Fig. 8.1 shows apparatus used to identify the products formed when an alkane fuel is burned in a good supply of air. Fig. 8.1 Tube X contains anhydrous copp...

This question is about identifying the products of burning a hydrocarbon and testing for them. The gases from the flame are drawn first through anhydrous copper(II) sulfate (tube X, tests for water) and then through limewater (tube Y, tests for carbon dioxide).

(a) The two products of complete combustion [2]. A hydrocarbon burned in plenty of air gives carbon dioxide [1] and water (vapour) [1]. The carbon in the fuel ends up as CO₂ and the hydrogen ends up as H₂O.

(b) Colour change in tube X [2]. Anhydrous copper(II) sulfate turns from white to blue [1]. This change is caused by water [1], which is being condensed and absorbed from the gas stream. This is the standard chemical test for the presence of water.

(c) Observation in tube Y [2]. The limewater turns milky (cloudy) [1]. The gas responsible is carbon dioxide [1], which reacts with the calcium hydroxide in limewater to form insoluble calcium carbonate.

(d) Balanced equation for complete combustion of hexane [2].

2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O

Award [1] for the correct products (CO₂ and H₂O) and [1] for the whole equation being balanced. Check the balance: 12 C on each side, 28 H on each side, and \(12\times2 + 14 = 38\) O atoms on the right, matching \(19\times2 = 38\) on the left. Using 2 molecules of hexane avoids a fractional coefficient in front of O₂.

(e) Why tube X comes before tube Y [1]. So that the water vapour is removed and tested before the gas reaches the limewater [1]. If limewater came first, water from the flame would mix with it and you could not test the two products separately.

(f) Carbon-containing products of incomplete combustion [2]. In a limited air supply the fuel forms carbon monoxide [1] and carbon (soot) [1].

(g) Why these form instead of carbon dioxide [2]. There is not enough oxygen for the carbon to be fully oxidised [1], so the carbon is only partly oxidised to carbon monoxide, or not oxidised at all and left as solid carbon [1]. Full oxidation to CO₂ needs the greatest amount of oxygen, so it is the first thing to be lost when air is restricted.

Exam tip: remember the pairing of tests, white to blue anhydrous copper(II) sulfate for water and milky limewater for carbon dioxide, and always balance combustion equations by doubling the hydrocarbon if an odd number of oxygen atoms appears.