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Equations (AQA GCSE Chemistry)

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How to write word equations

  • Word equations show the reactants and products of a chemical reaction using their full chemical names

reactants → products

  • The reactants are those substances on the left-hand side of the arrow
    • They can be thought of as the chemical ingredients of the reaction
  • They react with each other to form new substances, which are the products
  • The products are on the right-hand side of the arrow
  • The arrow (which is spoken as “to form” or “produces”) implies the conversion of reactants into products
    • Reaction conditions or the name of a catalyst (a substance added to make a reaction go faster) can be written above the arrow
  • An example is the reaction of sodium hydroxide (a base) and hydrochloric acid to produce sodium chloride (common table salt) and water:

sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water

Worked example

Word equations

  1. Ammonia reacts with nitric acid to form the fertiliser ammonium nitrate. Write a word equation for the reaction taking place.
  2. Iron(II) hydroxide and sodium sulfate are formed when iron(II) sulfate solution and sodium hydroxide react together. Write a word equation for the reaction taking place. 
  3. Carbon is the main element found in coal and burns in air to produce carbon dioxide. Write a word equation for the reaction taking place.

Answers:

  1. Ammonia + nitric acid → ammonium nitrate
    • This question has all the information in the correct order
      • Ammonia reacts with nitric acid
        • This becomes ammonia + nitric acid
      • to form 
        • This is the arrow in the equation
      • to form the fertiliser ammonium nitrate
        • This tells you that the product is ammonium nitrate
  2. Iron(II) sulfate + sodium hydroxide → iron(II) hydroxide + sodium sulfate 
    • Careful: This question has all the required information but the products are written first
      • Iron(II) hydroxide and sodium sulfate are formed
        • This becomes → iron(II) hydroxide + sodium sulfate 
      • when iron(II) sulfate solution and sodium hydroxide react together
        • This becomes Iron(II) sulfate + sodium hydroxide →
  3. Carbon + oxygen → carbon dioxide
    • Careful: Not all of the required information is given in the question
    • You are expected to know that burning in air means that the chemical is reacting with oxygen
      • Carbon... ...burns in air
        • This becomes carbon + oxygen
      • to produce
        • This is the arrow in the equation
      • to produce carbon dioxide
        • This tells you that the product is carbon dioxide

How to write balanced equations

How to write balanced equations

  • A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
  • When writing symbol equations, you should:
    • Ensure reactants are on the left of the equation and products are on the right
    • Write the following non-metals as molecules: H2, N2, O2, F2, Cl2, Br2 and I2 
    • Include state symbols
      • Solid = (s)
      • Liquid = (l)
      • Gas = (g)
      • Aqueous = (aq)
  • You need to be confident using the state symbols (s), (l), (g) and (aq)
    • You will not need to include them in all equations unless you are specifically asked to 
    • However, it is good practice to include state symbols in your equations so that you don't miss any marks
  • A symbol equation must be balanced to give the correct ratio of reactants and products:
    • For example, the combustion of sulfur: 

S (s) + O2 (g)→ SO2 (g)

    • This equation shows that one atom of sulfur, S, reacts with one molecule of oxygen, O2, to make one molecule of sulfur dioxide, SO2

Balancing equations

  • When balancing equations, there must be the same number of atoms of each element on either side of the equation following the Law of Conservation of Mass
  • To balance an equation you work across the equation from left to right, checking one element after another
    • If there is a group of atoms such as a nitrate group (NO3–) that has not changed from one side to the other, then count the whole group as one entity rather than counting the individual atoms
  • Examples of balanced symbol / chemical equations include:
    • Acid-base neutralisation reaction:

NaOH (aq) + HCl (aq)  ⟶ NaCl (aq) + H2O (l) 

    • Redox reaction:

2Fe2O3 (aq) + 3C (s) ⟶ 4Fe (s) + 3CO2 (g)

  • In each equation, there are equal numbers of each atom on either side of the reaction arrow so the equations are balanced
  • The best approach is to practice lot of examples of balancing equations
    • This can be by trial and error - changing the coefficients (numbers) in front of the formulae one by one and checking the result on the other side
    • Balance elements that appear on their own, last in the process

Worked example

Symbol equations 1

Aluminium reacts with copper(II) oxide to produce aluminium oxide and copper. Balance the symbol equation for the reaction taking place.

_Al (s) +  _CuO (s)  ⟶  _Al2O3 (s) +  _Cu (s)

Answer:

  • The balanced symbol equation is:

2Al (s) +  3CuO (s) ⟶  Al2O3 (s) +  3Cu (s)

  • Step 1 - balancing aluminium atoms
    • There are 2 aluminium atoms on the product side, so 2 aluminium atoms are needed on the reactant side
    • 2Al  +  _CuO  ⟶  _Al2O3  +  _Cu
  • Step 2 - balancing oxygen atoms
    • There are 3 oxygen atoms on the product side, so 3 oxygen atoms are needed on the reactant side
    • This means that 3 CuO will be needed as we cannot change the chemical formula 
    • 2Al  +  3CuO  ⟶  _Al2O3  +  _Cu
  • Step 3 - balancing copper atoms
    • There are 3 copper atoms on the reactant side, so 3 copper atoms are needed on the product side
    • 2Al  +  3CuO  ⟶  _Al2O3  +  3Cu
    • The equation is now balanced

Worked example

Symbol equations 2

When magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water. Write a symbol equation for this reaction. 

Answer:

  • The balanced symbol equation is:

MgO (s) + 2HNO3 (aq) ⟶ Mg(NO3)2 (aq) + H2O (l)

  • Step 1 - writing the unbalanced equation
    • Magnesium oxide, MgO, reacts with nitric acid, HNO3, it forms magnesium nitrate, Mg(NO3)2, and water
      • MgO + HNO3 ⟶ Mg(NO3)2 + H2O
    • The Mg and O atoms (not including the O in the NO3 group appear to be balanced), so we should focus on the H atoms and NO3 groups
  • Step 2 - balancing hydrogen atoms
    • There are 2 hydrogen atoms on the product side, so 2 hydrogen atoms are needed on the reactant side
    • This means that 2 HNO3 will be needed as we cannot change the chemical formula 
      • MgO + 2HNO3 ⟶ Mg(NO3)2 + H2O
    • This also balances the nitrate, NO3, groups
  • Step 3 - checking the equation
    • The equation appears balanced so we need to check that it is 
      • Reactant side:
        • 1 Mg atom
        • 1 O atom - not including those in the NO3 group
        • 2 H atoms
        • 2 NO3 groups - remember to keep groups as a single entity if they are unchanged on both sides of the equation
      • Product side:
        • 1 Mg atom
        • 2 NO3 groups - remember to keep groups as a single entity if they are unchanged on both sides of the equation
        • 2 H atoms
        • 1 O atom - not including those in the NO3 group
    • The equation is now balanced

Examiner Tip

  • Careful: A common mistake when balancing symbol equations is to add, change or remove small numbers in the chemical formula of a substance
    • You cannot do this because it changes what the substance is
    • For example, if a product was water, H2O, and you added a second oxygen to make it H2O2 then it is no longer water

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    Stewart

    Author: Stewart

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.