Reactivity Series of Metals
Metals above carbon → extracted by electrolysis. Between carbon and hydrogen → reduced with carbon. Below hydrogen → found native or reduced easily.
Displacement Reactions
A more reactive metal displaces a less reactive metal from its compound. This is a redox reaction. The more reactive metal is oxidised, the less reactive metal ion is reduced.
Metal-Metal Displacement Reactions
A more reactive metal displaces a less reactive metal cation from aqueous solution because it has a stronger tendency to lose electrons (more negative \(E^\circ\)):
\[\text{Zn}(\text{s}) + \text{Cu}^{2+}(\text{aq}) \rightarrow \text{Zn}^{2+}(\text{aq}) + \text{Cu}(\text{s})\]Metal-Acid Reactions
Metals above hydrogen in the reactivity series react with acids to produce hydrogen gas:
\[\text{Mg}(\text{s}) + 2\text{H}^+(\text{aq}) \rightarrow \text{Mg}^{2+}(\text{aq}) + \text{H}_2(\text{g})\]Metals below hydrogen (Cu, Ag, Au) do not react with dilute non-oxidising acids.
Metal Extraction Methods
| Position | Method | Example |
|---|---|---|
| Above carbon (K, Na, Ca, Mg, Al) | Electrolysis of molten compound | 2Al₂O₃(l) → 4Al(l) + 3O₂(g) |
| Between C and H (Zn, Fe, Sn, Pb) | Reduction with carbon/CO | Fe₂O₃ + 3CO → 2Fe + 3CO₂ |
| Below hydrogen (Cu, Ag, Au, Pt) | Found native / gentle heating | 2Ag₂O → 4Ag + O₂ |
Corrosion & Rusting
Corrosion is the electrochemical degradation of a metal by reaction with its environment. Rusting is the corrosion of iron. Both oxygen and water are required.
Electrochemical Rusting Mechanism
Rusting is an electrochemical redox process requiring both oxygen and water:
\[\text{Anode: } \text{Fe}(\text{s}) \rightarrow \text{Fe}^{2+}(\text{aq}) + 2\text{e}^-\] \[\text{Cathode: } \text{O}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l}) + 4\text{e}^- \rightarrow 4\text{OH}^-(\text{aq})\] \[\text{Rust: } \text{Fe}^{2+} \rightarrow \text{Fe}^{3+} \rightarrow \text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}(\text{s})\]Methods of Preventing Corrosion
- Barrier methods: Painting, greasing, plastic coating (prevents contact with \(\text{O}_2/\text{H}_2\text{O}\)).
- Sacrificial protection: Attaching a more reactive metal (e.g. Zn or Mg blocks on ship hulls) which oxidises preferentially.
- Galvanising: Coating iron with zinc (dual protection: physical barrier + sacrificial protection if scratched).
Predicting Reactions with E° (HL)
At HL, the reactivity series is quantified using standard electrode potentials. A more negative E° = stronger reducing agent (more reactive metal). Calculate E°cell = E°cathode − E°anode. If positive, the reaction is spontaneous.
Predicting Displacement using E° Values
\(E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\text{ V}\), \(E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\)
\[E^\circ_{\text{cell}} = +0.34 - (-0.76) = +1.10\text{ V} > 0 \implies \textbf{Spontaneous (Zn displaces Cu}^{2+}\textbf{)}\]Stoichiometry and Electrode Polarities
- Never multiply \(E^\circ\) values by stoichiometric coefficients when balancing half-equations (\(E^\circ\) is an intensive property).
- Electrode polarities: Remember voltaic cathode is \((+)\); electrolytic cathode is \((-)\).
Metal Extraction: Carbon vs Electrolysis
Iron extraction uses carbon reduction in a blast furnace because carbon is more reactive than iron. Aluminium is more reactive than carbon, so it must be extracted by electrolysis of molten bauxite.
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