Reactivity Series, Displacement Reactions, and Redox Chemistry

Reactivity Series of Metals

Definition and Arrangement

The reactivity series is a list of metals arranged in order of their reactivity, from most to least reactive. This series helps predict how metals will react with water, acids, and other compounds.

• Most reactive: Potassium, sodium, lithium, calcium

• Moderately reactive: Magnesium, aluminium, zinc, iron

• Least reactive: Copper, silver, gold

Key Point: The position of a metal in the series determines its ability to displace other metals from compounds and its reaction with water and acids.

Deduction Using Water and Acids

Metals above hydrogen in the reactivity series react with water or steam to produce hydrogen gas.

• Metals reacting with cold water: Potassium, sodium, lithium, calcium

• Metals reacting with steam: Magnesium, zinc, iron

• Metals below hydrogen: Do not react with water or steam (e.g., copper)

Example:

• Potassium + water → potassium hydroxide + hydrogen

• Magnesium + steam → magnesium oxide + hydrogen

Displacement Reactions

Definition and Explanation

A displacement reaction occurs when a more reactive metal displaces a less reactive metal from its compound.

• The more reactive metal replaces the less reactive metal in the compound.

• Used to deduce the position of metals in the reactivity series.

Example: Magnesium displaces copper from copper(II) oxide: $\text{Mg} + \text{CuO} \rightarrow \text{MgO} + \text{Cu}$ Additional info: Displacement reactions can also occur in aqueous solutions, where a metal displaces another from its salt.

Oxidation and Reduction (Redox)

Definitions

• Oxidation: Addition of oxygen or loss of electrons.

• Reduction: Removal of oxygen or gain of electrons.

• Redox reaction: A chemical reaction involving both oxidation and reduction.

• Oxidising agent: Substance that causes oxidation by accepting electrons.

• Reducing agent: Substance that causes reduction by donating electrons.

Mnemonic: OILRIG – Oxidation Is Loss (of electrons); Reduction Is Gain (of electrons).

Electron Transfer and Ionic Equations

• Reactions involving ions can be represented by ionic equations.

• Spectator ions are ions that do not participate in the reaction.

• Half equations show the electron transfer for individual substances.

Example:

• Oxidation: $\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^{-}$

• Reduction: $\text{Cu}^{2+} + 2e^{-} \rightarrow \text{Cu}$

Reactions of Metals with Water and Acids

Reactions with Water

• Metals above hydrogen react with water or steam to produce hydrogen.

• Cold water: Metal hydroxide and hydrogen are formed.

• Steam: Metal oxide and hydrogen are formed.

General equations:

• Metal + cold water → metal hydroxide + hydrogen

• Metal + steam → metal oxide + hydrogen

Reactions with Acids

• Metals above hydrogen react with dilute acids to produce salt and hydrogen.

• The higher the metal in the series, the more vigorous the reaction.

• Metals below hydrogen do not react with simple dilute acids.

Example:

• Magnesium + hydrochloric acid → magnesium chloride + hydrogen

• Iron + sulfuric acid → iron(II) sulfate + hydrogen

Rusting of Iron and Its Prevention

Conditions for Rusting

• Iron rusts in the presence of water and oxygen.

• Rusting is accelerated by electrolytes (e.g., salt).

• Rust is chemically represented as $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$.

Prevention Methods

• Barriers: Paint, oil, grease, plastic, or coating with less reactive metals (e.g., tin).

• Alloying: Making stainless steel with chromium or nickel.

• Galvanising: Coating iron with zinc.

Sacrificial Protection

• Zinc (more reactive than iron) corrodes instead of iron when used as a coating.

• Even if the zinc layer is scratched, zinc acts as a sacrificial anode and protects iron.

• Electrons from zinc flow to iron, preventing iron from forming ions and rusting.

Example: Zinc blocks are attached to boat hulls to protect iron from rusting.

Summary Table: Reactivity Series and Metal Reactions

Metal	Reaction with Water	Reaction with Acid	Position in Series
Potassium	Very vigorous	Too reactive	Top
Sodium	Very vigorous	Too reactive	Top
Lithium	Vigorous	Too reactive	Top
Calcium	Gentle	Can react if acid is very dilute	High
Magnesium	Almost no reaction (cold water), reacts with steam	Vigorous	Moderate
Zinc	Reacts with steam	Slow (cold), faster (hot)	Moderate
Iron	Reacts with steam	Slow (cold), faster (hot)	Moderate
Copper	No reaction	No reaction	Low
Silver	No reaction	No reaction	Bottom
Gold	No reaction	No reaction	Bottom

Additional info: The table summarizes the main reactions and positions of metals in the reactivity series, aiding prediction of their chemical behavior.