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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Related Articles:
- O Level Chemistry – Mole Concepts / Mole Calculations
- O Levels Chemistry Questions – Mole Calculations / Mole Concepts
- O Level Chemistry: Mole Concepts / Qualitative Analysis
- O-Level Chemistry: 5 Rules to Assign Oxidation States (Redox Reactions)
- O Level Chemistry – Rate of Reaction Mini Series Part 1
Thanks for encouragement. Keep learning!
Check out the archive blogposts on discussions based on Mole Concepts and Mole Calculations.
Rgds,
Sean Chua
it would have been more useful if u would have solved a tougher question concerning moles rather than the one answered above.
it has beinb very useful to me