AQA GCSE Chemistry

Revision Notes

3.1.3 Mass Changes when a Reactant or Product is a Gas

Gases in Reactions

  • Some chemical reactions may appear to involve a change in mass due to the presence of a gaseous reactant or product
  • If the reaction flask is open and a gaseous product is allowed to escape, then the total mass of the reaction flask will decrease as product mass is lost when the gas leaves the system
  • For example:
    • When a metal reacts with oxygen the mass of the oxide produced is greater than the mass of the metal you started with
    • When metal carbonates thermally decompose, carbon dioxide is produced and escapes into the atmosphere leaving the metal oxide as the only solid product

Exam Tip

Particles in the gas state still have mass! Remember mass is always conserved in chemical reactions

Explaining Observed Changes

  • By analysis of the balanced chemical equations and the corresponding state symbols, you should be able to deduce the changes in mass for non-enclosed reaction systems
  • 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 as carbon dioxide gas escapes to the atmosphere
    • So, the mass of the reaction mixture will decrease
  • If the mass of a reaction flask is found to increase then it is probably due to one of the reactants being a gas found in the air and all of the products are either solids or liquids
  • For example, the reaction of magnesium with oxygen produces magnesium oxide:

2Mg (s) + O2 (g) → 2MgO (s)

Exam Tip

If the reaction flask is completely closed, then no gases should be able to escape.

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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