Hydrogen Across IGCSE Chemistry

Hydrogen (H2) bridges acids-bases, electrochemistry, organic chemistry, and metals. This page traces every appearance so you can make connections across the syllabus.

1. Hydrogen in acid-base chemistry

Acid + metal -> salt + H2

Metals above hydrogen in the reactivity series react with dilute acids:

Mg + 2HCl -> MgCl2 + H2

Zn + H2SO4 -> ZnSO4 + H2

The hydrogen produced can be tested with a burning splint (squeaky pop).

Hydrogen ions define acids

An acid is a substance that produces H+(aq) ions in aqueous solution:

HCl(aq) -> H+(aq) + Cl-(aq)

The pH scale measures the concentration of H+ ions. Low pH = high [H+] = strong acid.

Hydrogen in neutralisation

In neutralisation, H+ ions react with OH- ions:

H+(aq) + OH-(aq) -> H2O(l)

See neutralisation reactions.

2. Hydrogen in the reactivity series

Hydrogen sits between iron and copper in the reactivity series. This reference point determines:

• Which metals react with dilute acids (metals above H)
• Which metals do not react with dilute acids (metals below H)

K > Na > Ca > Mg > Al > (C) > Zn > Fe > (H) > Cu > Ag > Au

3. Hydrogen in electrochemistry

Electrolysis product

Hydrogen is produced at the cathode when:

• Water or dilute acid is electrolysed
• A reactive metal salt solution is electrolysed (the reactive metal ion stays in solution; H+ from water is discharged instead)

Half-equation: 2H+(aq) + 2e- -> H2(g)

Examples:

• Electrolysis of dilute H2SO4: H2 at cathode, O2 at anode
• Electrolysis of brine (NaCl(aq)): H2 at cathode, Cl2 at anode

Hydrogen-oxygen fuel cell

2H2 + O2 -> 2H2O

A fuel cell converts chemical energy directly to electrical energy. Advantages: only water produced, no pollution. Disadvantages: H2 storage is difficult, most H2 currently from fossil fuels.

See hydrogen-oxygen fuel cells.

4. Hydrogen in organic chemistry

Hydrogenation

Adding H2 across the C=C double bond of an alkene:

C2H4 + H2 -> C2H6

Conditions: nickel catalyst, ~150 C. Used to harden vegetable oils into margarine.

As a component of hydrocarbons

All organic compounds contain hydrogen bonded to carbon. The hydrogen content determines properties and is involved in combustion:

CH4 + 2O2 -> CO2 + 2H2O

Fermentation

Glucose (C6H12O6) contains hydrogen, which ends up in ethanol (C2H5OH) during fermentation.

5. Hydrogen as a reducing agent

Hydrogen reduces metal oxides of metals below it in the reactivity series:

CuO + H2 -> Cu + H2O

Observation: black CuO turns pink-brown (copper). This is a redox reaction: CuO is reduced, H2 is oxidised.

Fe2O3 + 3H2 -> 2Fe + 3H2O

6. Production methods (summary)

Method	Context
Metal + acid	Laboratory preparation
Electrolysis of water/dilute acid	Cathode product
Electrolysis of brine	Industrial by-product
Steam reforming of methane	Industrial (Haber process feed)

7. Hydrogen as a fuel

Advantage	Disadvantage
Only product is water (clean)	Difficult to store (low boiling point, needs compression)
High energy per gram	Highly flammable/explosive
Can be renewable (electrolysis with renewable electricity)	Most current production uses fossil fuels

Name three different reactions that produce hydrogen gas. Give an equation for each. [6 marks]

• Metal + acid: Zn + H2SO4 -> ZnSO4 + H2 [2]
• Electrolysis of dilute acid: 2H+ + 2e- -> H2 (at cathode) [2]
• Group I metal + water: 2Na + 2H2O -> 2NaOH + H2 [2]

Hydrogen can act as a fuel and as a reducing agent. Explain the difference with equations. [4 marks]

• As a fuel: hydrogen burns in oxygen to produce water and release energy: 2H2 + O2 -> 2H2O [1]
• The purpose is to generate heat/electricity (e.g. in a fuel cell) [1]
• As a reducing agent: hydrogen removes oxygen from a metal oxide: CuO + H2 -> Cu + H2O [1]
• The purpose is to extract/obtain the metal; hydrogen is oxidised and the metal oxide is reduced [1]