Balancing Combustion Reactions

One of the more common combustion reactions is the combustion of an alkane in the presence of oxygen.

A general strategy for balancing these combustion reactions is to first balance the carbon (C) atom, followed by the hydrogen atoms (H) and then then oxygen (O) atoms.

Remember: C H O

The combustion of alkanes in the presences of oxygen will produce carbon dioxide and water. Here is the reaction for the combustion of propane , an alkane containing 3 carbon atoms.

Here is the balanced equation for the combustion of propane , an alkane
containing 3 carbon atoms:

C₃H₈ + 5 O₂ -----> 3 CO₂ + 4 H₂O

Let's examine how this combustion reaction was balanced.

Strategy for balancing a combustion reaction

1. Write the reaction.

C₃H₈ + O₂ -----> CO₂ + H₂O

2. Next balance the carbon atoms ( C) by placing a 3 in front of the CO₂ in the products.

C₃H₈ + O₂ -----> 3 CO₂ + H₂O

3. Then balance the hydrogen atoms (H) by placing a 4 in front of the H₂O in the products.

C₃H₈ + O₂ -----> 3 CO₂ + 4 H₂O

4. Finally balance the the oxygen atoms (O) by placing a 5 in front of the O₂ in the reactants.

C₃H₈ + 5 O₂-----> 3 CO₂ + 4 H₂O

If the alkane contains an even number of carbon atoms, you will need to alter your strategy slightly.

You still use C H O; however when you attempt to balance the oxygens in the end you will get stuck.

A simple strategy for balancing alkanes that have an even number of hydrogens is to simply place a 2 in front of the alkane before you begin to balance.

Balancing combustion reactions with an even number of carbons in the alkane.

1) Let's first write the reaction.

C₂H₆ + O₂ -----> CO₂ + H₂O

2) Arbitrarily place a 2 in front of the C₂H₆ in the reactants.

2 C₂H₆ + O₂ -----> CO₂ + H₂O

3) Now balance the carbon atoms by placing a 4 in front of CO₂ in the products.

2 C₂H₆ + O₂ -----> 4 CO₂ + H₂O

4) Balance the hydrogen atoms by placing a 6 in front of H₂O in the products.

2 C₂H₆ + O₂ -----> 4 CO₂ + 6 H₂O

5) Now balance the oxygen atoms by placing a 7 in front of the O₂ in the reactants.

2 C₂H₆ + 7 O₂ -----> 4 CO₂ + 6 H₂O