Chemistry 8.7 - 8.13 - Combustion and air pollutants

Complete combustion and evidence

A hydrocarbon contains carbon and hydrogen only. When a hydrocarbon burns with a plentiful supply of oxygen, it undergoes complete combustion. The carbon atoms from the fuel finish in carbon dioxide molecules, while the hydrogen atoms finish in water molecules.

At the macroscopic level, a flame transfers energy to its surroundings: nearby materials become warmer. Complete combustion is therefore exothermic. At the sub-microscopic level, fuel and oxygen particles react and atoms are rearranged; at the symbolic level, the general word equation is:

Energy is released overall because forming the new bonds in carbon dioxide and water releases more energy than is needed to break bonds in the reactants. Breaking a bond does not itself release energy.

Worked example: balancing complete combustion

Start with the correct formulae for methane combustion. Formula subscripts describe each substance, so they must not be altered to balance the equation.

1. Carbon is already balanced: one C atom on each side.
2. There are four H atoms on the left, so place a coefficient 2 before water.
3. The products now contain four O atoms in total: two in $\ce{CO2}$ and two across $\ce{2H2O}$. Place a coefficient 2 before oxygen.

The sense-check is conservation of atoms: both sides have 1 C, 4 H and 4 O atoms. The equation also agrees with the observable claim: carbon dioxide and water are produced while energy is given out.

Investigating the products

The purpose is to obtain evidence that burning a hydrocarbon produces water and carbon dioxide. Use a laboratory gas burner on a heatproof mat in a well-ventilated laboratory, with its air hole open to give a steady non-luminous blue flame. For water, hold a dry, chilled boiling tube just above the flame and look for liquid droplets condensing on its outer surface. For carbon dioxide, draw gases from above the flame through fresh limewater using an inverted glass funnel, heat-resistant connection, gas-washing bottle, empty safety trap and low-flow suction pump; limewater turning cloudy supports the presence of carbon dioxide.

The independent variable is the gas source: combustion gas from the lit burner or room air collected for the same time with the gas supply off. The dependent observations are whether droplets form and whether the limewater becomes cloudy. Keep collection time, apparatus position, starting temperature and dryness of the boiling tube, limewater amount and concentration, and room conditions constant. Repeat each condition with a dry tube and fresh limewater; record every result rather than selecting the clearest trial.

One possible set of observations is shown below; the control outcome must be recorded rather than assumed.

Wear eye protection because limewater can irritate eyes. Tie back hair, keep flammable materials away, check the burner and gas tubing before use, handle hot glass with tongs and allow it to cool, and turn off the gas supply after the test. Do not enclose the flame, smell the gases or attempt to prepare or test carbon monoxide; good ventilation and a small flame limit exposure to combustion products.

The unlit control matters because room moisture could otherwise be mistaken for product water. Droplets are supporting evidence rather than a chemical identity test, so starting with a completely dry tube and obtaining droplets only in the flame trial strengthens the inference. A funnel also collects some room air, which dilutes carbon dioxide; a better seal around the gas path, without restricting the flame's air supply, makes the limewater result clearer. The observations support the predicted products, but they do not prove that combustion was complete because a partly incomplete flame can still make some carbon dioxide and water.

When oxygen is limited

If oxygen cannot reach the flame fast enough, incomplete combustion occurs. Carbon atoms are not all fully oxidised to carbon dioxide, so the products can include carbon monoxide, $\ce{CO}$, and solid carbon, $\ce{C}$, as soot. Water is still formed from the hydrogen in the fuel, and a real flame may produce a mixture of carbon dioxide, carbon monoxide, carbon and water.

Balanced methane equations show two possible oxygen-limited routes: