Learning focus
Use chemical formulae and equations accurately, convert measured quantities into moles, apply balanced-equation ratios and present multi-step calculations with units and checks.
Definition
The limiting reactant is the reactant completely consumed first. It determines the maximum amount of product. Any other reactant present beyond the required ratio is in excess.
Comparing amounts
Convert each reactant quantity to moles and divide by its equation coefficient. The smaller value identifies the limiting reactant because it can support fewer complete reaction sets.
Alternative product method
Calculate the product amount each reactant could produce separately. The smaller product amount is physically possible and identifies the limiting reactant.
Consequences
Calculations of theoretical yield must use the limiting reactant. Using the excess reactant produces an impossible amount because insufficient partner reactant is available.
Worked example
2H2 + O2 -> 2H2O. With 3 mol H2 and 2 mol O2: H2/2=1.5; O2/1=2. H2 is limiting and forms 3 mol H2O.
Practical or data skill
Model reactions with counters in fixed ratios and observe which colour remains after all complete reaction groups are formed.
Examination tip
Do not choose the reactant with the smaller mass; compare moles relative to coefficients.
Review questions and suggested answers
Question 1
What determines the maximum product amount?
Suggested answer
The limiting reactant.
Question 2
For N2+3H2, compare 2 mol N2 and 3 mol H2. Which limits?
Suggested answer
H2, because 3 mol H2 reacts with only 1 mol N2.
Question 3
Can an excess reactant be completely consumed?
Suggested answer
No, some remains after the limiting reactant is used.
