9.3 – Reactivity
9.3.1 – Deduce a reactivity series based on the chemical behaviour of a groups of oxidising and reducing agents
When a metal is replaced by another metal in a compound, this is called a metal displacement reaction. For this to occur, the metal in the compound must be less reactive than the metal it is being replaced by. This new metal is the reducing agent, or electron donor. Although all solid metals act as reducing agents, some are more reactive than others.
We can compare the reactivity of different metals based on a number of factors, including reactivity with:
- Oxygen
- Water
- Dilute acids
- Other metal salts
These are then organised into the reactivity series:
Since all solid metals are oxidised, losing electrons, the reactivity series orders the metals in order of increasing ease of oxidation.
Since hydrogen occurs above copper on the reactivity series, any metals below it will not react with acids.
A more reactive metal will displace a less reactive metal from its salt. This is because the more reactive metal is a stronger reducing agent.
A more reactive halogen will displace a less reactive halogen from a compound. This is because the more reactive halogen is a stronger oxidising agent.
Some reactivity series will not only show metals, but oxidising agents as well (non-metals), including the halogens. The reactivity of metals and non-metals is compared using half-cells. These all have the same conditions:
- 1 mol dm-3 concentration of aqueous solutions
- 25°C temperature
- 1 atm pressure for gases.
Using these cells, the potential difference can be measured to determine which element is more reactive.
9.3.2 – Deduce the feasibility of a redox reaction from a given reactivity series
The greater the difference in reactivity of two metals, the more rapidly the reaction would occur. However, the metal acting as the reducing agent must occur higher in the reactivity series that the one being reduced. The one being displaced is the oxidising agent.
It is also important to remember that the ions of a less reactive metal will act as stronger oxidising agents. Therefore, they are more likely to oxidise the metals at the top of the series.
This may be done using these steps:
- Identify all reactants and find their half-equations on the electrochemical series. You should have some on each side (the oxidising and reducing agents)
- The oxidising agent on the left must be lower down the series than the reducing agent on the right.
- The half-equation for the reducing agent needs to be reversed, then the two half-equations can be balanced and added together