AQA GCSE Chemistry

Revision Notes

3.1.1 Conservation of Mass & Balanced Chemical Equations

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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) + 2NaCl (aq)

law-of-conservation-of-mass-igcse-and-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

  • 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.

Exam Tip

  • A large number before any chemical applies to that entire chemical
  • The last equation above starts with 2Fe2O3
    • The Fe2O3 suggests that there are two iron atoms and three oxygen atome
    • The large 2 in front applies to the whole Fe2O3, i.e. 2 x Fe2O3
      • Therefore, there are four iron atoms and six oxygen atoms involved in the reaction

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

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