2.4 Reactivity Series

REVISION NOTES

IGCSE Edexcel Chemistry

2.4 Reactivity Series

2.4.1 Understand how metals can be arranged in a reactivity series based on their reactions with:

Water
Dilute hydrochloric or sulfuric acid

REACTIVITY SERIES OF METAL

The reactivity series of metal can be determined by analysing their reactions with water and acids

Metals react with water and acids
Based on the observation made of the reactions, metals are placed in order of reactivity

Reaction 1: Metal + Water

Metal + Water → Metal hydroxide + Hydrogen

Ca (s) + 2H₂O (l) → Ca(OH)₂ (aq) + H₂(g)

Calcium + Water → Calcium hydroxide + Hydrogen

Reaction 2: Metal + Acid

Metal + Acid → Salt + Hydrogen

2K (s) + 2HCl (aq) → 2KCl (s) + H₂ (g)

Potassium + Hydrochloric acid → Potassium chloride + Hydrogen

Only metals above hydrogen in the reactivity series will react with acids
The more reactive the metal, the more vigorous the reaction

2.4.2 Understand how metals can be arranged in a reactivity series based on their displacement reactions between:

Metals and metal oxides
Metals and aqueous solutions of metal salts

The reactivity of metal decreases going down the reactivity series

A more reactive metal will displace a less reactive metals from its compounds
Two reactions will be discussed:
- Metal and metal oxide
- Metal and aqueous solutions of metal salts

REACTION 1 METAL + METAL OXIDE

Zn + CuO → ZnO + Cu
According to the reactivity series, Zn is more reactive than Cu
Therefore, Zinc can displace Cu in CuO and form ZnO

REACTION 2 METAL + AQUEOUS SOLUTION OF METAL SALTS

A more reactive metal displaces the less reactive metal in its aqueous solution
The more reactive metal slowly disappears from the mixture
Mg + CuSO₄ → MgSO₄ + Cu
Mg is more reactive than Cu, so Mg can displace Cu in CuSO₄
- The blue colour of CuSO₄ fades as colourless MgSO₄ solution is formed

2.4.3 Know the order of reactivity of these metals: potassium, sodium, lithium, calcium, magnesium, aluminium, zinc, iron, copper, silver, gold

2.4.4 Know the conditions under which iron rusts

RUSTING OF IRON

Rusting is a chemical reaction between iron, oxygen and water
Hydrated iron (III) oxide is formed
Required conditions:
- Oxygen
- Water
Rusting occurs faster in salty water as sodium chloride speeds up the reaction

Iron + Water + Oxygen → Hydrated Iron(III) Oxide

4Fe (s) + 3O2 (g) + xH2O (l) → 2Fe2O3・xH2O (s)

2.4.5 Understand how the rusting of iron may be prevented by:

Barrier methods
Galvanising
Sacrificial protection

Rusting of iron can be prevented by eliminating contact of iron with oxygen or water

METHOD 1 BARRIER METHOD

Rust can be prevented by coating iron with barriers
To prevent iron from contacting with water and oxygen
Common barrier methods:
- Grease
- Oil
- Paint
- Plastic
However, if the coatings are washed away or scratched, the iron is again exposed to water and oxygen and will rust

METHOD 2 GALVANISING

Rusting can be prevented by using metals higher in reactivity than iron
Galvanising: iron is protected by coating with a layer of zinc
Zn reacts with oxygen and carbon dioxide in the air, which protects the iron
- Zn loses electrons more readily than Fe
- Fe stays protected because it accepts the electrons released by Zn, so Fe does not undergo oxidation
If the coating is damaged or scratched is still protected from rusting because Zn preferentially corrodes as it is higher up in the reactivity series than iron

METHOD 3 SACRIFICIAL CORROSION

Sacrificial corrosion occurs when a more reactive metal is intentionally allowed to corrode

Example: ships’ hull

ships’ hulls sometimes have large blocks of magnesium attached
The blocks slowly corrode and provide protection to the hull in the same way the zinc does

2.4.6 Understand the terms:

Oxidation
Reduction
Redox
Oxidising agent
Reducing agent

in terms of gain or loss of oxygen and loss or gain of electrons.

REDOX REACTION (IN TERMS OF OXYGEN)

Zn + CuO → ZnO + Cu
This displacement reaction can be classified as a redox reaction
- Oxidation occurs together with reduction simultaneously
CuO loses oxygen, so it undergoes reduction
Zn gains oxygen, so it undergoes oxidation
CuO oxidises Zn, so it is an oxidising agent
Zn reduces CuO, so it is an reducing agent

REDOX REACTION (IN TERMS OF ELECTRON)

Displacement reaction can also be analysed in terms of electron transfer
An ionic equation is used to help better understand the electron transfer
Zn + CuO → ZnO + Cu
- Ionic equation: Zn + Cu²⁺ + O^2- → Zn²⁺ + O^2- + Cu
- O^2- is the spectator ion that appears on both sides, it is removed for the net ionic equation
- Net ionic equation: Zn + Cu²⁺ → Zn²⁺ + Cu
- The equation can be further split into two half equations that show oxidation and reduction individually:

Zn → Zn²⁺ + 2e^–

Cu²⁺+ 2e^– → Cu

Zn loses electrons, so it undergoes oxidation
Cu²⁺ gains electrons, so it undergoes reduction

2.4.7 Practical: investigate reactions between dilute hydrochloric and sulfuric acids and metals (e.g. magnesium, zinc and iron)

METHODS

Use a measuring cylinder to measure 5cm3 of dilute hydrochloric acid
Add same mass of magnesium ribbon, iron filings and zinc turnings to different test tubes
Observe what happens
Use a light splint to test for any gases given off
Repeat the experiment with dilute sulfuric acid

Observations