Balancing Chemical Equations (OCR Gateway GCSE Chemistry: Combined Science)

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
• The following nonmetals 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:
      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

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

Answer

Answer

• State symbols are written after formulae in chemical equations to show which physical state each substance is in:

• Sometimes it can be hard to know what the correct state symbol is and we have to look for clues in the identity of substances in a reaction
• Generally, unless they are in a solution:
  • Metal compounds will always be solid, although there are a few exceptions
  • Ionic compounds will usually be solids
• Non-metal compounds could be solids, liquids or gases, so it depends on chemical structure
• Precipitates formed in solution count as solids
• In the worked examples above the final equations with the state symbols would be

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

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