Video Transcript
A piece of magnesium oxide is heated with hydrogen until the magnesium is recovered. Is the magnesium oxide being oxidized or reduced?
In this experiment, we are being asked to identify if only magnesium oxide is being oxidized or reduced. In order to determine this, let us first write a chemical equation. We know our reactants are magnesium oxide and hydrogen gas. Now, let us create the chemical formula for magnesium oxide.
The magnesium ion has a charge of two plus and the oxide ion has a charge of two minus. For every one magnesium ion, there would need to be one oxide ion in the compound. Therefore, the formula would be MgO. We can also use a different method to derive the formula. When we cross the charges and drop the signs, the formula is Mg2O2. Since there are two magnesium ions for every two oxide ions, the formula can be reduced to MgO. Moving on to the other reactant, hydrogen gas, we would write H2 since hydrogen is diatomic, so it consists of two atoms chemically bonded together.
Now, let us determine the products within the reaction. In this experiment, the magnesium two plus cation would become elemental magnesium and hydrogen would bond with oxygen. Therefore, the products would be solid magnesium, Mg, and water, H2O. It should be noted that this reaction is already balanced and therefore no coefficients are required. Since this question is asking us to determine if magnesium oxide is being oxidized or reduced, we first need to discuss redox reactions, which is a chemical reaction that involves the transfer of electrons between chemical species.
As the question implies, since we need to determine if magnesium oxide is being oxidized or reduced, there are two terms associated with redox reactions: reduction, which is a chemical reaction where a chemical species gains electrons, and oxidation, which is a chemical reaction where a chemical species loses electrons. If you’re wondering how the term “redox reaction” was named, look at the terms “reduction” and “oxidation.” The first three letters of reduction and the first two letters of oxidation help to create the name of this type of reaction.
When looking at reduction as the species gains electrons which have a negative charge, the initial charge of the chemical species decreases, such as each atom and elemental oxygen O2 gaining two electrons, forming negatively charged O2−. Since each atom in a molecule of oxygen gains two electrons, the charge becomes more negative. When looking at oxidation, the species loses electrons which are negatively charged. So, the charge increases, such as elemental calcium with no charge forming an ion of calcium, Ca2+. Since elemental calcium lost two electrons, the charge becomes more positive.
Returning back to our chemical equation, the question is only asking us to identify if magnesium oxide is being oxidized or reduced. Therefore, we will only focus on magnesium and oxygen within the chemical equation. Let us first begin with oxygen. When looking at oxygen in magnesium oxide, oxygen will have a charge of two minus in the compound. Now, let us take a look at oxygen on the product side of the chemical equation.
Once again, oxygen is within a molecule and will have a charge of two minus. Since oxygen has the same charge throughout the chemical reaction, it did not gain or lose electrons and is therefore not being oxidized or reduced. This does not mean this reaction is not a redox reaction. We just have to look at the other chemical species in magnesium oxide, which is magnesium.
When looking at magnesium oxide, we need to remember that the compound must have an overall charge of zero since it is neutral. Since we know oxygen has a charge of two minus, magnesium must have a charge of two plus to create a neutral compound. When looking at magnesium on the product side, it is an elemental form, so the charge is zero. Since magnesium is going from a charge of two plus and ending with a charge of zero, that means two electrons were gained in order to decrease the charge by two.
When referring back to the terms of oxidation and reduction, the chemical reaction where a chemical species gains electrons is reduction. Therefore, magnesium oxide is being reduced. To ensure we are correct, we can take a look at hydrogen within the reaction. On the reactant side, elemental hydrogen gas has a charge of zero. And on the product side, each atom of hydrogen has a charge of one plus. Since hydrogen is going from a charge of zero to one plus, this means that each atom in H2 lost one electron, so a total of two electrons were lost.
Therefore, hydrogen is being oxidized. Since both oxidation and reduction is being shown within the chemical equation, this is a redox reaction. Referring back to magnesium oxide, since the charge of oxygen remained the same and hydrogen was oxidized, the correct answer choice is magnesium oxide is being reduced.
