Chemistry - Balancing Equations (AQA Syllabus A)

An understanding of the fundamental ideas in Chemistry is vital for students of GCSE Science. This is the last of six quizzes which recap these ideas and it focusses on chemical formulae and reactions, and on balancing chemical equations.

Symbol equations are often the things that put people off chemistry, but if you know a few simple rules you find that they make life so much easier. They are just a shorthand way of putting a huge amount of information about a chemical reaction down on paper. Once you know how the code works, it isn't that difficult. It can be fiddly at times but even for the most complicated reactions, persistence pays off.

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All that happens in a chemical reaction is that the atoms present in the reactants (starting chemicals) are rearranged to form new materials. For example, when you react the metal calcium with water, the atoms of calcium and the atoms that are in water just rearrange themselves to form hydrogen and calcium hydroxide. Word equations are useful as a starting point but they don't always help you to make sense of a chemical reaction. Balanced symbol reactions show you exactly what is going on.

The key rule for balancing equations is called the law of conservation of matter. Put simply, it means that whatever atoms you start with, you must end up with. So if you start a chemical reaction with 1 atom of calcium, 5 atoms of carbon and 10 atoms of oxygen you must end up with 1 atom of calcium, 5 atoms of carbon and 10 atoms of oxygen but arranged differently to how they were at the start.

The first step is to write down the correct chemical formulae for each of the chemicals involved. These CANNOT be changed to help with the balancing.You need to count the individual atoms on the side of reactants and then on the side of the products. You can then compare the two and decide where you need to make changes. You can only make changes by putting numbers in front of formulae. Adding extra numbers within formulae would make them incorrect.

Take the example of calcium reacting with water. The word equation and correct formulae are:

Calcium + water → calcium hydroxide + hydrogen

Ca + H2O → Ca(OH)2 + H2

So if you count up the atoms on the left you have one Ca, two H and one O. But counting up on the right you have one Ca, four H and two O. Yes? Just double check for yourself. The H and the O are out of balance.

That breaks the unbreakable law of conservation of matter. But in this case, it is easy to put right. Writing a 2 in front of the H2O would give you the same number of atoms on both sides.

But before you can do balanced symbol equations you need to be able to read and understand chemical formulae so the first few questions should get you warmed up for the main show...

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  1. How many atoms of hydrogen, sulfur and oxygen are there in sulfuric acid, formula H2SO4?
    A small number after the symbol for an atom in the formula tells you how many of that atom are present. Seeing the symbol just on its own means that you only have one of that sort of atom
  2. What non-metals are found in sodium silicate?
    The name of a chemical will often indicate what elements are present. Sodium is a metal so, for this question, can be ignored. The non-metal part is the 'silicate'. The 'silic' indicates the element silicon and the 'ate' is a code that indicates that it is combined with oxygen. This rule works with most chemicals that you will meet at GCSE so something like phosphate indicates you have phosphorous combined with oxygen. Other useful parts of this chemical code are 'ite', which also indicates oxygen is in there, and 'ide' which indicates that the non-metal is there on its own
  3. What do their names tell you about sodium sulfide and sodium sulfate?
    Chemical names do not tell you any of the physical or chemical properties of a compound so you can immediately dismiss the first 3 options
  4. The main gas used in a stink bomb is the highly toxic hydrogen sulfide. What elements does this compound contain?
    Easy if you were paying attention to the useful comment of question 2! You should have been able to dismiss answers 2 and 4 because there is no such element as sulfide and water is a compound, not an element
  5. How many atoms of oxygen in this thermal decomposition - CaCO3 ⇌ CaO +CO2
    So the equation is balanced
  6. You might find it easier to have a pen and paper handy for this and the rest of the questions. A student is balancing an equation that involves Na2SO4. She needs to have 6 atoms of sodium, how would she write this in her answer?
    Writing a large number in front of a formula multiplies everything in the formula by that number. Na2 tells her that there were 2 sodium atoms there to begin with so multiplying the formula by 3 would give her the 6 she needed. That would also increase the number of sulfur and oxygen atoms
  7. Bill needs to balance an equation that involves aluminium oxide, formula Al2O3. He requires a total of 12 oxygen atoms to balance his equation. What should he write?
    OK, getting the gist of things now? You should have immediately dismissed answers 2 and 3 as they change the formula
  8. Balance the equation Na + H2O → NaOH + H2.
    There are 2 Na atoms, 4 H atoms and 2 O atoms on each side of the equation. All the other answers have different numbers of the atoms on both sides of the equation
  9. Balance the equation for the reaction CuO + Al → Cu + Al2O3
    We hope that you didn't choose the fourth one - remember, do not change formulae to balance equations. In the exam, any formulae you are given for balancing equations will be correct. If you alter them, they will then be wrong
  10. In a blast furnace, there are several chemical reactions. The main one for smelting the iron from the iron ore is Fe2O3 + CO → Fe + CO2.
    Which numbers should be put in front of each formula in order to balance the equation?
    Remember, start off by counting individual atoms on each side then see what you need to get the same on both sides of the equation

Author: Kev Woodward

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