Conservation of Mass & Balancing Chemical Equations (AQA GCSE Chemistry)

• The Law of Conservation of Mass states that no matter is lost or gained during a chemical reaction.
• Mass is always conserved, therefore the total mass of the reactants is equal to the total mass of the products, which is why all chemical equations must be balanced
• The sum of the relative atomic/molecular masses of the reactants will be the same as the sum of the relative atomic/molecular masses of the products
• A precipitation reaction is one in which two solutions react to form an insoluble solid called a precipitate
• If the reaction flask is closed and no other substance can enter or leave the system, then the total mass of the reaction flask will remain constant
• For example, the reaction between calcium chloride and sodium sulfate produces a precipitate of calcium sulfate.
• If carried out in a closed system then the mass before and after the reaction will be the same
• The balanced equation is:

CaCl₂ (aq) + Na₂SO₄ (aq) ⟶ CaSO₄ (s) + 2NaCl (aq)

Diagram showing the conservation of mass in a precipitation reaction

• If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will change as product mass is lost when the gas leaves the system
• For example, the reaction between hydrochloric acid and calcium carbonate produces carbon dioxide gas:

2HCl (aq) + CaCO₃ (s) ⟶ CaCl₂ (aq) + H₂O (l) + CO₂ (g)

• Mass will be lost from the reaction flask unless it is closed
• If the mass of a reaction flask is found to increase then it may be due to one of the reactants being a gas found in the air and all of the products are either solids or liquids

• The numbers involved in chemical formulae and equations give a lot of information about the chemicals involved
• In chemical formulae:
  • If there is no subscript number after an element, then there must be one of that particular element
    • e.g. CO contains one carbon and one oxygen atom
  • If there is a subscript number after an element, then that number belongs to the element just before it
    • e.g. CO₂ contains one carbon atom and two oxygen atoms
  • If there is a subscript number after brackets, then that number belongs to all of the elements inside the bracket
    • e.g. Ca(OH)₂ contains one calcium atom, two oxygen atoms AND two hydrogen atoms
  • The most complicated examples contain a subscript number inside the bracket as well as outside,
    • e.g. Ca(NO₃)₂ contains one calcium atom
    • Inside the bracket, there is one nitrogen atom and three oxygen atoms but the subscript 2 outside the bracket applies to the nitrogen and oxygen inside the bracket
    • This means that there are two nitrogen atoms and six oxygen atoms
• 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 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:

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
• 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