Metal Displacement Reactions
This topic explains how to use simple displacement reactions to rank metals in order of their reactivity. The principle is that a more reactive metal can 'push out' a less reactive metal from a solution of its salt, which allows for a direct comparison of their chemical activities.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- A more reactive metal will displace a less reactive metal from an aqueous solution of its salt.
- During displacement, the more reactive metal atom is oxidised (loses electrons) and the less reactive metal ion is reduced (gains electrons).
- If no reaction occurs when a metal is added to a salt solution, the metal is less reactive than the metal in the salt.
- By systematically testing different metal and salt solution pairs, an unknown reactivity series can be constructed.
- The general ionic equation for a displacement reaction is: MoreReactiveMetal(s) + LessReactiveMetalIon(aq) → MoreReactiveMetalIon(aq) + LessReactiveMetal(s).
Formulae
MA(s) + MBn+(aq) → MAn+(aq) + MB(s) To represent the general ionic equation for a metal displacement, where metal MA is more reactive than metal MB.
Definitions
- Displacement Reaction
- A reaction in which a more reactive element takes the place of a less reactive element within a compound.
- Reactivity Series
- A list of metals arranged in descending order of their tendency to react and form positive ions.
Worked example
A student performs two experiments to determine the relative reactivity of three metals: J, K, and L. The observations are: 1. A strip of metal J is placed in a solution of KSO4(aq). A reaction occurs and a dark coating forms on the strip. 2. A strip of metal L is placed in a solution of KSO4(aq). No reaction is observed. Based ONLY on these results, what can be concluded about the order of reactivity?
- 1
Analyse experiment 1:
Metal J reacts with a solution of K ions.
This means J displaces K.
Therefore, J is more reactive than K (J > K).
- 2
Analyse experiment 2:
Metal L does not react with a solution of K ions.
This means L cannot displace K.
Therefore, L is less reactive than K (K > L).
- 3
Combine the two deductions:
We have established that J > K and K > L.
- 4
Form the final order:
By combining the inequalities, we can construct the full reactivity order from most to least reactive:
J > K > L.
Answer: The order of reactivity from most to least reactive is J > K > L.
Common mistakes
- ×Attempting to determine a complete reactivity order with incomplete information. If you find that X > Z and Y > Z, you cannot order X and Y relative to each other without a direct experimental comparison.
- ×Forgetting that no reaction is also a valid and useful result. It tells you that the solid metal is less reactive than the metal ion in the solution.
- ×Confusing the reactivity of the solid metal with the metal ion. The displacement is driven by the greater tendency of the more reactive solid metal to lose electrons and form ions.
No-calculator tips
- ✓Use greater-than (>) and less-than (<) symbols to quickly sketch out the relationships as you read the question. This turns the problem into a simple logic puzzle.
- ✓Focus on the core principle: 'more reactive displaces less reactive'. This is sufficient to answer most questions without needing to write out full ionic or redox equations.
- ✓Visually check your final reactivity chain to ensure all the experimental observations are consistent with your proposed order.