Metals and displacement reactions
Displacement in solutions
A more The tendency of a substance to undergo a chemical reaction.Shiny element that is a good conductor of electricity and heat, and which forms basic oxides. can Take the place of another substance in a chemical reaction. For example, a metal can displace a less reactive metal from its oxide, removing oxide ions from the less reactive metal and becoming an oxide itself. a less reactive metal from its A substance formed by the chemical union of two or more elements.. For example, magnesium is more reactive than copper. It displaces copper from copper sulfate Mixture formed by a solute and a solvent.:
magnesium + copper sulfate → magnesium sulfate + copper
Mg(s) + CuSO4(aq) → MgSO4(aq) + Cu(s)
In this A reaction that occurs when a more reactive element replaces a less reactive element in a compound.:
- magnesium becomes coated with copper
- the blue colour of the solution fades as blue copper sulfate solution is replaced by colourless magnesium sulfate solution
Determining a reactivity series
A A list of elements in order of their reactivity, usually from most reactive to least reactive. can be deduced by carrying out several displacement reactions. A piece of metal is dipped into a The substance formed when the hydrogen ion in an acid is replaced by a metal ion. solution. Different combinations of metal and salt solution are tested. The table shows the results of one of these investigations.
| Magnesium sulfate solution | Copper sulfate solution | Iron sulfate solution | Reactions | |
| Magnesium | Not done | Brown coating | Black coating | 2 |
| Copper | No visible reaction | Not done | No visible reaction | 0 |
| Iron | No visible reaction | Brown coating | Not done | 1 |
| Magnesium | |
|---|---|
| Magnesium sulfate solution | Not done |
| Copper sulfate solution | Brown coating |
| Iron sulfate solution | Black coating |
| Reactions | 2 |
| Copper | |
|---|---|
| Magnesium sulfate solution | No visible reaction |
| Copper sulfate solution | Not done |
| Iron sulfate solution | No visible reaction |
| Reactions | 0 |
| Iron | |
|---|---|
| Magnesium sulfate solution | No visible reaction |
| Copper sulfate solution | Brown coating |
| Iron sulfate solution | Not done |
| Reactions | 1 |
Question
Use the results in the table to deduce an order of reactivity, starting with the most reactive metal.
The order of reactivity is: magnesium > iron > copper. This is because magnesium displaced copper and iron, iron displaced copper only, but copper could not displace magnesium or iron.
Question
Explain why three combinations of metal and salt solution were not done in the investigation.
A metal cannot displace itself from a solution of one of its salts. There would be no reaction, so these combinations were not done.
Displacement reactions as redox reactions - Higher
A A chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow. for the reaction between magnesium and copper sulfate solution can be written in terms of the Electrically charged particle, formed when an atom or molecule gains or loses electrons. involved:
Mg(s) + Cu2+(aq) + SO42-(aq) → Mg2+(aq) + SO42-(aq) + Cu(s)
Sulfate ions, SO42-, appear on both sides of the equation. They do not take part in the reaction and are called An ion that is exactly the same on both sides of an ionic equation.. The equation can be rewritten without them:
Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
This equation is an example of a balanced A chemical equation that shows how positively charged ions join with negatively charged ions to make a compound.. It can be split into two An equation, involving ions and electrons, that describes the process happening at an electrode.:
Mg(s) → Mg2+(aq) + 2e- (The gain of oxygen, or loss of electrons, by a substance during a chemical reaction.)
Cu2+(aq) + 2e- → Cu(s) (The loss of oxygen, gain of electrons, or gain of hydrogen by a substance during a chemical reaction.)
Notice that:
- magnesium The smallest part of an element that can exist. lose Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons. - they are oxidised
- copper ions gain electrons - they are reduced
Reduction and oxidation happen at the same time, so the reactions are called When reduction and oxidation take place at the same time. reactions.
Displacement reactions are just one example of redox reactions. The decomposition (breakdown) of a compound using an electric current. reactions are also redox reactions.
Note that the reaction of metals with acids can also be described as a displacement reaction or a redox reaction. Only metals above hydrogen in the reactivity series will react and displace hydrogen from acids.
For example:
Zinc + sulfuric acid → zinc sulfate + hydrogen
Written with the ions involved:
Zn(s) + 2H+(aq) + SO42-(aq) → Zn2+(aq) + SO42-(aq) + H2(g)
Removing the spectator ions this becomes an ionic equation:
Zn(s) + 2H+(aq) → Zn2+(aq) + H2(g)
The half equations are:
Zn(s) → Zn2+(aq) + 2e-
2H+(aq) + 2e- → H2(g)
