Balancing Equations

Balancing symbol equations is actually easier than remembering word equations and usually worth more marks. Many students are put off because symbols look complicated but once you understand what the symbols really mean, you can see that they are just a shorthand way of describing what s happening in a reaction. The numbers involved are rarely larger than about 10, so no complicated maths is required.

For example, if you take a piece of shiny sodium metal and drop it into a jar of yellow-green, poisonous chlorine gas, there will be a violent reaction with a bright orange flame and then a flash and then clouds of white smoke. Eventually, if you have enough chlorine gas, all the sodium metal will have disappeared, any excess chlorine gas will have floated away (hopefully into a fume cupboard as any remaining chlorine will still be poisonous). The white solid is called sodium chloride and incredibly, it the same compound that we sprinkle on our food.

While it can be fun, to describe a reaction in a lot of detail, we are all busy and it s good to have a shorthand. Balanced symbol equations can do this quickly and accurately and as a bonus, they also show how much sodium chloride we can expect from a given amount of sodium. The trick is to learn what the symbols mean. All the ingredients (reactants) are represented by the symbols of the elements they are made from, and these are all found on the periodic table. It can really helpful to just learn the first 20 elements and their symbols by heart as these feature most often in the common materials met at GCSE and even A level. For example, sodium is 11th and chlorine is 17th. Whenever an element has two letters in its symbol (like Cl for Chlorine), the second letter is always lower case. This is how you can tell the difference between CO (carbon monoxide) and Co (cobalt). The plus sign just means and and the arrow means changes into . So in the word equation below, sodium and chlorine change into sodium chloride.

Sodium + Chlorine Sodium Chloride

Word equations are quite useful but you have to remember to write chloride instead of chlorine and with some names like ammonia, it s not immediately obvious which elements are involved: the formula for ammonia is NH_3. This tells you that the elements are nitrogen and hydrogen and there are three times as many atoms of hydrogen as nitrogen.

As well as the letters, there are also numbers. The big numbers before an element symbol like the two in 2Na below mean multipliers - 2Na means that there are two units (atoms, molecules or ions) of sodium compared to only one molecule or unit of Cl₂ (Chlorine gas). When there is only one of something, we don t bother to write the 1, it s just assumed.

The small subscri pt numbers after an element symbol tell you about the structure of chlorine molecules. There are always two atoms of chlorine in a molecule of chlorine gas. Cl₃ can t exist because three atoms of chlorine don t stick together properly (which is because of their electronic structure but that s another story). Anyway the formula of chlorine gas is Cl₂. If we need two molecules of chlorine, we would write 2Cl₂

2Na + Cl₂ 2NaCl

If we now look at the equation below, we can see that we start with two atoms of sodium and one molecule of chlorine (which contains two atoms of chlorine). It makes sense that we should have the same number of atoms at the end of the reaction so we have to balance the equation

2Na + Cl₂ 2NaCl

The equation above is already balanced but supposing we wanted to react sodium with bromine gas (also poisonous). The formula of bromine gas is Br₂ , sodium bromide is NaBr (which is also because of their electronic structure) and sodium is still Na, so if we just write the reactants first and then the product that they change into after the arrow, we get

Na + Br₂ NaBr but this equation doesn t balance because there are two bromine at the start but only one at the end (we have lost a bromine somewhere).

The solution is to make two units of sodium bromide (2NaBr), but to do this we need two units of sodium, 2Na. This now gives the balanced equation below:

2Na + Br₂ 2NaBr

Since there are two atoms of sodium at the beginning, there must be two atoms of sodium at the end. The formula of sodium chloride is just NaCl which contains only one unit of sodium, so we need two units of sodium chloride to get two units of sodium. If we now look at chlorine, we start with one

CH₄ + 2O₂ CO₂ + 2H₂O

There are just two more things to remember. If we need to show charged ions, this is done by a superscri pt - eg Na⁺ or Mg²⁺ and if a formula is in brackets, it is treated like a fixed unit - eg (NH₄)₂SO₄ , ammonium sulphate has one Sulphur, four oxygens and two units of the ammonium, NH₄ which is in the bracket. The ammonium has to be treated as a set of four hydrogens and one nitrogen which stick together - so they are kept together in brackets. The numbers outside the bracket relate to how many units of everything inside the bracket are needed. (In NH₄)₂SO₄, there are two nitrogens eight hydrogens, one sulphur and 4 oxygens. An example of an equation involving brackets would be:

Mg(OH)₂ + Na NaOH + Mg - not balanced

It s always important to check the balance by counting the atoms of each element at the start (before the arrow) and checking that there are the same number at the end (after the arrow).

So for the example above, there is one atom of magnesium at the beginning and one magnesium at the end - done,

Two oxygens at the beginning but only one at the end. We could solve this by having two units of NaOH (2NaOH) but this would also give two units of oxygen and hydrogen.

That s actually ideal as there are two units of oxygen and hydrogen in magnesium hydroxide - Mg(OH)₂

Mg(OH)₂ + Ca Ca(OH)₂ + Mg So the balanced equation would be: