IGCSEChemistry: Double ScienceEdexcelRevision Notes1. Principles of ChemistryChemical Formulae, Equations, CalculationsWord & Chemical Equations

Word & Chemical Equations (Edexcel IGCSE Chemistry: Double Science)

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Writing equations

  • New substances are made during chemical reactions
    • However, the same atoms are always present before and after reaction
    • They have just joined up in different ways
    • Atoms cannot be created or destroyed, so if they exist in the reactants then they absolutely must be in the products!

  • Because of this the total mass of reactants is always equal to the total mass of products
  • This idea is known as the Law of Conservation of Mass

Conservation of mass

  • The Law of Conservation of Mass enables us to balance chemical equations, since no atoms can be lost or created
  • You should be able to:
    • Write word equations for reactions outlined in these notes
    • Write formulae and balanced chemical equations for the reactions in these notes

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

  • 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

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

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

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    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.