3.7 Limiting Reactant

Introduction to Limiting Reactants

  • Concepts of Stoichiometry: Up until this point, basic problems involved using single reactants or products for calculations. Now, problems will involve multiple starting materials.

  • Limiting Reactants: A limiting reactant runs out first in a chemical reaction, determining the extent of the reaction and how much product can be formed.

Limiting Reactants Explained

  • Definition: If one reactant is depleted before others, it limits the amount of product created.

  • Importance: Understanding limiting reactants is crucial for accurately predicting product yields.

Everyday Example: Hot Dogs and Buns

  • Stoichiometric Ratio: For making hot dogs, the ratio is 1 hot dog (Frank) to 1 bun.

  • Problem Example: Given 5 hot dogs and 4 buns, the maximum number of complete hot dogs made is 4, making buns the limiting reactant.

Chemical Reaction Example: Water Formation

  • Balanced Equation: Hydrogen (H2) + Oxygen (O2) → Water (H2O).

  • Molecular Representation: Oxygen molecules react with hydrogen to form water; the number of waters formed is limited by the availability of reactants.

  • Limiting Reactant Determination: In the example, H2 runs out first, making it the limiting reactant.

Methodology for Determining Limiting Reactants

Method 1: Compare Required vs. Available Moles

  • Calculate moles of each reactant available.

  • Determine moles needed based on the stoichiometry of the balanced equation.

  • Compare available moles to needed moles to find the limiting reactant.

Method 2: Calculating Product from Each Reactant

  • Convert amounts of each reactant to the amount of product they would produce if reacted completely.

  • The reactant producing the least amount of product is the limiting reactant.

Method 3: Ratio of Moles to Coefficients

  • Convert to moles and divide by their respective coefficients in the balanced equation.

  • The smallest ratio indicates the limiting reactant.

Example Calculation: 5g H2 and 10g O2 Reaction

  • Convert grams to moles for both H2 and O2.

  • Determine necessary moles for complete reaction using stoichiometry.

  • Conclude which reactant runs out first (limiting reactant) and calculate the theoretical yield of water.

Excess Reactants

  • Definition: The reactant that remains after the reaction has gone to completion.

  • To find the excess reactant quantity, subtract the amount used from the initial amount available.

Theoretical Yield vs. Actual Yield

  • Theoretical Yield: Maximum product amount possible from the complete reaction of reactants.

  • Actual Yield: The amount actually produced during the experiment.

  • Percent Yield Calculation: Percent yield = (Actual Yield / Theoretical Yield) × 100.

Example: Theoretical and Percent Yields in Practice

  1. Reaction Situation: Evaluate reaction limits and calculate theoretical yield given 10g of oxygen reacts with hydrogen.

  2. Calculation Steps: Based on limiting reactant findings, theoretical yield of water can be determined.

  3. Percent Yield Example: If the actual yield is 8.56g of water, calculate percent yield based on the theoretical yield from earlier calculations.

Final Concept Check Example

  • Review Statements: Validate understanding of the reactions, theoretical yields, and limiting reactants. Utilize previous examples to confirm knowledge.