AQA GCSE Chemistry

Revision Notes

3.1.1 Conservation of Mass & Balanced Chemical Equations

The Law of Conservation of Mass

  • 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) + NaCl (aq)

Law of Conservation of Mass, IGCSE & GCSE Chemistry revision notes

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

Exam Tip

Matter cannot be created or destroyed, so the total amount of matter before and after a reaction is the same. What changes is the chemical and physical properties of the reactants as they transform into products.

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

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 notesBalancing Equations WE1 2, 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 notesBalancing Equations WE 2 2, downloadable IGCSE & GCSE Chemistry revision notes

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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