapour.)
If we look at equation \(\eqref{water}\) from the point of view of oxygen, we can think of oxygen as the oxidising agent which oxidises hydrogen, but in doing so it has been reduced. Because the two always occur together, these are called redox reactions. That term is a contraction of reduction–oxidation and was first used in 1928.
$$\ce{hydrogen + oxygen -> water}$$ $$\begin{equation} \ce{2H2(g) + O2(g) -> 2H2O(g)} \end{equation}\label{water}$$
(I've made the water a gas here because when you burn hydrogen in oxygen, things get hot, and the water is produced as vapour.)
If we look at equation \(\eqref{water}\) from the point of view of oxygen, we can think of oxygen as the oxidising agent which oxidises hydrogen, but in doing so it has been reduced. Because the two always occur together, these are called redox reactions. That term is a contraction of reduction–oxidation and was first used in 1928.
Let's look at another reaction to see what I mean. An important industrial one is reduction of iron III oxide (Fe2O3) using carbon monoxide (CO).
$$\ce{iron\ III\ oxide + carbon\ monoxide → iron + carbon\ dioxide}$$ $$ \begin{equation} \ce{Fe2O3(s) +3CO(g) -> 2Fe(l) + 3CO2(g)} \end{equation}\label{ironore}$$ (I've made the iron a liquid here, because at the high temperatures needed for this reaction, molten iron is produced.)
Oxygen has been removed from the iron III oxide to leave pure iron, so the iron III oxide has been reduced. Oxygen has been added to the carbon monoxide to make carbon dioxide, so that has been oxidised. We have redox.
Test yourself 1: when oxygen reacts with methane \(\ce{CH4}\) to make water and carbon dioxide, is that oxidation or reduction of the methane molecules? What about the oxygen molecules?
At GCSE level, the most used definition is:
Oxidation Is Loss (of electrons) = OIL
Reduction Is Gain (of electrons) = RIG
By remembering OILRIG you will be able to work out whether a substance has been oxidised or reduced.
Another technique for understanding redox is to use oxidation states. Very useful introduction here.
Ionic equations
Splitting a chemical equation into ionic or half equations is another useful way to consider redox. Our earlier iron ore example: $$\begin{equation} \tag{\ref{ironore}} \ce{Fe2O3(s) +3CO(g) -> 2Fe(l) + 3CO2(g)} \end{equation}$$ can be split into two ionic equations: $$\begin{equation} \ce{Fe^{3+} + 3e- -> Fe } \end{equation}\label{iron3}$$ $$\begin{equation} \ce{O^{2-} + CO -> CO2 + 2e- } \end{equation}\label{co}$$ When we do that we can see that the iron III ion has gained electrons (equation \(\eqref{iron3}\)) and so has been reduced; In the other ionic equation \(\eqref{co}\) electrons from the oxide ion have been lost and so oxidation has occurred.Answers to test yourself questions
- From the point of view of methane, hydrogen has been removed, and oxygen added, so that is oxidation. From the point of view of oxygen, hydrogen has been added, so that is reduction.
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