Complete Combustion

The Step-by-Step Oxidation Process

When a hydrocarbon undergoes complete combustion, a specific sequence of chemical events takes place:

• First, the hydrocarbon fuel is mixed with an excess of oxygen.
• Next, the reaction overcomes its activation energy, allowing the existing chemical bonds in the fuel and oxygen molecules to break.
• Then, both of the elements in the fuel undergo oxidation, which is the chemical gain of oxygen. The carbon atoms are fully oxidised to form carbon dioxide, and the hydrogen atoms are fully oxidised to form water.
• Finally, new bonds are formed to create the products. Because this is a highly exothermic reaction, the energy released by forming these new bonds is much greater than the energy used to break the old ones, releasing maximum heat and light to the surroundings.

Word and Symbol Equations

To describe complete combustion in an exam, you can use the following general word equation:

$$\text{hydrocarbon} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water} \text{ (+ energy)}$$

You must also be able to balance symbol equations for specific alkanes. For example, methane is the main component of natural gas:

$$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(g)$$

Propane is commonly used in bottled camping gas:

$$C_3H_8(g) + 5O_2(g) \rightarrow 3CO_2(g) + 4H_2O(l)$$

Note on balancing: Sometimes, balancing hydrocarbons (like ethane, $C_2H_6$) initially results in a decimal for oxygen (e.g., $3.5O_2$). To fix this, simply double every coefficient in the equation to achieve whole numbers.

Testing for the Products

If you need to prove that complete combustion has taken place, you can test the gases produced by the flame:

• Testing for carbon dioxide: Bubble the exhaust gases through limewater. If carbon dioxide is present, the limewater will turn cloudy or milky.
• Testing for water: Pass the gases over anhydrous copper(II) sulfate. If water is present, the powder will change colour from white to blue.

Both of these products are greenhouse gases, meaning that while complete combustion is an excellent source of energy, it directly contributes to global warming.