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.
  • 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 decreases.
  • 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)

AQA GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.