Theoretical yield
The theoretical yieldThe maximum possible mass of a product that can be made in a chemical reaction. is the maximum possible massThe amount of matter an object contains. Mass is measured in kilograms (kg) or grams (g). of a productA substance formed in a chemical reaction. that can be made in a chemical reaction.
It can be calculated from:
- the balanced chemical equationA chemical equation written using the symbols and formulae of the reactants and products, so that the number of units of each element present is the same on both sides of the arrow.
- the mass and relative formula massThe sum of the relative atomic masses of the atoms in a chemical formula. of the limiting reactantThe reacting substance that is completely used up in a chemical reaction and which determines how much product is made., and
- the relative formula mass of the product
An actual yieldThe actual mass of a product made in a chemical reaction. is the mass of a product actually obtained from the reaction. It is usually less than the theoretical yield. The reasons for this include:
- incomplete reactions, in which some of the reactantsSubstances present at the start of a chemical reaction. do not react to form the product
- practical losses during the experiment, such as during pouring or filteringThe process of passing a mixture through a device - soluble substances pass through the filter as a 'filtrate' but insoluble substances or unwanted material will stay in the filter as a 'residue'.
- side reactions (unwanted reactions that compete with the desired reaction)
- reversible reactionA chemical reaction which can go both ways.
- impurityA substance, usually unwanted, that is present in another substance. in reactants
Calculating theoretical yield
Reacting masses may be used to calculate the theoretical yield. Theoretical yield can also be worked out using a moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 脳10虏鲁 number of particles). calculation.
Worked example
If heated, calcium carbonate decomposeIf a substance decomposes, it breaks down into simpler compounds or elements. to form calcium oxide and carbon dioxide.
Calculate the maximum theoretical yield of calcium oxide that can be produced from 250 g of calcium carbonate.
- Write down the balanced chemical equation:
- CaCO3\(\rightarrow\) CaO + CO2
- Calculate the relative formula masses of the substances in the question:
- Mr of CaCO3 = 40.1 + 12.0 + (3 脳 16.0) = 100.1
- Mr of CaO = 40.1 + 16.0 = 56.1
- Use the balanced chemical equation to work out the reacting masses based on the relative formula masses:
- 100.1 g CaCO3 produces 56.1 g of CaO
- Work out the reacting masses for 1 g (or 1 kg or 1 tonne if different mass units are used):
- \(\frac{100.1}{100.1}\) = 1 g of CaCO3 produces \(\frac{56.1}{100.1}\) g of CaO
- Scale up the reacting masses to match the given reacting mass of the reactant:
- 250 g of CaCO3 produces \(\frac{56.1}{100.1}\) x 250
- = 140 g of CaO