Reactivity Series, Metal Displacement & Applications of Gold

Use of Gold in Communication Satellites

• Gold’s exceptional chemical inertness
  • One of the most unreactive metals; will not oxidise in air → minimal maintenance.
• Radiation‐shield applications
  • Gold-coated plastic sheets line satellite exteriors & astronauts’ suits.
  • Reflects infrared (IR) and ultraviolet (UV) radiation → protects delicate on-board instruments from thermal & photochemical damage.
• Mechanical advantages
  • Softer & more malleable than most metals → easy to roll, plate or vapor-deposit as ultrathin films.
• Electrical & optical uses
  • Excellent conductor → used in micro-processors & circuitry.
  • Mirrors in space telescopes receive a nanometre-thick Au layer to enhance reflectivity across a broad EM spectrum.

Reactivity of Metals & Reactivity Series

• Oxidising agents examined in earlier chapters (water, $\text{O}_2$, acids); now extend to metal cations.
• Reactivity definition
  • Ease with which a metal loses electrons (is oxidised) determines its reducing strength.
• Trend within the series (Figure 12.2.1)
  • Right-hand list (metals): from least reactive $\text{Au}$ at top → most reactive $\text{K, Li}$ at bottom.
  • Left-hand list (metal cations): inverse trend in oxidising ability.
• Key takeaways
  • Down the series:
  • Metals → stronger reducing agents (lose e⁻ more readily).
  • Their cations → weaker oxidising agents (harder to gain e⁻).
  • The strongest reducing agents sit bottom-right.
  • The strongest oxidising agents sit top-left.
• Energy perspective
  • Fewer valence electrons + lower ionisation energy ⇒ easier electron loss.

Condensed list of reduction half-equations (selection)

• $\text{Au}^{3+}(aq) + 3e^- \;\rightarrow\; \text{Au}(s)$
• $\text{Ag}^+(aq) + e^- \;\rightarrow\; \text{Ag}(s)$
• $\text{Cu}^{2+}(aq) + 2e^- \;\rightarrow\; \text{Cu}(s)$
• …
• $\text{Li}^+(aq) + e^- \;\rightarrow\; \text{Li}(s)$

Discovery of Metals Through the Ages (Case Study)

• Low-reactivity metals (Au, Pt) discovered first; occur native.
• Campfire reduction → discovery of Cu & Sn → alloy bronze.
  • Bronze = harder than stone, resharpenable; decisive in warfare (e.g.
    bronze-tipped spears during Trojan War).
• More reactive metals (Pb, Fe) required higher extraction temperatures.
  • Advent of charcoal furnaces enabled economical iron production, ushering the Iron Age.
• Highly reactive metals (Al, Na) only became accessible post-1800s with electricity.
  • 1855: Napoleon III showcased an Al bar with crown jewels.
  • 1857: French 20-franc coins struck in both Au & Al.
• Modern extraction
  • Huge electrolytic cells (Figure 12.2.3) reduce molten ores (e.g.
    $\text{Al}<em>2\text{O}</em>3$ → Al).
  • High electricity cost ⇒ recycling Al is economically & environmentally imperative.

Metal Displacement Reactions

• Definition: Redox processes where a more reactive metal displaces a less reactive metal from its salt solution.
• General rule (Figure 12.2.4)
  • If $\text{M}<em>1$ (metal) lies below/right of $\text{M}</em>2^{n+}$ (cation) in the series, reaction proceeds:
  • $\text{M}_1$ = reducing agent (is oxidised).
  • $\text{M}_2^{n+}$ = oxidising agent (is reduced).
• Example: Copper wire in AgNO₃ (Figures 12.2.5 & 12.2.6)
  • Observation: Silver crystals form; solution turns blue (Cu²⁺).
  • Ionic equation: $\text{Cu}(s) + 2\text{Ag}^+(aq) \;\rightarrow\; \text{Cu}^{2+}(aq) + 2\text{Ag}(s)$
• Prediction protocol
  1. Locate species in series.
  2. Confirm metal is lower-right vs cation.
  3. Write half-equations directly from series.
  4. Balance electrons; combine.

Worked Example 12.2.1: Zn vs Cu²⁺ (Figure 12.2.7)

1. Positions: $\text{Zn}$ is below $\text{Cu}^{2+}$.
2. Half-equations:
   • Reduction: $\text{Cu}^{2+}(aq)+2e^- \rightarrow \text{Cu}(s)$
   • Oxidation: $\text{Zn}(s) \rightarrow \text{Zn}^{2+}(aq)+2e^-$
3. Overall: $\boxed{\text{Zn}(s)+\text{Cu}^{2+}(aq)\;\rightarrow\;\text{Zn}^{2+}(aq)+\text{Cu}(s)}$
   • Visual cue: Brown Cu deposits on Zn; blue colour fades as $[\text{Cu}^{2+}]$ drops.

Try-Yourself Example 12.2.1: Co vs Cu²⁺

• Series shows $\text{Co}$ is above $\text{Cu}$ → No displacement; solution remains unchanged.

Practical & Conceptual Implications

• Corrosion control: Choosing metals high in series (e.g.
  Zn) for sacrificial anodes protects Fe structures.
• Resource management: High electro-extraction energy of Al, Na etc. underlines need for sustainable electricity & recycling policies.
• Space technology: Gold’s inertness + superb IR/UV reflectivity exemplifies how reactivity concepts influence material selection in hostile environments.