Predicting Redox Reactivity
Quick Notes
- Metals are usually oxidized (lose electrons) and become positive ions.
- Non-metals are usually reduced (gain electrons) and become negative ions.
- Metal reactivity increases down a group (e.g. Group 1).
- Non-metal reactivity decreases down a group (e.g. Group 17).
- Displacement reactions show relative ease of reduction or oxidation:
- More reactive metals displace less reactive metal ions from solution.
- More reactive halogens displace less reactive halide ions from solution.
Full Notes
Oxidation and Reduction Recap
- Oxidation = loss of electrons (common for metals).
- Reduction = gain of electrons (common for non-metals).
- Oxidising agents: get reduced themselves by oxidising something else.
- Reducing agents: get oxidised themselves by reducing something else.
The more easily oxidized a metal is, the more reactive it is as a reducing agent.
Reactivity of Metals (Group Trends)
Reactivity increases down Group 1 or 2:
- Valence electrons are further from the nucleus.
- Less nuclear attraction meaning outermost electrons are lost more easily.
Example: K > Na > Li (increasing ease of oxidation).
Metal + Metal Ion Displacement Reactions
A more reactive metal displaces a less reactive metal from its compound by reducing its ions and becoming oxidised itself.
This can be used to compare the relative reactivity of metals.
Example Zinc and copper sulfate
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
- Zinc displaces copper, showing Zn is more reactive (oxidized more easily).
- Zn → Zn2+ (oxidation), Cu2+ → Cu (reduction).
Observations:
- Solution changes from blue to colourless.
- Reddish copper metal forms.
Reactivity of Halogens (Group 17)
Down Group 17, halogen reactivity decreases:
- Atomic radius increases.
- Electron affinity and electronegativity decrease making it harder to gain electrons.
Displacement reactions: A more reactive halogen displaces a less reactive halogen from its compounds.
Example Chlorine and potassium bromide
Cl2 + 2KBr → 2KCl + Br2
- Chlorine displaces bromide → Cl2 is more reactive.
- Observation: Colourless solution turns orange-brown as Br2 forms.
Predicting Redox Behaviour
- A metal higher in the reactivity series will displace one lower down.
- A halogen higher in the group will displace one lower down from its halide solution.
Summary
- Metals usually lose electrons (oxidised) and non-metals gain electrons (reduced).
- Metal reactivity increases down groups, halogen reactivity decreases.
- Displacement reactions show relative redox reactivity.
- Observations like colour changes or deposits indicate redox behaviour.
Linked Course Questions
Why does metal reactivity increase, and non-metal reactivity decrease, down the main groups of the periodic table?
Metal Reactivity (e.g. Group 1): Increases down the group because atoms become larger and the outer electrons are further from the nucleus and more shielded. This makes it easier to lose electrons — a key step in metal reactivity.
Non-metal Reactivity (e.g. Group 17): Decreases down the group because atoms become larger and the attraction between the nucleus and incoming electrons weakens. This makes it harder for the atom to gain electrons — a key feature of non-metal reactivity.
What observations can be made when metals are mixed with aqueous metal ions, and solutions of halogens are mixed with aqueous halide ions?
Metals with Aqueous Metal Ions (Metal Displacement): If a more reactive metal is added to a solution containing ions of a less reactive metal: a colour change may be seen as the metal ion is reduced to a solid metal (e.g. blue Cu2+ solution fades as brown copper metal forms).
Halogens with Aqueous Halide Ions (Halogen Displacement): If a more reactive halogen is added to a solution containing halide ions of a less reactive halogen: a colour change may occur as the displaced halogen is formed (e.g. orange colour from displaced Br2).