Metal oxides: metals gain oxygen when they react
Definition
Metal oxide
A compound in which a metal is chemically combined with oxygen.
- When a metal reacts with oxygen, it forms a metal oxide.
- A metal oxide is a compound in which a metal is chemically combined with oxygen.
- The general reaction is metal+oxygen→metal oxide\text{metal} + \text{oxygen} \rightarrow \text{metal oxide}metal+oxygen→metal oxide
- Oxygen exists as O2\text{O}_2O2 molecules, so it is always written as O2\text{O}_2O2 in a symbol equation.
- Magnesium burns in oxygen with a bright white flame to form white magnesium oxide.
- 2Mg(s)+O2(g)→2MgO(s)2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s)2Mg(s)+O2(g)→2MgO(s)
Example
- Iron slowly gains oxygen from damp air, which is why an iron nail forms orange iron oxide (rust).
- Reactive metals such as sodium tarnish within seconds because their surface is oxidised almost immediately.
Oxidation and reduction: follow the oxygen
Definition
Oxidation
The gain of oxygen by a substance during a chemical reaction.
Definition
Reduction
The loss of oxygen by a substance during a chemical reaction.
- Oxidation is the gain of oxygen by a substance.
- A metal is oxidised when it reacts with oxygen, because oxygen is added to it.
- Reduction is the loss of oxygen by a substance.
- A metal oxide is reduced when oxygen is removed from it.
Exam technique
- To say which substance is oxidised or reduced, name the substance and state whether it gained or lost oxygen.
- A one-word answer such as "oxidation" drops the mark; write "the metal is oxidised because it gains oxygen".
Redox: one substance is oxidised as another is reduced
- When copper oxide is heated with hydrogen, oxidation and reduction happen together in a redox reaction.
- CuO(s)+H2(g)→Cu(s)+H2O(l)\text{CuO}(s) + \text{H}_2(g) \rightarrow \text{Cu}(s) + \text{H}_2\text{O}(l)CuO(s)+H2(g)→Cu(s)+H2O(l)
- Copper oxide is reduced because it loses oxygen to form copper.
- Hydrogen is oxidised because it gains oxygen to form water.
- The oxygen is transferred from the copper oxide to the hydrogen.
Common Mistake
- Do not decide a substance was oxidised just because it contains oxygen after the reaction.
- Compare each substance before and after to check whether it gained or lost oxygen.
- Do not reverse the definitions: oxidation gains oxygen and reduction loses oxygen.
Identifying oxidation and reduction in any equation
- Locate the oxygen: find which substances contain oxygen before and after the reaction.
- Follow the oxygen: decide which substance gained it and which lost it.
- Name the changes: the substance that gained oxygen was oxidised and the one that lost oxygen was reduced.
- Because the oxygen simply moves from one substance to another, every reduction is matched by an oxidation.
Self review
- What type of compound forms when a metal reacts with oxygen?
- Define oxidation and reduction in terms of oxygen.
- In the reaction between copper oxide and hydrogen, which substance is reduced and why?
- Why is this reaction described as a redox reaction?
- Write the balanced symbol equation for magnesium burning in oxygen.
