Metal Extraction, Reactivity Series, and Electrolysis Study Notes

Metal Ores, Oxidation, and Reduction

Oxidation:
- Defined as the process of gaining oxygen.
- Example chemical equation: $Fe + O \rightarrow Fe_2O_3$
- The common chemical name for rust or iron ore is iron oxide ( $Fe_2O_3$ ).
Reduction:
- Defined as the process of losing oxygen.
- Example chemical equation: $2MgO \rightarrow 2Mg + O_2$
- In this reaction, magnesium oxide ( $MgO$ ) loses oxygen and is reduced to pure elemental magnesium ( $Mg$ ).
Unreactive Metals:
- Metals such as gold ( $Au$ ), silver ( $Ag$ ), and platinum ( $Pt$ ) are found in the Earth's crust as pure, native metals.
- Because they are chemically unreactive, they do not naturally combine with oxygen to form oxides, meaning no chemical reduction process is required to extract them.

The Reactivity Series and Extraction Methods

Reactivity series diagram listing elements from most to least reactive alongside review items

The Reactivity Series Order (from most reactive to least reactive):
1. Potassium ( $K$ ) — Most reactive
2. Sodium ( $Na$ )
3. Calcium ( $Ca$ )
4. Magnesium ( $Mg$ )
5. Aluminium ( $Al$ )
6. Carbon ( $C$ ) — Non-metal reference point
7. Zinc ( $Zn$ )
8. Iron ( $Fe$ )
9. Tin ( $Sn$ )
10. Lead ( $Pb$ )
11. Hydrogen ( $H_2$ ) — Non-metal reference point
12. Copper ( $Cu$ )
13. Silver ( $Ag$ )
14. Gold ( $Au$ )
15. Platinum ( $Pt$ ) — Least reactive
Extraction via Reduction with Carbon:
- Applicability: Used for metals that are less reactive than carbon (such as zinc, iron, tin, lead, and copper).
- Process: This is the cheapest and easiest method to extract a metal from its oxide. The metal oxide is heated with carbon.
- Chemical Principle: Carbon is more reactive than the metal, so it removes oxygen from the metal oxide, reducing the metal oxide to pure metal while carbon itself is oxidized.
- Example: When iron oxide is heated with carbon, pure iron is extracted because carbon is more reactive than iron. The reaction yields pure iron and carbon dioxide.
- Limitation: Metals that are more reactive than carbon (such as aluminium, magnesium, calcium, sodium, and potassium) cannot be extracted by carbon reduction because carbon is not reactive enough to displace them from their oxides.
Extraction via Electrolysis:
- Applicability: Mandatory for metals that are more reactive than carbon.
- Economic Evaluation: Electrolysis is significantly more expensive than reduction with carbon because it consumes massive amounts of electrical energy and heat energy.

Principles of Electrolysis and Ion Movement

Diagram showing electrolysis apparatus and ion movement during copper chloride electrolysis

Definition of Electrolysis: A chemical process that uses a potential difference (direct current power supply) to separate positive metal ions from negative non-metal ions.
Requirement for Dissolving or Melting:
- A metal compound must be dissolved in water (aqueous solution) or melted (molten state) before electrolysis can take place.
- Scientific Reason: In a solid lattice structure, ions are locked in fixed positions. Dissolving or melting the compound frees the ions, allowing them to move freely to carry electrical charge as an electric current.
Components of an Electrolysis Setup:
- Direct Current (d.c.) Power Supply: Drives electron flow and creates a potential difference across the electrodes.
- Inert Carbon Electrodes:
  - Cathode: The negatively charged electrode connected to the negative terminal.
  - Anode: The positively charged electrode connected to the positive terminal.
- Electrolyte: The liquid solution or molten ionic substance containing mobile ions.

Practical Extraction: Electrolysis of Copper Chloride

Target Compound: Copper chloride ( $CuCl_2$ ).
Ionization Reaction: In solution, copper chloride breaks down into mobile positive and negative ions: $CuCl_2 \rightarrow Cu^{2+} + 2Cl^-$
Reaction at Cathode (Negative Electrode):
- Positively charged copper ions ( $Cu^{2+}$ ) are attracted to the negatively charged electrode.
- At the cathode, copper ions gain electrons (discharge) to form pure solid copper metal: $Cu^{2+} + 2e^- \rightarrow Cu$
- Observation: Solid reddish-brown copper deposit collects on the electrode.
Reaction at Anode (Positive Electrode):
- Negatively charged chloride ions ( $2Cl^-$ ) are attracted to the positively charged electrode.
- At the anode, chloride ions lose electrons (discharge) to form chlorine gas: $2Cl^- \rightarrow Cl_2 + 2e^-$
- Observation: Effervescence/bubbles of chlorine gas produced at the electrode.

Industrial Applications, Economics, and Review Questions

Elemental Composition of Tin Oxide:
- Tin oxide consists of the chemical elements tin ( $Sn$ ) and oxygen ( $O$ ).
Tin Coating on Steel Cans:
- Steel is composed primarily of iron ( $Fe$ ).
- Steel cans are coated with tin ( $Sn$ ) because tin acts as a barrier or is less reactive, preventing the underlying steel/iron from rusting or reacting with contents such as food or moisture.
Reactivity Comparison in Iron Extraction:
- Heating iron oxide with carbon extracts pure iron because carbon is more reactive than iron, allowing carbon to displace iron from its oxide.
Why Aluminium is Extracted Despite High Cost:
- Aluminium is more reactive than carbon, so cheap carbon reduction cannot extract it; it must be extracted using electrolysis.
- Despite the high electrical energy costs, aluminium is extracted because its physical properties—such as low density, lightweight strength, and corrosion resistance—make it essential for modern structural and industrial uses.
Economic and Environmental Rationale for Recycling Aluminium:
- Recycling aluminium consumes only a small fraction of the energy required to extract raw aluminium from ore via electrolysis.
- Recycling significantly reduces electricity expenditure, conserves natural ore deposits, and lowers overall carbon emissions.