CIE A Level Chemistry复习笔记1.2.4 Balancing Equations

• 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

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, without showing the spectator ions

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)

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)