The Limiting Reactant Lab
The limiting reactant, the one that runs out first and limits how much product forms, the reactant left in excess, and how to find the limiting one by comparing moles against the equation ratio.
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When one runs out first
Mix two reactants and they rarely finish at the same moment. One runs out first and stops the reaction, so it decides how much product can form. That one is the limiting reactant, and the leftover one is in excess. This module shows what the limiting reactant is and how to work out which reactant it is.
Reaction words to know
Learn these before you compare the reactants.
Limiting against excess
The two reactants play opposite roles.
Compare moles, not raw amounts
Never guess the limiting reactant from the masses given. Work out the moles of each reactant, then compare them with the balanced-equation ratio. Whichever reactant provides the smaller share for that ratio runs out first and is limiting. The amount of product then follows from that limiting reactant alone.
Match the term
- the limiting reactant
- the reactant in excess
- the product
- the equation ratio
- runs out first and limits the product
- has some left over at the end
- the substance the reaction makes
- the amounts that react together
Match the step
- find the moles of each reactant
- compare them to the ratio
- pick the smaller share
- work out the product
- the first step before any decision
- checks which reactant is short
- that reactant is the limiting one
- use the limiting reactant to find it
What does the limiting reactant do?
What effect does the limiting reactant have on a reaction?
- It runs out first and limits how much product can form.
- It is always left over at the end.
- It has no effect on the product.
- It only changes the colour of the product.
True about excess
Select the TWO true statements about a reactant in excess.
- Some of it is left over at the end
- It does not limit the amount of product
- It runs out before the other reactant
- It is not really a reactant at all
Find the leftover
Reactants A and B react in a 1 to 1 ratio. You start with 5 mol of A and 2 mol of B, so B is limiting. Subtract the moles of B from the moles of A to find how many moles of A are left over. What is the answer?
Order the method
Put the steps of finding the limiting reactant in order, earliest first.
- Balance the equation
- Find the moles of each reactant
- Compare the moles to the ratio
- Name the limiting reactant
Complete the rules
The reactant that runs out first is the _____ reactant, and it decides how much _____ can form. The reactant left over is said to be in _____. To find the limiting one you compare the moles of each reactant against the _____ in the balanced equation.
Spot the limiting reactant clues
Tap the TWO things that are true of the limiting reactant.
- it runs out first
- it limits the product
- it is left over at the end
- it makes no difference
Work out the reaction
Read each case and choose the best answer.
- A and B react 1 to 1. You have 3 mol of A and 8 mol of B. Which reactant is limiting?
- In that reaction, which reactant is in excess?
- A student decides the limiting reactant just from the masses on the label, without finding moles. What is wrong?
Explain the limiting reactant
Explain what a limiting reactant is and how to work out which reactant is limiting in a reaction.
- Say what a limiting reactant is and what a reactant in excess is
- Explain why the limiting reactant decides the amount of product
- Describe how to find the limiting reactant using moles and the ratio