O Level Chemistry: Mole Calculations – Limiting & Excess Reactants

Concepts of Limiting & Excess Reactants are so commonly tested in school exams as well as ‘O’ Levels Chemistry examinations. However, many students seemed to have very weak foundation of the essential concepts surrounding it.

Before we even visit any question, it is important to understand the definition of both reactants.

Limiting Reactant:

- The reactant that is completely used up in a chemical reaction

- It determines or limits the amount of products formed in a reaction

Excess Reactant:

- The reactant that are not used up in a chemical reaction

- There is always an excess at the end of the reaction, stays unreacted

Example:

Reaction of hydrogen gas with chlorine according to the equation below:

H₂ (g) + Cl₂ (g) –> 2HCl (g)

We can see that 1 mole of H₂ reacts with 1 mole of Cl₂ to form 2 moles of HCl

Scenario A: When 1 mole of H₂ added to 1 mole of Cl₂

Results: 2 moles of HCl are produced and all H₂ & Cl₂ are used up.

Conclusion: No Limiting & Excess Reactants.

Scenario B: When 2 moles of H₂ added to 1 mole of Cl₂

Results: 2 moles of HCl are produced and 1 mole of H₂ stays unreacted.

Conclusion: Cl₂ is the Limiting Reactant & H₂ is the Excess Reactant

Scenario C: When 1 mole of H₂ added to 2 moles of Cl₂

Results: 2 moles of HCl are produced and 1 mole of Cl₂ stays unreacted.

Conclusion: Cl₂ is the Excess Reactant & H₂ is the Limiting Reactant

Impt Notes:

1. Must always have a balanced chemical equations first
2. Compare ”No. of Moles” between the 2 reactants to determine which is Limiting / Excess
3. If all reactants & products are in gaseous state, you can compare by volume directly (if given)

Let me know if the above explanation is useful to you. I would love to hear from you.

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