Edexcel IGCSE Chemistry: Double Science

Revision Notes

1.5.1 Word & Chemical Equations

Test Yourself

Writing Equations

Nothing created - nothing destroyed

  • 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

Word Equations

  • These show the reactants and products of a chemical reaction using their full chemical names
  • The reactants are those substances on the left-hand side of the arrow and can be thought of as the chemical ingredients of the reaction
  • They react with each other and form new substances
  • The products are the new substances which are on the right-hand side of the arrow
  • The arrow (which is spoken as “goes to” 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 produces sodium chloride (common table salt) and water:

sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water

Representing reactions as equations

  • Chemical equations use the chemical symbols of each reactant and product
  • When balancing equations, there has to be the same number of atoms of each element on either side of the equation in accordance with the Law of Conservation of Mass
  • A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction
  • A symbol equation must be balanced to give the correct ratio of reactants and products:   

S + O2 → SO2

  • This equation shows that one atom of sulfur (S) reacts with one  molecule of oxygen (O2) to make one molecule of sulfur dioxide (SO2)
  • The following non-metals must be written as molecules: H2, N2, O2, F2, Cl2, Br2 and I2
  • 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, for example 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 chemical equations:
    • Acid-base neutralisation reaction:

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

    • Redox reaction:

2Fe2O(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

  • Don't forget to add state symbols when writing balanced equations:

Using state symbols table, IGCSE & GCSE Chemistry revision notes

Balancing Equations

The best approach is to practice lot of examples of balancing equations

  • By trial and error change the coefficients (multipliers) in front of the formulae, one by one checking the result on the other side
  • Balance elements that appear on their own, last in the process

Worked example

Example 1:

Balance the following equation:

aluminium + copper(II)oxide ⟶ aluminium oxide + copper 

Unbalanced symbol equation:

Al + CuO ⟶ Al2O3 + Cu

Answer:

Balancing Equations WE1 1, downloadable IGCSE & GCSE Chemistry revision notes

Worked example

Example 2:

Balance the following equation:

magnesium oxide + nitric acid ⟶ magnesium nitrate + water 

Unbalanced symbol equation:

MgO + HNO3 ⟶ Mg(NO3)2 + H2O

Answer:

Balancing Equations WE2 1, downloadable IGCSE & GCSE Chemistry revision notes

Exam Tip

Chemical equations do not contain an equals sign between the left and right-hand sides but are written with an arrow instead. The arrow means that the reactants have reacted together and formed the product(s).

Writing ionic equations

  • In aqueous solutions ionic compounds dissociate into their ions, meaning they separate into the component ions that formed them, e.g. hydrochloric acid and potassium hydroxide dissociate as follows:

HCl (aq) →  H+ (aq) + Cl-(aq) 

KOH (aq)  → K+ (aq)  + OH- (aq) 

  • It is important that you can recognise common ionic compounds and their constituent ions
  • These include:
    • Acids such as HCl and H2SO4
    • Group I and Group II hydroxides e.g. sodium hydroxide
    • Soluble salts e.g. potassium sulfate, sodium chloride
  • Follow the example below to write ionic equations 

Worked example

Write the ionic equation for the reaction of aqueous chlorine and aqueous potassium iodide.

Answer:

  • Step 1: Write out the full balanced equation:

2KI (aq) +  Cl2 (aq) → 2KCl (aq) + I2 (aq)

  • Step 2: Identify the ionic substances and write down the ions separately

2K+ (aq) + 2I- (aq) +  Cl2 (aq) → 2K+ (aq) + 2Cl- (aq) + I2 (aq)

  • Step 3: Rewrite the equation eliminating the ions which appear on both sides of the equation (spectator ions ) which in this case are the K+ ions:

 2I- (aq) +  Cl2 (aq) → 2Cl- (aq) + I2 (aq)

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