Hydrogen

Hydrogen (H2) runs through the whole 0620 syllabus: in the gas test, in acid + metal reactions, as a reducing agent, in electrolysis and fuel cells, and as a feedstock for the Haber process. It is an element that exists as diatomic molecules.

Key facts

Property	Value
Formula	H2
Mr	2(1) = 2
Bonding	One single covalent bond, H-H (diatomic molecule)
Key property	Lightest, least dense gas; flammable; insoluble in water
Gas test	Squeaky pop with a lighted splint
Main uses	Fuel and fuel cells; hydrogenation of oils; Haber process

Where hydrogen appears in 0620

• Tests for gases: the squeaky-pop test.
• Reactions of metals and the reactivity series: acid + metal -> salt + hydrogen, and hydrogen’s reference position in the series.
• Hydrogen-oxygen fuel cells: clean electricity.
• Electrolysis of aqueous solutions: the cathode product when a reactive metal ion is in solution.
• Alkenes: hydrogenation to make margarine.

Key reactions

Combustion (the gas test)

2H2(g) + O2(g) -> 2H2O(l)

Metal + acid (laboratory preparation)

Zn(s) + H2SO4(aq) -> ZnSO4(aq) + H2(g)

Any metal above hydrogen in the reactivity series works; copper and silver do not react.

Hydrogen as a reducing agent

CuO(s) + H2(g) -> Cu(s) + H2O(l)

The black copper(II) oxide is reduced to pink-brown copper. See redox reactions.

At the cathode / in the Haber process

2H+(aq) + 2e- -> H2(g)

N2(g) + 3H2(g) ⇌ 2NH3(g)

Industrially, most hydrogen is made by steam reforming of methane (CH4 + H2O -> CO + 3H2), which is also the source of the hydrogen fed into the Haber process.

Hydrogen as a fuel

A hydrogen-oxygen fuel cell converts hydrogen and oxygen straight into electricity, with water as the only product (2H2 + O2 -> 2H2O). Advantages over petrol: no carbon dioxide or pollutant gases at the point of use, and water is the only exhaust. Drawbacks: hydrogen is hard to store (needs high pressure or very low temperatures), is highly flammable, and is mostly made from fossil fuels at present. See fuel cells and the hydrogen economy.

Hydrogenation of alkenes

C2H4(g) + H2(g) -> C2H6(g) (nickel catalyst)

Adding hydrogen across a C=C double bond turns an unsaturated compound into a saturated one. Industrially this hardens vegetable oils into margarine.

Collecting hydrogen

• Over water (downward displacement of water), because hydrogen is insoluble.
• By downward delivery into an upturned tube, because hydrogen is less dense than air.

Hydrogen is passed over heated copper(II) oxide. State the colour change, write the equation, and explain in terms of oxygen which substance is oxidised. (4 marks)

Mark scheme

• Colour change: black to pink-brown (copper) [1]
• Equation: CuO + H2 -> Cu + H2O [1]
• Hydrogen is oxidised [1]
• because it gains oxygen (while the copper(II) oxide is reduced as it loses oxygen) [1]

Examiner note: define oxidation and reduction in terms of oxygen for this question — gain and loss of oxygen. Naming copper as oxidised, or muddling gain/loss, throws away two marks.

Common exam mistakes

• Describing the wrong test. Hydrogen gives a squeaky pop with a lighted splint; a glowing splint relighting is the test for oxygen.
• Claiming a hydrogen fuel cell is completely pollution-free. Water is the only exhaust, but the hydrogen is often produced from fossil fuels, releasing CO2 upstream.
• Getting the redox the wrong way round in CuO + H2. Hydrogen gains oxygen (oxidised); copper oxide loses oxygen (reduced).
• Leaving the combustion equation unbalanced. It is 2H2 + O2 -> 2H2O, not H2 + O2 -> H2O.