Limiting Reactant

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Additional Worked Out Examples/ Practice

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  • Separation techniques: Selected and explaining limitation of appropriate separation

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Core Concept

In a chemical reaction, the limiting reactant (or limiting reagent) is the substance that is completely used up first, stopping the reaction from continuing and thus determining the maximum amount of product that can be formed. The excess reactant is the reactant that remains after the limiting reactant is fully consumed.

Practice Tips

  • Always Balance the Equation First: Mole ratios are essential for identifying the limiting reactant.

  • Convert to Moles First: Always convert masses to moles before using mole ratios.

  • Compare Based on Products: Calculate the amount of product each reactant can form, and the smaller amount indicates the limiting reactant.

  • Double-Check Units: Be mindful of units (grams, moles) to avoid calculation errors.

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Key Concepts

  1. Limiting Reactant:

    • The reactant that is entirely consumed in a reaction, limiting the formation of products.

    • Determines the maximum amount of product that can be formed.

  2. Excess Reactant:

    • The reactant that remains after the limiting reactant is used up.

    • Does not affect the amount of product formed but can be calculated to determine the leftover quantity.

  3. Significance in Stoichiometry:

    • Identifying the limiting reactant allows you to accurately calculate the theoretical yield of the reaction, which is the maximum amount of product possible based on the limiting reactant.

Steps to Identify the Limiting Reactant

  1. Write and Balance the Chemical Equation:

    • Ensure the equation is balanced with the correct coefficients.

  2. Convert Masses to Moles (if necessary):

    • Convert the given amounts of reactants (often in grams) to moles using their molar masses.

  3. Use Mole Ratios to Compare Reactants:

    • Use the mole ratios from the balanced equation to determine the amount of product each reactant could theoretically produce.

    • The reactant that produces the smallest amount of product is the limiting reactant.

  4. Calculate the Amount of Product:

    • Use the limiting reactant to determine the theoretical yield of the reaction.

  5. Calculate Excess Reactant (if needed):

    • If required, calculate how much of the excess reactant remains after the reaction.

Example Problem: Identifying the Limiting Reactant

Problem: Given the reaction below, if 5.0 g of H₂ reacts with 20.0 g of O₂, identify the limiting reactant and calculate the amount of water (H₂O) produced.

2H₂ + O₂ → 2H₂O

Solution:

1. Write and Balance the Equation: The equation is already balanced.

2. Convert Masses to Moles:

  • Molar mass of H₂ = 2.02 g/mol Moles of H₂ = 5.0 g / 2.02 g/mol = 2.48 mol

  • Molar mass of O₂ = 32.00 g/mol Moles of O₂ = 20.0 g / 32.00 g/mol = 0.625 mol

3. Use Mole Ratios to Compare Reactants:

From the balanced equation, 2 mol H₂ reacts with 1 mol O₂.

To react with 0.625 mol O₂, we would need: 0.625 mol O₂ × (2 mol H₂ / 1 mol O₂) = 1.25 mol H₂

We have 2.48 mol H₂, which is more than enough to react with the available 0.625 mol O₂. Therefore, O₂ is the limiting reactant.

4. Calculate the Amount of Product:

Using O₂ as the limiting reactant: 0.625 mol O₂ × (2 mol H₂O / 1 mol O₂) = 1.25 mol H₂O

Molar mass of H₂O = 18.02 g/mol: 1.25 mol H₂O × 18.02 g/mol = 22.53 g H₂O

Answer: The limiting reactant is O₂, and the maximum amount of H₂O produced is 22.53 g.

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