Lower secondary · 12 min

Burning hydrocarbons: signs of a chemical change

Ignite methane, ethylene, acetylene and benzene in turn; compare the flame and the products each fuel leaves behind.

Goal

Recognise combustion as a chemical change: new substances ( $\ce{CO2}$ , $\ce{H2O}$ , soot) and a flame whose colour tracks the carbon content.

Apparatus and reagents

Four gaseous hydrocarbon samples and an oxygen supply (virtual); a flame-colour chart.

Procedure

Select $\ce{CH4}$ and ignite it. Note the flame colour and what collects in the exhaust.
Switch to $\ce{C2H4}$ and $\ce{C2H2}$ . Watch the flame brighten and the first soot particles appear.
Finish with $\ce{C6H6}$ and rank the four fuels by sootiness.
For each fuel, verify the atoms balance: every C ends up in $\ce{CO2}$ , every H in $\ce{H2O}$ when oxygen is plentiful.

What to observe

$\ce{CH4}$ burns with a clean blue flame; richer fuels give luminous, then smoky flames — $\ce{C6H6}$ is the smokiest.
All four complete combustions share the same products, $\ce{CO2}$ and $\ce{H2O}$ — the sign of a chemical change is the appearance of new substances.

Explanation

A hydrocarbon only contains C and H, so complete combustion in excess $\ce{O2}$ always yields $\ce{CO2}$ + $\ce{H2O}$ : $\ce{CH4 + 2O2 -> CO2 + 2H2O}$ , $\ce{C6H6 + 15/2 O2 -> 6CO2 + 3H2O}$ . When oxygen is locally scarce, carbon is released as soot (C) and $\ce{CO}$ instead — the yellow glow is incandescent soot. The %C by mass rises from $\ce{CH4}$ (75 %) to $\ce{C6H6}$ (92 %), so benzene flames are the smokiest.

History of the experiment

Lavoisier showed in the 1770s that burning is combination with oxygen, not a loss of "phlogiston" — combustion of hydrocarbons, weighed carefully, always conserves mass.

Chemists behind it

Antoine Lavoisier