Limiting Reagents
- A chemical reaction stops when one of the reagents is used up
- The reagent that is used up first is the limiting reagent, as it limits the duration and hence the amount of product that a reaction can produce
- The amount of product is therefore directly proportional to the amount of the limiting reagent added at the beginning of a reaction
- The limiting reagent is the reactant which is not present in excess in a reaction
- In order to determine which reactant is the limiting reagent in a reaction, we have to consider the ratios of each reactant in the balanced equation
- When performing reacting mass calculations, the limiting reagent is always the number that should be used as it indicates the maximum possible amount of product
- The steps are:
- Write the balanced equation for the reaction
- Calculate the moles of each reactant
- Compare the moles & deduce the limiting reactant
Worked example
9.2 g of sodium is reacted with 8.0 g of sulfur to produce sodium sulfide, Na2S. Which reactant is in excess and which is the limiting reactant?
Answer:
- Step 1: Write the balanced equation and determine the molar ratio
2Na + S → Na2S so the molar ratios is 2 : 1
- Step 2: Calculate the moles of each reactant
Moles = Mass ÷ Ar
Moles Na = 9.2/23 = 0.40
Moles S = 8.0/32 = 0.25
- Step 3: Compare the moles
To react completely 0.40 moles of Na requires 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.
Examiner Tip
An easy way to determine the limiting reactant is to find the moles of each substance and divide the moles by the coefficient in the equation. The lowest number resulting is the limiting reactant
- In the example above:
- divide 0.40 moles of Na by 2, giving 0.20
- divide 0.25 moles of S by 1, giving 0.25, so Na is limiting
