In chemistry there are some "Siamese twins". In the neutralisation process, there must be an acid and a base. You cannot have just one.
In the reduction process the reduction and oxidation occur simultaneously. When there is reduction there is oxidation. One compound is oxidised and the other reduced. So much so we have coined a term "redox reaction" for the process.
It actually started of as the hydrogenation-oxidation twin. In the reaction hydrogen is used to remove oxygen from the metal oxide. So hydrogen is added to the oxygen, or looking from the oxygen perspective the oxygen is added to the hydrogen, to give water, leaving behind the metal.
Now with the understanding that reactions are about giving and taking of electrons we like to see this process in its proper context. Let us illustrate this with the reaction of CuO with hydrogen.
We would consider copper in its natural state, Cu, as neutral. Meaning it does not take or give electron, it maintains its own number of electron. So we assigned it an oxidation state of 0. In CuO we considered that Cu gave two electrons to the oxygen to form the oxide. So we write it as Cu(II), or copper in the oxidation state of +2. Consequently oxygen must be in the -2 oxidation state, to maintain the charge balance.
Similarly the oxidation state in H2 is considered to be 0. The oxidation state of hydrogen atom in the water molecule is then +1, since oxygen as oxides has a -2 oxidation state.
In the reaction the Cu(II) is converted to Cu, from an oxidation state of +2 to 0, while hydrogen is converted from 0 to +1.
A reducing agent is then defined as an electron donor, and an oxidising agent as an electron acceptor. Hydrogen gave an electron in the reaction so it is the reducing agent. CuO accepts electrons to give Cu and so it is the oxidising agent.
Alternatively we say hydrogen is being oxidised and CuO is being reduced.
HYDROGENATION of HYDROCARBONS
For covalently bonded compounds the concept of giving and taking does not exist, since the electrons become the joint property of the two atoms bonded. So here we have to use our imagination.
We have already assigned hydrogen in hydrogen molecule a zero oxidation state, and oxygen in its oxide state a -2 oxidation state.
Now considering carbon monoxide as C=0, then carbon with a π-bond must have an oxidation state of +2, purely on a zero sum concept. CO is neutral, the oxide is -2 so carbon here must be +2.
Similarly by considering methane, CH4, carbon with a σ-bond should be assigned an oxidation state of +1, since each hydrogen attached to it has an oxidation state of -1.
So in the hydrogenation process, hydrogen is converted from a zero state to a -1 oxidation state, and the carbon from a +2 oxidation state to a +1 oxidation state. So this is a redox reaction and the carbon is reduced.
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