How do I balance an equation for a reaction, such as complete combustion of ethene?
Balancing equations is a simple conservation of the elements in a reaction: what you start with is what you finish with. With that in mind, let's take a look at the equation:C2H4 + O2 -> CO2 + H2OThe process for this is the same as for any equation: pick an element and count its atoms on each side. Let's start with Carbon, there are 2 in the reactants and 1 in the products. Clearly there aren't enough in the products, so we double the number of CO2 molecules produced. Note we can only increase the number of molecules involved, not individual atoms.C2H4 + O2 -> 2CO2 + H2OYou can see this changes the number of oxygen atoms in the products, but we have elemental oxygen in the reactants so we can worry about that last. There are 4 hydrogen atoms in the products and 2 in the reactants, which means we need to double the number of water molecules produced:C2H4 + O2 -> 2CO2 + 2H2ONow that carbon and oxygen are balanced, we can work out the oxygen. Note that we have 2CO2 molecules. Each molecule has 2 oxygens of its own and we have 2 molecules which gives 4 oxygens from, and then 2 more oxygen atoms from the water. Therefore we need 3 oxygen molecules and the final equation is:C2H4 + 3O2 -> 2CO2 + 2H2O
