7 - LR and Yield

Theoretical Yield vs. Actual Yield

  • Theoretical Yield: Best case scenario when a reaction goes to completion with no losses.

    • Based on the law of conservation of mass.

    • Example: In a reaction, maximum products expected when all reactants are consumed.

  • Actual Yield: Amount of product obtained from a reaction.

    • Often lower than theoretical yield due to loss in procedures or contamination.

    • Can be higher if impurities are present in calculations.

  • Percent Yield: Measurement of the efficiency of a reaction.

    • Formula: Percent Yield = (Actual Yield / Theoretical Yield) × 100%

Calculating Theoretical Yield

  • Example Problem: Iodine and Chlorine Reaction

    • Reaction: I2 + Cl2 -> ICl3

    • Given: 2 grams of I2 and 3 grams of ICl3 produced.

    • Determine the theoretical yield from 2 grams of I2.

    • Convert grams to moles and apply stoichiometry to find the mass of ICl3.

    • Theoretical yield: 3.68 grams of ICl3.

Practical Problem Solving

  1. Calculate the actual yield (given) and theoretical yield (calculated).

  2. To find percent yield: (3 grams / 3.68 grams) × 100% yields 81.5%.

  3. Assess how much Cl2 was consumed using the stoichiometric relationship with I2.

    • Result: 1.68 grams of Cl2 used in the reaction.

Limiting Reactant Concept

  • Limiting Reactant: Reactant that gets fully consumed in a reaction, determining the extent of the reaction.

  • Excess Reactant: Reactant that remains when a reaction ceases.

  • Example: If you are manufacturing cars, having fewer car bodies than tires limits the production of cars.

Finding Limiting Reactants

  • Two methods for finding limiting reactants:

    1. Calculate how much of the other reactant is needed based on one reactant amount.

    2. Calculate product formation potential from each reactant.

  • Example 1 (Ammonia and Oxygen Reaction):

    • Given: 2 grams of NH3 and 4 grams of O2.

    • Determine how much O2 is needed for 2 grams of NH3 (4.70 grams required).

    • Conclusion: O2 is the limiting reactant as only 4 grams are present.

Percent Yield Application

  • Percent yield calculation requires theoretical yield, which is derived from stoichiometric relationships and can affirm efficiency in laboratory settings.

  • Example: If a reaction produces 32 grams of O2 with a percent yield of 65%, calculate the grams of KClO3 needed using theoretical yield derived from stoichiometry.

Responding to Exam Problems

  • Familiarity with reaction equations, stoichiometry, and conversions between grams and moles is critical.

  • Ensure all relevant reactions are balanced before calculations.

  • Practice calculating both limiting reagents and theoretical yields for variations in problems.