When a piece of magnesium oxide is heated with hydrogen, magnesium metal is produced. Is the hydrogen oxidized or reduced?
Let’s start by using the information given in the question to write a kind of chemical equation. We are told that the reactants are magnesium oxide and hydrogen. Magnesium oxide is given the formula MgO. And hydrogen is H₂. If you weren’t sure about magnesium oxide, remember that magnesium is in group two of the periodic table. And so it usually forms a plus two oxidation state. Oxygen, on the other hand, normally forms a minus two oxidation state. So with plus two for magnesium and minus two from oxygen, we get an overall neutral compound MgO.
The question also tells us that one of the products is magnesium metal, which simply has the formula Mg. But what about the other product? In the reactants, we still have some hydrogen atoms and oxygen atoms left over. The easiest way to combine these into a stable product is as water, H₂O. So this is the most likely chemical reaction equation. And it’s even balanced already.
The question is asking us whether the hydrogen in this reaction is oxidized or reduced. So let’s remind ourselves what we mean by oxidation and reduction. You may have come across the acronym OIL RIG. This stands for Oxidation Is Loss and Reduction Is Gain. What we mean by this, of course, is the loss and gain of electrons. Using our reaction equation, along with the oxidation states of each element, we can work out which are reduced and which are oxidized.
Let’s start with magnesium. When it’s part of magnesium oxide, the magnesium has a plus two oxidation state. We know this because oxygen is most likely to have a negative two oxidation state. And, of course, these two states balance each other out to give an overall neutral magnesium oxide. H₂, hydrogen gas, is also neutral. And this means that each hydrogen atom within the molecule must be neutral as well. So we would say it has an oxidation state of zero on each hydrogen atom.
Now let’s look to the products. Magnesium metal is Mg on its own, which has no charge. This means that the magnesium atoms must have a neutral zero oxidation state. The oxygen in a water molecule, again, has a negative two oxidation state. And, finally, we come to the hydrogen. But be careful, hydrogen in water must have overall plus two oxidation state to balance out the oxygen. But, there are two hydrogen atoms. So this overall plus two is divided equally between each hydrogen atom. This means that each hydrogen atom has a plus one oxidation state.
Now, to work out which elements are oxidized or reduced, we need to look at the change between the state in the reactants and the state in the products. In the case of magnesium, it has moved from a plus two oxidation state to a zero oxidation state. This is a drop of two and can be explained by adding two negatively charged electrons. In this case, the magnesium has gained electrons. And if it has gained electrons, that means it must have been reduced.
Now let’s move to the oxygen. We can see that oxygen has a negative two oxidation state in both the reactants and the products. So it is neither reduced nor oxidized. So we can ignore this one for now. Finally, we come to the hydrogen, where the hydrogen atoms on the reactant side have a zero oxidation state. But on the product side, they have a plus one oxidation state. This can be accounted for by the loss of one electron. Since the hydrogen has lost an electron, it must have been oxidized. Because, remember, oxidation is loss.
Because the magnesium is reduced while the hydrogen is oxidized, we often call this a redox reaction. Meaning, both a reduction and oxidation reaction in the same overall reaction. You may also come across the terms oxidizing agent and reducing agent. In this case, the magnesium is the oxidizing agent. Remember that an oxidizing agent is itself reduced, the opposite of oxidation. Likewise, the hydrogen here, which is oxidized, is the reducing agent. This is because the hydrogen reduces the magnesium by giving it its electrons.
So, to answer the question of whether the hydrogen is oxidized or reduced, we can see that the hydrogen is oxidized because it loses an electron. So this is our answer.