Complete combustion oxidises the hydrocarbon

1. Burning an alkane or an alkene in plenty of oxygen is complete combustion.
2. Every carbon atom is oxidised to carbon dioxide.
3. Every hydrogen atom is oxidised to water.
4. The hydrocarbon has gained oxygen, which is what oxidation means here.
5. Energy is given out, so the reaction is exothermic.

Writing the equation for an alkane

1. Methane burns completely to give: $${CH}_4+2O_2\to {CO}_2+2H_{2}O$$
2. Ethane needs more oxygen for the same reason: $$2C_2{H}_6+7O_2\to 4{CO}_2+6H_{2}O$$
3. Each carbon in the formula gives one carbon dioxide molecule.
4. Each pair of hydrogens gives one water molecule.
5. Oxygen is balanced last, once the products are fixed.

Writing the equation for an alkene

1. Ethene burns completely to give: $$C_2{H}_4+3O_2\to 2{CO}_2+2H_{2}O$$
2. The products are the same two substances as for an alkane.
3. The double bond makes no difference to what is formed.
4. An alkene has fewer hydrogens, so it needs less oxygen per carbon than the matching alkane.
5. The same three-step balancing works for either family.

Burning in a limited supply

1. With too little oxygen the combustion is incomplete.
2. Carbon monoxide and soot can form, as with any hydrocarbon.
3. Alkenes tend to burn with a smokier flame than alkanes of similar size.
4. That smoke is unburnt carbon, released because the molecule has proportionally more of it.
5. A clean blue flame indicates that the supply of oxygen is plentiful.