What to do if you get stuck trying to balance a chemical equation

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What to do if you get stuck trying to balance a chemical equation

If you’re taking senior chemistry, you already know how to balance chemical equations. But sometimes, perhaps, you still get stuck. You balance the oxygens, but that messes up the carbons. You balance the carbons, but now the nitrogens are out-of-whack. Before you know it, you’ve got coefficients of 21 and 56 floating around and you know you’ve done something wrong.

If you don’t know what I’m talking about, then you either haven’t practised enough or you’re 100% competent and you can stop reading now. But if you have experienced digging yourself into a proverbial hole of endlessly multiplying coefficients, then read on!

The key is to remember that order is important. Always balance the element that appears in the LEAST number of chemical species first.

Example

Let’s take an example: the fermentation of glucose into carbon dioxide and ethanol.

C6H12O6 (aq) ➔ CO2 (aq) + C2H5OH (aq)

If we count the atoms, we see that carbon, hydrogen and oxygen are all unbalanced. So which one do we balance first?

Well, carbon appears in 3 species: glucose, carbon dioxide and ethanol. Oxygen also appears in all three. But hydrogen only appears in glucose and ethanol. So we will balance hydrogen first.

With 12 hydrogens on the left and 6 on the right, ethanol will need a coefficient of 2.

C6H12O6 (aq) ➔ CO2 (aq) + 2 C2H5OH (aq)

Next, it doesn’t matter whether we go for carbons or oxygens first. If we look at the carbons, there are 6 on the left in glucose, 1 on the right in carbon dioxide and 4 on the right in ethanol. We need one more on the right. We’ve already put down a coefficient for ethanol which we know balances out the hydrogens, so we want to leave that alone. Let’s put down a coefficient of 2 for carbon dioxide.

C6H12O6 (aq)2 CO2 (aq) + 2 C2H5OH (aq)

Now if we count the atoms, the equation is completely balanced – 6 carbons on each side, 12 hydrogens and 6 oxygens.

So the next time you find yourself lost in a haze of uncooperative atoms and rapidly increasing coefficients, start from scratch and figure out which element appears in the least number of species. Go from there and soon, everything will fall into place.

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