Metal oxides: metals gain oxygen when they react
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)
- 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
Oxidation
The gain of oxygen by a substance during a chemical reaction.
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.
- 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.
- 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.
- 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.
