Limiting Reactant
The reactant that runs out first limits the product.
About this topic
The limiting reactant is the one that is completely used up first and so limits the amount of product. The other reactant is in excess. To find it, convert the amounts of each reactant to moles and divide by their coefficients; the smaller result belongs to the limiting reactant, or compare how much product each would make.
Think of making sandwiches with 10 slices of bread and 3 slices of cheese. Each sandwich needs 2 bread and 1 cheese. The bread could make 5 sandwiches, but the cheese only 3, so cheese is limiting and 4 slices of bread are left over. In chemistry the process is the same but uses moles and coefficients. The amount of product is always calculated from the limiting reactant.
Worked example: 4.0 mol of H2 and 3.0 mol of O2 react in 2H2 + O2 → 2H2O. H2 allows 4.0 / 2 = 2.0 units and O2 allows 3.0 / 1 = 3.0 units, so H2 is limiting and 4.0 mol of water forms, leaving 1.0 mol O2.
Industry often adds an excess of the cheaper reactant to make sure the expensive one is fully used. Rocket engineers balance fuel and oxidiser. Misconceptions: the limiting reactant is not the one with the smaller mass or the smaller number of moles; you must account for the ratio in the equation.
Key ideas
- Compare mole ratios
- Excess reactant remains
- Theoretical yield from the limiting one
Quick check
Use cases: where Limiting Reactant helps
- Homework: review the key idea behind Limiting Reactant before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Limiting Reactant without notes.
- Visual learning: play with the interactive lab to see how Limiting Reactant changes when you adjust its inputs.
- Teaching a friend: explain Limiting Reactant in your own words, then compare your version with the key ideas.
Chemistry explains medicine, food, cleaning products, fuels and building materials.
Frequently asked questions
What is Limiting Reactant?
The reactant that runs out first limits the product.
What will I learn in Limiting Reactant?
Compare mole ratios. Excess reactant remains. Theoretical yield from the limiting one.
Is Limiting Reactant part of Chemistry?
Yes. It is one of 150 Chemistry topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Limiting Reactant lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Limiting Reactant?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.