Chemical Reactions · Grade 10

Why may you change a coefficient in an equation but not a number inside a formula?

Because the coefficient says how many units take part, while the number inside the formula says what the substance is. Changing a coefficient means taking more of the same substance; changing a number inside a formula means swapping it for a different substance. You balance by adding units, not by inventing new compounds.

Learning objectives

An equation is a sentence

Left of the arrow are the reactants, right of it the products, and the arrow reads 'becomes'. 'Methane reacts with oxygen to give carbon dioxide and water' becomes CH4 + O2 → CO2 + H2O.

Two things must be right before you balance anything: every formula must be the substance's real formula, and everything formed must appear on the right. An unbalanced equation with correct formulas can be fixed; a balanced equation with a wrong formula is a tidy falsehood.

The balancing itself follows directly from the previous lesson. Atoms do not vanish, so every atom on the left must appear on the right, in exactly the same number.

Coefficient against subscript

In 2H2O the coefficient 2 means 'two water molecules'. In H2O2 the 2 after the oxygen means 'two oxygen atoms in the molecule', and that is no longer water but hydrogen peroxide — a different substance with different properties.

So balancing is done with coefficients only. Someone who changes a subscript to make the numbers come out has produced equal counts and written an equation for a reaction that did not happen.

A method that nearly always works: balance the elements that appear in only one substance on each side first, leave oxygen and hydrogen until last, and if you get stuck on a fraction, double the whole equation.

State symbols

After each formula you can say what state the substance is in: (s) solid, (l) liquid, (g) gas and (aq) dissolved in water.

They are not decoration. The difference between NaCl(s) and NaCl(aq) is the difference between a crystal that does not conduct electricity and a solution that does — and, as the periodic table hub showed, the reason is whether the ions are free to move.

Worked examples

  1. Balance: H2 + O2 → H2O

    1. Two oxygen atoms on the left, one on the right
    2. Double the water: H2 + O2 → 2H2O
    3. Now four hydrogen atoms on the right and two on the left
    4. Double the hydrogen

    Answer: 2H2 + O2 → 2H2O

  2. Balance the burning of methane: CH4 + O2 → CO2 + H2O

    1. Carbon is already balanced, one on each side
    2. Four hydrogen atoms on the left need 2H2O on the right
    3. The right now has 2 + 2 = 4 oxygen atoms
    4. So the left needs 2O2

    Answer: CH4 + 2O2 → CO2 + 2H2O

  3. A student balanced the first equation as H2 + O2 → H2O2. What is wrong?

    1. The atom counts do come out right
    2. But H2O2 is hydrogen peroxide, not water
    3. A subscript was changed instead of a coefficient

    Answer: It is balanced and describes a completely different reaction

Common mistakes

Changing a subscript to make it balance
That swaps the substance. H2O and H2O2 are both clear liquids and only one is water; an equation balanced around the wrong substance is worse than one not balanced at all.
Balancing oxygen first
Oxygen usually appears in several products at once, so every correction undoes what you already balanced. Leave it until last and it falls into place in one move.
Leaving out the state symbols
They say what is actually happening in the flask — a precipitate settling, a gas escaping, ions in solution. Without them the equation is correct and silent.

What to remember

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