Overview
This lecture focuses on the concept of the limiting reagent in chemical reactions, its definition, importance for exams, and provides step-by-step solutions to typical limiting reagent problems.
Limiting Reagent Concept
- The limiting reagent is the reactant that limits or stops the progress of a chemical reaction.
- In any reaction, the limiting reagent determines the maximum amount of product formed.
- Excess reagent refers to the reactant present in more than required quantity, which does not limit the reaction.
- Real-life analogy: Like running out of flour before water when making chapatis—flour is the limiting factor.
How to Identify the Limiting Reagent
- Write and balance the chemical equation for the reaction.
- Convert the mass of each reactant to moles using: moles = mass / molar mass.
- For each reactant, calculate: (number of moles) / (stoichiometric coefficient from the balanced equation).
- The reactant with the lowest value from the above calculation is the limiting reagent.
Example 1: Nitrogen and Hydrogen Reaction
- Reaction: N₂ + 3H₂ → 2NH₃, with 5g N₂ and 10g H₂.
- Moles of N₂: 5/28 = 0.18, moles of H₂: 10/2 = 5.
- Calculation: N₂ (0.18/1 = 0.18), H₂ (5/3 ≈ 1.67).
- Lower value is 0.18 (N₂), so N₂ is the limiting reagent.
- Amount of ammonia formed calculated using the limiting reagent.
Example 2: Magnesium and Oxygen Reaction
- Reaction: 2Mg + O₂ → 2MgO, with 1g Mg and 0.5g O₂.
- Moles of Mg: 1/24 ≈ 0.042, moles of O₂: 0.5/32 ≈ 0.016.
- Calculation: Mg (0.042/2 = 0.021), O₂ (0.016/1 = 0.016).
- Lower value is 0.016 (O₂), so O₂ is the limiting reagent.
- Amount of MgO formed is based on limiting reagent using stoichiometry.
Calculating Product Formed
- Always use the amount of limiting reagent to calculate product.
- Use the stoichiometric ratios from the balanced chemical equation.
- For mass, multiply moles of product formed by its molar mass.
Key Terms & Definitions
- Limiting Reagent — The reactant that is completely consumed first, limiting the amount of product formed.
- Excess Reagent — The reactant that remains after the reaction is complete.
- Stoichiometric Coefficient — The number before a compound or element in a balanced chemical equation.
- Mole — A unit representing 6.022 × 10²³ particles of a substance.
Action Items / Next Steps
- Practice additional limiting reagent problems.
- Review the concept of concentrations in solutions for the next lecture.