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Chemistry 0620 How-to guide 7 min read Free

How to balance chemical equations

A five-step method for every kind of 0620 equation, from burning fuels to ionic equations, with a worked example marked line by line.

Start reading See the worked example
On this page
  1. Why marks are lost
  2. The five-step method
  3. Worked example
  4. Mistakes that cost marks
  5. Where to go next
Start here

Why equations lose marks

A balanced equation has the same number of each type of atom on both sides. Atoms are rearranged in a reaction, never created or destroyed, so the count must match.

The 0620 syllabus asks you to write word and symbol equations with state symbols. Extended candidates must also write ionic equations and work out an equation from information in the question.

Three mistakes cost the most marks. The formulae are wrong before balancing starts. The equation is balanced by changing a subscript. Or the state symbols are missing or guessed.

The idea
Balancing is bookkeeping. Get the formulae right, then adjust only the numbers in front until every atom is accounted for.
The method

The five-step method

Work through these steps in order, every time.

  1. Write the correct formulae

    Turn the word equation into formulae. Use ion charges for ionic compounds: Al3+ and Cl− give AlCl3. Remember the diatomic elements.

    Tip. Once a formula is written, treat it as locked. You will only ever add numbers in front of it.

  2. Count the atoms on each side

    Make a quick tally under the equation, one column per element. A coefficient multiplies everything in the formula after it, so 2H2O contains 4 H and 2 O.

  3. Balance one element at a time

    Pick an element that appears in only one substance on each side. Add a coefficient so its count matches. If the counts are 2 and 3, aim for 6, the lowest common multiple.

  4. Save lone elements for last

    Elements that appear on their own, such as O2, H2 or a metal, are the easiest to adjust. Changing them disturbs nothing else, so balance them at the end.

    Tip. If you need half a molecule, such as 3½ O2, double every coefficient to clear the fraction.

  5. Recount, then add state symbols

    Check every element again on both sides. Then add (s), (l), (g) or (aq) after each formula, using the information in the question.

    Tip. A coefficient of 1 is not written. Make sure the coefficients cannot all be divided by a common number.

For a fuel burning in oxygen, a useful order is carbon, then hydrogen, then oxygen. Propane gives C3H8 + 5O2 → 3CO2 + 4H2O. Ethane needs 3½ O2 at first, so double it: 2C2H6 + 7O2 → 4CO2 + 6H2O.

Shortcut for ions
If a group such as NO3 or SO4 stays together on both sides, count it as one unit. Ca(OH)2 + 2HNO3 → Ca(NO3)2 + 2H2O balances quickly this way. Count NO3 groups, not separate N and O atoms. The OH group breaks up to form water, so count its H and O separately.

Extended: ionic equations. These show only the ions that change. Atoms and total charge must both balance. Mixing silver nitrate and sodium chloride solutions gives Ag+(aq) + Cl−(aq) → AgCl(s). Charge is 0 on each side. The neutralisation of an acid by an alkali is H+(aq) + OH−(aq) → H2O(l).

Barium chloride and sodium sulfate solutions form a white precipitate in the same way: Ba2+(aq) + SO42−(aq) → BaSO4(s). The charges are 2+ and 2−, so the total is 0 on both sides.

See it in action

Worked example

This question gives a reaction in words. You must write the formulae, balance them and add state symbols.

Worked example · metal and acid[3 marks]

Aluminium reacts with dilute hydrochloric acid. The products are aluminium chloride solution and hydrogen gas. Write the balanced symbol equation for this reaction, including state symbols.

Al3+Cl−hydrogen is diatomicacid is dilute, so aqueous
Mark 1
Al + HCl → AlCl3 + H2
Correct formulae for all four substances. The charges 3+ and 1− give AlCl3.
Mark 2
Cl: 1 → 3, so try 3HCl. H is now 3 on the left and 2 on the right. The LCM of 3 and 2 is 6: 6HCl and 3H2. Cl is now 6, so 2AlCl3. Finally 2Al.
2Al + 6HCl → 2AlCl3 + 3H2
Correct balancing. Check: 2 Al, 6 H and 6 Cl on each side.
Mark 3
2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
Correct state symbols on every species.
2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
Mark split is our own illustration of how a mark scheme typically rewards each stage, not taken from a real paper.

Look at the second line. Fixing hydrogen changed chlorine, so chlorine had to be fixed again. That back-and-forth is normal. Keep going until a full recount comes out equal.

Try it yourselfSame method · 3 marks

Sodium reacts with water to form sodium hydroxide solution and hydrogen gas. Write the balanced symbol equation, including state symbols.

Show the answer
Formulae: Na + H2O → NaOH + H2
H is 2 on the left and 3 on the right, so try 2H2O and 2NaOH. Then Na needs 2Na.
Check: 2 Na, 4 H, 2 O on each side.
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Exam advice

Mistakes that cost marks

Changing a subscript to make it balance
Turning H2O into H2O2 balances the oxygen but makes hydrogen peroxide. Only coefficients may change.
Single atoms for diatomic elements
Hydrogen gas is H2 and oxygen is O2. Writing H or O as a product is marked as a wrong formula.
Wrong formula from the ion charges
Calcium chloride is CaCl2, not CaCl. Check that the charges in an ionic compound add up to zero before you balance.
Forgetting that a coefficient multiplies the whole formula
3Ca(OH)2 contains 3 Ca, 6 O and 6 H. Count the atoms inside brackets as well.
Guessing state symbols
Water at room temperature is (l), not (aq). A precipitate is (s). A salt dissolved in solution is (aq). Read the question for clues such as solution, gas or precipitate.
Unbalanced charge in an ionic equation
Atoms can balance while charges do not. Add up the charges on each side as the final check.
Keep going

Where to go next

How-to guideMole calculationsUse the ratio from your balanced equation to find reacting masses.Read the guide →Free Revision notesChemistry 0620 notesFormulae and equations are taught in full in the stoichiometry topic.Read the notes →Free Past papersChemistry past papersPractise writing equations with state symbols on the theory papers.Browse papers →Free
Balancing is the first step. The notes cover every calculation after it.
Formulae, equations, the mole and reacting masses, with worked examples and exam advice. $10, one-time, no subscription.
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