Reactions in closed systems
No substances can enter or leave a closed systemIn chemistry, a system in which no substances can enter or leave during a reaction., such as a stoppered flask. Sometimes, reactions that happen in open beakers are closed systems, for example acid-alkali neutralisationThe reaction between an acid and a base to form a salt plus water. reactions. Since all the reactants and products stay in the beaker, the total mass of the beaker and the substances in it stay the same during the reaction.
Reactions in non-enclosed systems
Substances can enter or leave a non-enclosed systemIn chemistry, a system in which gases or other substances can enter or leave during a reaction.. These systems include open flasks, boiling tubes or crucibles that let gases enter or leave. For example:
- copper carbonate decomposes on heating to make solid copper oxide, which stays in the boiling tube, and carbon dioxide gas, which escapes
- magnesium reacts with oxygen, gained from the air, to produce magnesium oxide
If a gas escapes, the total mass will look as if it has decreased. If a gas is gained, the total mass will look as if it has increased. However, the total mass stays the same if the mass of the gas is included.
Worked example
Question
10.0 g of calcium carbonate, CaCO3, was heated in a thermal decompositionType of reaction in which a compound breaks down to form two or more substances when it is heated. reaction. 5.60 g of solid remained after heating. The equation below represents the reaction:
CaCO3(s) → CaO(s) + CO2(g)
Explain the change in mass.
The mass appears to have decreased because one of the products, carbon dioxide, escapes to the air. The total mass of the solid and gas products will be 10.0 g.
