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Edexcel GCSE Chemistry · 1CH0
Edexcel 1CH0 · 5.12C Check the specification (PDF) (opens in a new tab)
A balanced chemical equation predicts the greatest amount of a chosen product that could form from the available reactants. This is based on the —the reactant whose available amount limits how much product can form. The prediction assumes that this reactant is converted completely through the intended reaction and that all the desired product is recovered.
The actual yield is the amount of desired product really collected or measured. It is usually lower because the ideal assumptions are rarely met perfectly. Atoms have not disappeared: either less desired product forms or some of the product that forms is not recovered.
Actual yield is usually lower because conversion may stop short of the theoretical extent, competing reactions may form unwanted products, or desired product may be lost before collection. A deliberately excess reactant may remain even after the limiting reactant is fully consumed.
A reaction is incomplete when it does not proceed to the theoretical extent expected from the limiting reactant. For example, warming may be stopped too soon, leaving some limiting reactant that could still have formed more desired product. Because less of that reactant follows the intended pathway, less product forms.
A reversible reaction may reach equilibrium with reactants and products both present. The forward and reverse reactions continue at equal rates, so complete conversion of the limiting reactant may not occur. The actual amount of product available can therefore be below the ideal theoretical prediction.
Simply observing leftover reactant is not enough to prove that a reaction was incomplete. A reactant may have been added deliberately in excess; some of it should remain once the limiting reactant has been used up. The important question is whether conversion stopped short of the amount predicted from the limiting reactant.
Even if the desired substance forms, not all of it may reach the final container. Product can be spilt, remain on a stirring rod or stick to glassware during transfer. Processes involving several vessels create more opportunities for these losses.
Product may also be lost during separation and purification. Some crystals may remain dissolved when the solution is discarded; fine solid may pass through a filter or remain on the filter paper; a gas may escape before it is collected. These are practical losses: the desired product formed, but it was not included in the measured actual yield.
The intended reaction may not be the only possible pathway. Under particular conditions, a side reaction can compete for the same reactants and form unwanted products. Reactant diverted into this pathway is no longer available to form as much of the desired product.
For example, heating for too long can encourage some side reactions. Choosing a suitable temperature and reaction time can reduce this problem, although it may not eliminate it.
An unwanted product from a competing reaction is not the same as a by-product already shown in the balanced equation for the intended reaction. Side reactions affect how much desired product is obtained; the products specified by the intended equation are considered separately when studying atom economy.
The causes can combine. Conversion may stop short of the theoretical extent, some reactant may enter a side reaction, and some desired product may then be lost during filtration. Each reduces the amount of pure, dry desired product finally collected.
Actual yield is usually, not always apparently, below theoretical yield. A measured mass that seems too high may include water or impurities if a product was not properly dried or purified. That is a measurement problem, not a genuine yield above the theoretical maximum.
An apparent yield above the theoretical value usually indicates wet or impure product, or a measurement error.
When explaining a low actual yield, name the cause and link it to less desired product being obtained: for example, “some product remained dissolved, so it was discarded with the solution”.
Classify evidence carefully. Leftover reactant only indicates incomplete conversion if that reactant was expected to be consumed, such as the limiting reactant. A reactant deliberately added in excess can remain after the reaction is complete.
Do not confuse percentage yield with atom economy. Yield compares the desired product actually obtained with the theoretical amount; atom economy concerns how reactant atoms are distributed between products in the balanced equation.
A measured yield above 100% usually indicates wet or impure product, or a measurement error—not genuine production beyond the theoretical maximum.
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Theoretical yield
The maximum amount of a specified product predicted from the balanced equation and the limiting reactant, assuming complete conversion and no loss of product.
Actual yield
The amount of a specified product that is actually collected or measured in an experiment.
Incomplete reaction
A reaction that has not proceeded to the theoretical extent expected from the limiting reactant, so less desired product forms.
Practical loss
The loss of desired product while it is transferred, separated, purified or collected.
Side reaction
A competing reaction that consumes reactants and forms an unwanted product instead of some of the desired product.
Limiting reactant
The reactant that is used up first according to the balanced equation and therefore limits the theoretical amount of product.
Put your knowledge into practice — try past paper questions for Chemistry
Theoretical yield
The maximum amount of a specified product predicted from the balanced equation and the limiting reactant, assuming complete conversion and no loss of product.
Actual yield
The amount of a specified product that is actually collected or measured in an experiment.
Incomplete reaction
A reaction that has not proceeded to the theoretical extent expected from the limiting reactant, so less desired product forms.
Practical loss
The loss of desired product while it is transferred, separated, purified or collected.
Side reaction
A competing reaction that consumes reactants and forms an unwanted product instead of some of the desired product.
Limiting reactant
The reactant that is used up first according to the balanced equation and therefore limits the theoretical amount of product.