1.2.4 Reaction Yields

• 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:

• The 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

• 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

Answer

Step 1: The symbol equation is:

Zn (s)    +    CuSO₄ (aq)     →     ZnSO₄ (aq)    + Cu (s)

 Step 2: Calculate the amount of zinc reacted in moles

Step 3: Calculate the maximum amount of copper that could be formed from the molar ratio:

Since the ratio of Zn(s) to Cu(s) is 1:1 a maximum of 0.10 moles can be produced

Step 4: Calculate the maximum mass of copper that could be formed (theoretical yield)

mass =   mol   x   M_r

mass =   0.10 mol x 64 g mol^-1

Step 5: Calculate the percentage yield of copper

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

Answer

Step 1: Calculate the moles of each reactant

Step 2: Write the balanced equation and determine the molar ratio

2Na + S → Na₂S

The molar ratio of Na: Na₂S is 2:1

Step 3: Compare the moles and determine the limiting reagent

So to react completely 0.40 moles of Na require 0.20 moles of S and since there are 0.25 moles of S, then S is in excess. Na is therefore the limiting reactant.

Once all of the Na has been used up, the reaction will stop, even though there is S left.