Metals: Properties, Reactivity Series and Corrosion

What you'll learn

This revision guide covers the physical and chemical properties of metals, the reactivity series (activity series), and corrosion processes as specified in the CXC CSEC Integrated Science syllabus. You will learn how to predict metal reactions, explain displacement reactions, and understand how metals deteriorate in the Caribbean environment. These concepts are fundamental to understanding Caribbean industries including bauxite processing in Jamaica and Guyana, and the maintenance of marine vessels and infrastructure.

Key terms and definitions

Metal — an element that typically forms positive ions (cations), conducts electricity and heat, and has a characteristic lustre when polished

Reactivity series — the arrangement of metals in order of their reactivity with water, acids, and oxygen, from most reactive (potassium) to least reactive (gold)

Displacement reaction — a chemical reaction where a more reactive metal removes a less reactive metal from its compound

Corrosion — the gradual destruction of a metal by chemical reaction with substances in its environment, particularly oxygen and water

Oxidation — the loss of electrons by a substance; in metals, this results in the formation of positive ions

Alloy — a mixture of two or more metals, or a metal with a non-metal, designed to improve properties such as strength or resistance to corrosion

Galvanizing — the process of coating iron or steel with a protective layer of zinc to prevent rusting

Ore — a naturally occurring rock or mineral from which a metal can be extracted profitably

Core concepts

Physical properties of metals

Metals share characteristic physical properties that distinguish them from non-metals:

Electrical conductivity: Metals contain delocalized electrons that move freely through the metallic structure. These mobile electrons allow metals to conduct electricity efficiently. Copper is widely used in Caribbean electrical infrastructure due to its excellent conductivity.

Thermal conductivity: The same delocalized electrons that conduct electricity also transfer thermal energy rapidly through metals. Aluminium cooking pots, manufactured from Caribbean-processed bauxite, demonstrate this property.

Malleability and ductility: Metals can be hammered into sheets (malleability) and drawn into wires (ductility) without breaking. This occurs because layers of metal atoms can slide over one another while maintaining metallic bonding.

Lustre: Polished metal surfaces reflect light, creating a characteristic shine. This property is exploited in Caribbean jewelry made from gold mined in Guyana.

High melting and boiling points: Most metals have strong metallic bonds requiring significant energy to break. However, there are exceptions—mercury is liquid at room temperature.

High density: Metals typically have high density because metal atoms pack closely together in regular lattice structures.

Sonority: Metals produce a ringing sound when struck, a property utilized in steel pan instruments originating from Trinidad and Tobago.

The reactivity series and metal reactions

The reactivity series arranges metals from most reactive to least reactive:

Most reactive:

• Potassium (K)
• Sodium (Na)
• Calcium (Ca)
• Magnesium (Mg)
• Aluminium (Al)
• Zinc (Zn)
• Iron (Fe)
• Lead (Pb)
• Copper (Cu)
• Silver (Ag)
• Gold (Au)
• Platinum (Pt)

Least reactive

Reactions with oxygen: More reactive metals combine with oxygen more readily. Magnesium burns with a brilliant white flame in oxygen:

2Mg + O₂ → 2MgO

Iron reacts slowly with oxygen, forming rust over time. Gold does not react with oxygen even when heated.

Reactions with water: Highly reactive metals like potassium and sodium react vigorously with cold water, producing metal hydroxides and hydrogen gas:

2Na + 2H₂O → 2NaOH + H₂

Less reactive metals like magnesium react slowly with cold water but vigorously with steam:

Mg + H₂O (steam) → MgO + H₂

Metals below zinc in the series do not react with water.

Reactions with acids: Metals above hydrogen in the reactivity series react with dilute acids, producing salts and hydrogen gas:

Mg + 2HCl → MgCl₂ + H₂ Zn + H₂SO₄ → ZnSO₄ + H₂

Metals below hydrogen (copper, silver, gold) do not react with dilute acids.

Displacement reactions: A more reactive metal displaces a less reactive metal from its compound. This principle is fundamental to metal extraction and explains many industrial processes:

Zn + CuSO₄ → ZnSO₄ + Cu

The blue copper sulfate solution becomes colourless as zinc sulfate forms, and reddish-brown copper metal is deposited.

Extraction of metals from ores

The method used to extract a metal from its ore depends on its position in the reactivity series:

Metals below carbon (copper, lead, iron): These can be extracted by heating the ore with carbon (reduction). Carbon displaces the less reactive metal:

2Fe₂O₃ + 3C → 4Fe + 3CO₂

Metals above carbon (aluminium, sodium, calcium, magnesium): These require electrolysis to extract because they cannot be reduced by carbon.

Bauxite processing in the Caribbean: Jamaica and Guyana possess significant bauxite deposits (aluminium ore). The extraction process involves:

1. Mining bauxite (Al₂O₃·2H₂O)
2. Purifying bauxite to produce alumina (Al₂O₃) using the Bayer process
3. Electrolyzing molten alumina mixed with cryolite at approximately 1000°C
4. Collecting pure aluminium metal at the cathode

This process requires enormous amounts of electricity, which is why some Caribbean countries have investigated establishing aluminium smelters near hydroelectric facilities.

Corrosion of metals

Corrosion is the deteriorating reaction between metals and environmental substances. In the Caribbean, high humidity, salt spray, and elevated temperatures accelerate corrosion processes.

Rusting of iron: This is the most economically significant form of corrosion. Rust is hydrated iron(III) oxide (Fe₂O₃·xH₂O). Both oxygen and water must be present:

4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O

Factors affecting rusting rate:

• Presence of salt: Coastal Caribbean locations experience accelerated rusting because salt dissolved in water increases its conductivity, facilitating electron transfer. Marine equipment in ports like Kingston Harbour or Bridgetown requires frequent maintenance.

• Humidity: High Caribbean humidity means moisture is constantly available for rust formation.

• Acidic conditions: Acid rain or industrial pollutants increase rusting rates by providing H⁺ ions that accelerate oxidation.

• Contact with other metals: When iron contacts a more reactive metal (like zinc), the iron is protected. When it contacts a less reactive metal (like copper), rusting accelerates.

Prevention of rusting:

1. Barrier methods:

   • Painting creates a physical barrier between iron and atmospheric moisture
   • Oiling or greasing protects tools and machinery
   • Plastic coating is used for outdoor furniture and fencing

2. Galvanizing: Iron is coated with zinc. Zinc is more reactive than iron and preferentially corrodes, protecting the iron beneath. Galvanized roofing sheets are common throughout the Caribbean.

3. Sacrificial protection: A more reactive metal (usually zinc or magnesium) is attached to the iron structure. The reactive metal corrodes instead of the iron. Ship hulls and underwater pipelines use zinc blocks as sacrificial anodes.

4. Alloying: Stainless steel is an alloy of iron with chromium and nickel. The chromium forms a protective oxide layer that prevents further corrosion. Stainless steel is expensive but essential for high-quality Caribbean restaurant equipment and medical facilities.

5. Electroplating: A thin layer of non-reactive metal (chromium, nickel, silver) is deposited on iron using electrolysis, providing both protection and aesthetic appeal.

Uses of metals and alloys

Pure metals and their applications:

• Copper: Electrical wiring, water pipes, roofing (develops a protective green patina)
• Aluminium: Aircraft manufacturing, cooking utensils, beverage cans, window frames
• Iron: Construction, manufacturing machinery (usually as steel)
• Gold: Jewelry, electronics, dental work
• Zinc: Galvanizing, battery production

Alloys and their advantages:

Pure metals often have limitations—they may be too soft, corrode easily, or have unsuitable melting points. Alloys overcome these limitations:

• Steel (iron + carbon): Stronger than pure iron; used in construction and vehicle manufacturing
• Stainless steel (iron + chromium + nickel): Corrosion-resistant; used in cutlery, medical instruments, industrial equipment
• Brass (copper + zinc): Harder than copper with an attractive gold colour; used in musical instruments, door fittings, decorative items
• Bronze (copper + tin): Hard and corrosion-resistant; used in marine equipment, statues, bells
• Solder (lead + tin): Low melting point; used for joining electrical components

The Caribbean steel pan is traditionally made from oil drums, which are steel alloys. The metal must be strong enough to withstand repeated striking yet malleable enough to be shaped.

Worked examples

Example 1: Predicting displacement reactions (3 marks)

Question: A student has samples of magnesium, copper, and iron. She also has solutions of magnesium sulfate, copper sulfate, and iron sulfate. Predict whether a reaction will occur when:

a) Iron is added to copper sulfate solution b) Copper is added to magnesium sulfate solution
c) Magnesium is added to iron sulfate solution

Mark scheme answer:

a) Reaction occurs ✓ Iron is more reactive than copper and will displace it ✓

b) No reaction ✓ Copper is less reactive than magnesium and cannot displace it ✓

c) Reaction occurs ✓ Magnesium is more reactive than iron and will displace it ✓

Examiner note: Always refer to the reactivity series. A more reactive metal will displace a less reactive metal from its compound.

Example 2: Explaining rusting prevention (5 marks)

Question: A Caribbean shipping company maintains cargo vessels that transport bauxite from Jamaica to North America. Explain TWO methods the company could use to prevent rusting of the steel hulls and explain how each method works.

Mark scheme answer:

Method 1: Galvanizing/Zinc coating The steel hull is coated with zinc ✓ Zinc is more reactive than iron ✓ Zinc preferentially oxidizes/corrodes instead of the iron ✓

Method 2: Sacrificial protection Blocks of zinc or magnesium are attached to the hull ✓ These more reactive metals corrode first ✓ This protects the iron in the steel from oxidation ✓

Alternative acceptable methods: painting, electroplating, using stainless steel in critical areas

Examiner note: For full marks, you must both name the method AND explain the chemistry behind why it works. Simply stating "paint the hull" without explaining that paint forms a barrier to oxygen and water would earn only 1 mark.

Example 3: Metal extraction (4 marks)

Question: Explain why aluminium is extracted from bauxite using electrolysis but iron is extracted from iron ore using carbon.

Mark scheme answer:

Aluminium is more reactive than carbon ✓ Therefore carbon cannot displace aluminium from its oxide/ore ✓ Iron is less reactive than carbon ✓ Therefore carbon can reduce iron oxide, displacing iron from its compound ✓

Examiner note: This question tests understanding of the relationship between the reactivity series and extraction methods. The key principle is that only metals less reactive than carbon can be extracted by reduction with carbon.

Common mistakes and how to avoid them

• Confusing reactivity order: Students often place iron above zinc or aluminium below sodium. Create a memory aid: "Please Send Charlie's Monkeys And Zebras Into Large Cages Securely Guarded Please" (Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Lead, Copper, Silver, Gold, Platinum).

• Stating metals react with "air" instead of "oxygen": Be specific. Air is a mixture; the reactive component is oxygen. Write "reacts with oxygen" in your answers.

• Saying rust is "iron oxide" without including water: Rust is specifically hydrated iron(III) oxide. Both oxygen AND water are required for rust formation. Stating only one factor is incomplete.

• Reversing displacement reaction logic: Remember: the MORE reactive metal displaces the LESS reactive metal from solution. Copper cannot displace zinc from zinc sulfate because copper is less reactive.

• Confusing galvanizing with electroplating: Galvanizing specifically means coating with zinc. Electroplating is a general term for coating with any metal using electrolysis.

• Incomplete explanations of sacrificial protection: Don't just state that a reactive metal is attached—explain that it corrodes preferentially, protecting the iron by being oxidized instead.

Exam technique for "Metals: Properties, Reactivity Series and Corrosion"

• "Explain" questions require reasons: If asked to explain why zinc protects iron, you must state that zinc is more reactive and preferentially oxidizes. Simply naming the method earns minimal marks.

• Use chemical equations where possible: For 4+ mark questions on reactions, include balanced symbol equations. Even if not explicitly requested, they demonstrate understanding and may earn additional marks.

• Caribbean context questions: CXC often includes regional contexts (bauxite processing, maritime industries, steel pan manufacturing). Extract the core chemistry principle—don't be distracted by unfamiliar scenarios.

• Comparison questions: When comparing metals (e.g., "Compare the reactivity of magnesium and copper"), make direct comparative statements: "Magnesium is more reactive than copper" earns marks; separate statements about each metal may not.

Quick revision summary

Metals are elements with characteristic properties: they conduct electricity and heat, are malleable and ductile, and typically have high melting points. The reactivity series arranges metals by their reactivity with oxygen, water, and acids. More reactive metals displace less reactive metals from compounds. Metals more reactive than carbon require electrolysis for extraction; less reactive metals can be reduced using carbon. Corrosion, especially rusting of iron, is accelerated in humid, salty Caribbean environments. Prevention methods include barrier protection, galvanizing, sacrificial protection, alloying, and electroplating.