Full & Ionic Equations (Edexcel International AS Chemistry)

• A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of each atom in the reactants and products
• A word equation is a longer way of describing a chemical reaction using only words to show the reactants and products

Balancing equations

• During chemical reactions, atoms cannot be created or destroyed
• The number of each atom on each side of the reaction must therefore be the same
  • E.g. the reaction needs to be balanced

• When balancing equations remember:
  • Not to change any of the formulae
  • To put the numbers used to balance the equation in front of the formulae
  • To balance firstly the carbon, then the hydrogen and finally the oxygen in combustion reactions of organic compounds

• When balancing equations follow the following the steps:
  • Write the formulae of the reactants and products
  • Count the numbers of atoms in each reactant and product
  • Balance the atoms one at a time until all the atoms are balanced
  • Use appropriate state symbols in the equation

• The physical state of reactants and products in a chemical reaction is specified by using state symbols
  • (s) solid
  • (l) liquid
  • (g) gas
  • (aq) aqueous

Formulae for Ionic Compounds

• The formulae of simple ionic compounds can be calculated if you know the charge on the ions
• Below are some common ions and their charges:

Common Ions & Their Charges Table

• For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges = 0
• Example: what is the formula of aluminium sulfate?
  • Write out the formulae of each ion, including their charges
  • Al³⁺ SO₄^2-

• Balance the charges by multiplying them out:

  Al³⁺ x 2 = +6 and SO₄^2- x 3 = -6; so +6 – 6 = 0

• So the formula is Al₂(SO₄)₃

Answer:

Step 1: Write out the symbol equation showing reactants and products

Mg + O₂ → MgO

Step 2: Count the numbers of atoms in each reactant and product

Step 3: Balance the atoms one at a time until all the atoms are balanced

2Mg + O₂ → 2MgO

This is now showing that 2 moles of magnesium react with 1 mole of oxygen to form 2 moles of magnesium oxide

Step 4: Use appropriate state symbols in the fully balanced equation

2Mg (s) + O₂ (g) → 2MgO (s)

Ionic equations

• In aqueous solutions ionic compounds dissociate into their ions
• Many chemical reactions in aqueous solutions involve ionic compounds, however only some of the ions in solution take part in the reactions
• The ions that do not take part in the reaction are called spectator ions
• An ionic equation shows only the ions or other particles taking part in a reaction, and not the spectator ions

Answer 1:

Step 1: To balance the equation, write out the symbol equation showing reactants and products

Zn  + CuSO₄  → ZnSO₄ + Cu

Step 2: Count the numbers of atoms in each reactant and product. The equation is already balanced

Step 3: Use appropriate state symbols in the equation

Zn (s)  + CuSO₄ (aq)  → ZnSO₄ (aq) + Cu (s)

Answer 2:

Step 1:  The full chemical equation for the reaction is

Zn (s)  + CuSO₄ (aq)  → ZnSO₄ (aq) + Cu (s)

Step 2:  Break down reactants into their respective ions

Zn (s)  + Cu²⁺ +  SO₄^2- (aq)  → Zn²⁺+ SO₄^2- (aq) + Cu (s) 

Step 3:  Cancel the spectator ions on both sides to give the ionic equation

Zn (s)  + Cu²⁺ + SO₄^2- (aq)  → Zn²⁺+ SO₄^2- (aq) + Cu (s)

Zn (s)  + Cu²⁺(aq)  → Zn²⁺ (aq) + Cu (s)

• Chemical equations give information about the reaction that is taking place
  • Balanced equations show the number of particles participating in the reaction and the number of products being formed
  • Balanced equations can be used to calculate the number of moles involved in reactions 
  • Balanced equations can, also, be used to calculate masses and volumes involved in reactions
  • Ionic equations only show the reacting particles 
  • Ionic equations allow you to identify spectator ion

Types of reaction

• Chemical equations can be used to determine the type of reaction taking place

Displacement reactions

Br₂ (aq) + 2KI (aq) → I₂ (aq) + 2KBr (aq)

•  In this reaction, the more reactive bromine displaces the less reactive iodide in potassium iodide
• This can also be seen in the ionic equation for the reaction

 Br₂ (aq) + 2I^- (aq) → I₂ (aq) + 2Br^- (aq)

Neutralisation reactions

• These can be identified by the presence of reactant acids and bases as well as the formation of a neutral solution salt and water (and sometimes other compounds such as carbon dioxide)

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

Na₂CO₃ (aq) + 2HNO₃ (aq) → 2NaNO₃ (aq) + H₂O (l) + CO₂ (g)

•  The ionic equations can more clearly demonstrate the neutralisation of an acid and a base:

H⁺ (aq) + OH^- (aq) → H₂O (l)

2H⁺ (aq) + CO₃^2- (aq) → H₂O (l) + CO₂ (g)

Precipitation reactions

• These are shown by the reaction of two aqueous solutions to form products which include one solid

BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq)

•  The ionic equation shows the precipitation reactions more clearly as there are no other products considered

Ba²⁺ (aq) + SO₄^2- (aq) → BaSO₄ (s)