# Limiting reactant

The reactant that is used up first, which stops the reaction and limits how much product forms. The other reactant is in excess.

Chemistry (school) · Part 05: Moles and calculations · Card 6 of 9 · Last updated 2026-10-19

## Analogy

If you have 10 slices of bread and 3 slices of cheese, you can only make 3 cheese sandwiches. The cheese is limiting.

## In practice

Chemists add extra of a cheap reactant to make sure the expensive one is all used up.

## Practice

### Flashcards

- **What is a limiting reactant?** The reactant used up first, which stops the reaction.

### Interview questions

**Explain what a limiting reactant is and why it matters to a manufacturer.**

- It is used up first
- It limits the amount of product
- The other reactant is in excess
- Using excess of the cheaper reactant makes sure the costly one is used completely

## Questions people ask

### How do you find the limiting reactant?

Convert each reactant to moles. Divide by its coefficient in the balanced equation. The smaller result is the limiting reactant. For 2H2 + O2 -> 2H2O with 4 mol of hydrogen and 3 mol of oxygen, hydrogen gives 2 and oxygen gives 3, so hydrogen limits.

### What does excess mean?

A reactant in excess is present in a larger amount than needed. Some will be left over when the reaction ends. In a reaction between magnesium and acid, if the magnesium is in excess, bubbling stops when the acid is used up and magnesium remains in the flask.

## Related concepts

- [Percentage yield](https://studyvisually.com/learn/science/chemistry-school/chemistry-school-part-5/chemistry-school-part-5-concept-7)

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