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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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