When a hydrocarbon fuel burns, the products depend on how much oxygen is available. (a) State the two products formed when a hydrocarbon undergoes complete ...

This question compares complete and incomplete combustion and focuses on carbon monoxide, the dangerous product of burning a hydrocarbon in too little oxygen. Complete combustion needs plenty of oxygen and gives only carbon dioxide and water; a limited oxygen supply gives carbon monoxide and/or soot instead.

(a) Complete combustion of a hydrocarbon gives two products: carbon dioxide [1] and water. [1]

(b) Two products of incomplete combustion, other than water: carbon monoxide [1] and carbon (soot). [1] These form because there is not enough oxygen to fully oxidise all the carbon to CO₂.

(c) Carbon monoxide is dangerous because it is toxic (poisonous); [1] it combines with haemoglobin in the blood, so the blood carries less oxygen around the body, starving the tissues of oxygen. [1]

(d) Because it is colourless and has no smell, a person cannot see or smell it and is not aware of the leak, so they breathe it in without realising and take no action to escape. [1]

(e) Completing the incomplete-combustion equation of methane:

2CH₄ + 3O₂ → 2CO + 4H₂O

[1] for CO and [1] for H₂O. Count the atoms to check: 2 C, 8 H and 6 O on each side. The carbon ends up as carbon monoxide (not CO₂) because oxygen is limited.

(f) One home appliance that could release carbon monoxide if faulty: a gas boiler (also acceptable: gas heater, gas cooker or gas fire). [1]

(g) One way to reduce the risk: fit a carbon monoxide detector (also acceptable: provide good ventilation, or have gas appliances serviced regularly). [1]

Exam tip: for why carbon monoxide is dangerous you need both marks, "toxic" and the haemoglobin/oxygen-carrying point. Just writing "it is poisonous" scores only one.