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 a copper(II) sulfate Mixture formed by a solute and a solvent.:
magnesium + copper(II) sulfate 鈫 magnesium sulfate + copper
Mg(s) + CuSO4(aq) 鈫 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 the blue copper(II) sulfate solution is replaced by a 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 worked out 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(II) sulfate solution | Iron(II) sulfate solution | Number of 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(II) sulfate solution | Brown coating |
| Iron(II) sulfate solution | Black coating |
| Number of reactions | 2 |
| Copper | |
|---|---|
| Magnesium sulfate solution | No visible reaction |
| Copper(II) sulfate solution | Not done |
| Iron(II) sulfate solution | No visible reaction |
| Number of reactions | 0 |
| Iron | |
|---|---|
| Magnesium sulfate solution | No visible reaction |
| Copper(II) sulfate solution | Brown coating |
| Iron(II) sulfate solution | Not done |
| Number of reactions | 1 |
Example
Use the results in the above 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 could displace copper and iron, iron could only displace copper, 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.
Writing ionic equations for displacement 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(II) sulfate solution is:
Mg(s) + CuSO4(aq) 鈫 MgSO4(aq) + Cu(s)
It can be written using the Electrically charged particle, formed when an atom or molecule gains or loses electrons. involved:
Mg(s) + Cu2+(aq) + SO42-(aq) 鈫 Mg2+(aq) + SO42-(aq) + Cu(s)
Notice that sulfate ions, SO42-(aq), appear on both sides of the equation. They do not take part in the reaction. The equation can be rewritten without them:
Mg(s) + Cu2+(aq) 鈫 Mg2+(aq) + Cu(s)
This equation is an example of balanced A chemical equation that shows how positively charged ions join with negatively charged ions to make a compound.. It shows only the The smallest part of an element that can exist. and Electrically charged particle, formed when an atom or molecule gains or loses electrons. that change during the reaction by losing or gaining Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons..
Question
The balanced chemical equation for the displacement reaction between iron and copper(II) sulfate is
Fe(s) + CuSO4 (aq) 鈫 FeSO4 (aq) + Cu (s)
Write the balanced ionic equation for the same reaction.
Fe(s) + Cu2+(aq) 鈫 Fe2+(aq) + Cu(s)
Ionic equations:
- show only the ions that change in the reaction
- show the gain or loss of electrons
- are useful for representing displacement reactions because they show what happens to the metal ions