How do we identify the reactant that runs out first?

Support map

Model diagram

Use only paper, a calculator and supplied data. Do not carry out reactions, generate gases, heat substances or mix household chemicals.

Situation and question

A paper model for 2H₂ + O₂ → 2H₂O supplies 3 mol of H₂ and 2 mol of O₂. Both numbers are positive: will everything run out together? Group cards in a 2:1 ratio and see which supply ends first. Do not use actual gases.

New terms, fully explained

The limiting reactant sets the maximum product amount in this complete-conversion model. An excess is the portion of another reactant left afterward. For one specified reaction, compare n/ν for each reactant, where ν is its positive coefficient. The smallest ratio sets the limit. Equal ratios describe a stoichiometric mixture.

Predict first

Can the limiting reactant be selected from the smaller mass or smaller mole number alone? For 3 mol of H₂ and 2 mol of O₂, compare 3/2 with 2/1, not simply 3 with 2. Explain why division by the coefficient is needed.

Investigate or analyse given data

Write initial amount, consumption and remainder on separate rows. H₂ has coefficient 2 and O₂ coefficient 1. The ratios 1.5 mol and 2 mol normalize each supply to the same reaction basis. The number 1.5 is not automatically the amount of every product; its coefficient must also be used.

Worked example

The minimum n/ν is 1.5 mol, so H₂ limits the reaction. It consumes 3 mol of H₂ and 1.5 mol of O₂ and forms 3 mol of H₂O. Oxygen remaining is 2−1.5=0.5 mol. That remainder is not added to the amount of water.

A common mistake

Mistake: choosing O₂ because 2 is smaller than 3, or converting all the initial O₂ into product. Both reactant supplies constrain the outcome. A negative remainder signals a calculation or model error. We assume one reaction and complete conversion of the limiting reactant.

Transfer to a new case

Transfer: the same model now supplies 4 mol of H₂ and 1 mol of O₂. Find the limiting reactant, H₂O formed and H₂ left. Then check 2 mol of H₂ with 1 mol of O₂: is any excess left in the ideal model?

Answer and recall

Check: O₂ limits the first case; 2 mol of H₂O forms and 2 mol of H₂ remains. The second mixture is stoichiometric, with no excess. Check atoms in the remaining reactants as well as in the products: leftovers are part of the final system.

Exercise

For 2H₂ + O₂ → 2H₂O, we have 1 mol H₂ and 1 mol O₂. Which limits? How much O₂ remains?

Answers

When to revisit

Close the page and, after 10 minutes, reconstruct the map, define the terms in your own words and solve the new case. Repeat without hints after a day and a week. If an error persists, return to the worked example and write down the missing step.

Sources and limits

• OpenStax: limiting reactants and yields

How can we calculate product mass from a reaction equation?

Why can the collected mass differ from the calculated mass?