OK I do requests; one of my readers wanted me to explain the best way to balance chemical equations

My golden rule is a chemical reaction never changes the number of atoms or electrons; all it does is change the way that atoms and electrons are arranged. If you forget everything else you read on my blog today then just try to remember that rule.

Lets take an example, light the gas and cook something.

We can start with an equation which shows the starting materials and the products but is not balanced.

CH_{4} + O_{2} –> CO_{2} + H_{2}O

My advice is to check how many carbons appear on each side, we are lucky as there is one carbon on each side.

Next check the oxygens, on the left are two and the right are three so we know it is not balanced.

Next look at the hydrogens, on the left are four and the right two. As we can not destroy atoms we need to do something to fix this problem.

What I would suggest is to double the number of water molecules on the right so the equation now reads

CH_{4} + O_{2} –> CO_{2} + 2H_{2}O

Now recount all the atoms,

Carbon atoms still are the same on both sides of the equation (1)

Hydrogen atoms are now in equal numbers on both sides of the equation (4)

Oxygens are not in equal numbers on the left are 2 and on the right 4.

So what we can do is to double the number of oxygen molecules on the left.

CH_{4} + 2O_{2} –> CO_{2} + 2H_{2}O

Now

Carbon atoms still are the same on both sides of the equation (1)

Hydrogen atoms are now in equal numbers on both sides of the equation (4)

Oxygens are now in equal numbers on the left are 4 and on the right 4.

So the equation is now balanced.

It should be

CH_{4} + 2O_{2} –> CO_{2} + 2H_{2}O

Now we feel good, now let us consider the gas cooker of doom which has a dirty yellow flame which makes soot.

The starting equation is now

CH_{4} + O_{2} –> CO + H_{2}O

OK the carbons are in equal numbers on both sides, so we can stop thinking about them for a moment.

It is clear that the number of hydrogens on the left is double the number on the right, so we should double the number of waters on the right

Now we have

CH_{4} + O_{2} –> CO + 2H_{2}O

It is still not balanced as the number of oxygens on the left is two and on the right it is three. What we can do is to add an extra oxygen on the left.

CH_{4} + O + O_{2} –> CO + 2H_{2}O

While it might be balanced, it does not make sense as oxygen is normally in the form of molecules which have two oxygen atoms in them.

CH_{4} + 1½O_{2} –> CO + 2H_{2}O

As we are dealing with 1½ oxygen molecules the best thing to do is to double everything to give us

2CH_{4} + 3O_{2} –> 2CO + 4H_{2}O

By the way the reason why I call this cooker which makes CO (carbon monoxide) the “cooker of doom” is that carbon monoxide is a nasty poison gas with no colour or smell. If you do get a sooty or yellow flame on a natural gas (or LPG) appliance then my advice is to call a gas fitter to check the item.

Filed under: Chemistry, Teaching, university |

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