3.3.1a Percentage yield
Real reactions rarely give all the product they could
Product
A product is a substance formed during a chemical reaction.
- The yield is the mass of product you actually collect from a reaction.
- The yield is almost always less than the maximum the equation predicts, even when the method is careful.
Percentage yield compares what you actually got with the most you could ever get, so it can never be more than 100%.
Three reasons a yield falls short of the maximum
Reversible reaction
A reaction in which the products can react to form the original reactants.
- The reaction may not finish, which is common in a reversible reaction that reaches a balance before all reactants are used.
- Some product is lost when it is separated, for example left behind on filter paper or spilled during transfer.
- Some reactants take part in unwanted side reactions, making different products instead of the one you want.
When a salt is made by filtering and drying crystals, a little always stays stuck to the filter paper, so the yield drops below the maximum.
Percentage yield compares actual mass with the maximum
Percentage yield
The mass of product actually made, written as a percentage of the maximum theoretical mass that could form.
- The formula is percentage yield =mass of product mademaximum theoretical mass×100= \dfrac{\text{mass of product made}}{\text{maximum theoretical mass}} \times 100=maximum theoretical massmass of product made×100.
- Both masses must be in the same unit so that the ratio is correct.
- A percentage yield above 100% means a mistake, usually a product that was still damp when weighed.
- Put the actual mass on top and the theoretical mass on the bottom, as swapping them is the most common error.
Worked example: calculating a percentage yield
- A reaction could make a maximum of 8 g8\ \text{g}8 g of product, but only 6 g6\ \text{g}6 g is collected.
- The percentage yield is 68×100=75%\dfrac{6}{8} \times 100 = 75\%86×100=75%.
- Give three reasons a percentage yield is less than 100%.
- Write down the formula for percentage yield.
- A reaction makes 9 g9\ \text{g}9 g out of a possible 12 g12\ \text{g}12 g; what is the percentage yield?
- Why would a damp product give a percentage yield above 100%?
3.3.1b Theoretical mass of a product
The theoretical mass is the most product you could possibly make
Theoretical mass
The maximum mass of a product that could be made if all the limiting reactant is converted and none is lost.
- The theoretical mass assumes every bit of the limiting reactant is converted and no product is lost.
- It is worked out from the balanced equation, so it is a prediction rather than a measured result.
The theoretical mass is the target the reaction is measured against, so it is the bottom of a percentage yield calculation.
Find it with the mole method
Mole
The amount of a substance, measured in mol, where one mole has a mass in grams numerically equal to its relative formula mass.
Relative formula mass
The sum of the relative atomic masses of all the atoms shown in a chemical formula.
- Change the mass of the reactant into moles using n=mMrn = \dfrac{m}{M_r}n=Mrm.
- Use the mole ratio from the balanced equation to find the moles of product.
- Change those moles into a mass using m=n×Mrm = n \times M_rm=n×Mr.
- Base the calculation on the limiting reactant, not on any reactant that is in excess.
- Show your moles and ratio clearly, so each stage scores even if the final figure is wrong.
Calcium oxide from a carbonate
- Find the theoretical mass of calcium oxide from 50 g50\ \text{g}50 g of calcium carbonate in CaCO3→CaO+CO2\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2CaCO3→CaO+CO2.
- Moles of calcium carbonate =50100=0.5 mol= \dfrac{50}{100} = 0.5\ \text{mol}=10050=0.5 mol.
- The ratio is 1:11:11:1, so 0.5 mol0.5\ \text{mol}0.5 mol of calcium oxide forms.
- Mass of calcium oxide =0.5×56=28 g= 0.5 \times 56 = 28\ \text{g}=0.5×56=28 g, which is the theoretical mass.
- If only 21 g21\ \text{g}21 g were actually collected, this 28 g28\ \text{g}28 g would be the value you divide by to find the percentage yield.
- What two assumptions does a theoretical mass make?
- List the three steps of the mole method for finding a theoretical mass.
- Find the theoretical mass of magnesium oxide from 6 g6\ \text{g}6 g of magnesium in 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}2Mg+O2→2MgO.
- Why should the calculation be based on the limiting reactant?
