Reaction Yields (AQA AS Chemistry): Revision Note

Percentage Yield

• In a lot of reactions, not all reactants react to form products which can be due to several factors:

  • Other reactions take place simultaneously

  • The reaction does not go to completion

  • Reactants or products are lost to the atmosphere

• The percentage yield shows how much of a particular product you get from the reactants compared to the maximum theoretical amount that you can get:

percentage yield = $\frac{\mathbf{actual}\mathbf{yield}}{\mathbf{theoretical}\mathbf{yield}}$

• Where actual yield is the number of moles or mass of product obtained experimentally

• The predicted yield is the number of moles or mass obtained by calculation

• You will often have to use the following equation to work out the reacting masses, to calculate the predicted yield

number of mol = $\frac{\mathbf{mass}\mathbf{of}\mathbf{a}\mathbf{substance}\mathbf{in}\mathbf{grams}\left(g\right)}{\mathbf{molar}\mathbf{mass}\left(g{\mathrm{mol}}^{-1}\right)}$

• It is important to be clear about the type of particle you are referring to when dealing with moles

  • Eg. 1 mole of CaF₂ contains one mole of CaF₂ formula units, but one mole of Ca²⁺ and two moles of F^- ions

Limiting & Excess Reagents

Limiting & Excess reagents

• Sometimes, there is an excess of one or more of the reactants (excess reagent)

• The reactant which is not in excess is called the limiting reagent

• To determine which reactant is limiting:

  • The number of moles of each reactant should be calculated

  • The ratio of the reactants shown in the equation should be taken into account e.g.

2Na + S → Na₂S

• Here, the ratio of Na : S is 2 : 1, and this should be taken into account when doing calculations

• Once all of one reactant has been used up, the reaction will stop, even if there are moles of the other reactant(s) leftover

  • The reactant leftover is in excess, the reactant which causes the reaction to stop once it is used up is the limiting reagent