Study Notes on Limiting Reactants, Reaction Yield, and Types of Chemical Reactions

Limiting Reactants

Overview

  • Limiting Reactants play a crucial role in chemical reactions, determining the quantity of product formed based on the reactants available.

Topics Covered

  • Determining the Limiting Reactant

  • Reaction Yield

  • Types of Chemical Reactions

Understanding Limiting Reactants

Concept of Limiting Reactants

  • Definition: The limiting reactant is the substance that gets used up first in a chemical reaction. It limits the amount of product that can be formed because once it is exhausted, the reaction stops.

  • Excess Reactants: These are present in quantities that exceed what is necessary to fully react with the limiting reactant. Some of this reactant will remain unreacted after the reaction is complete.

Example of Limiting Reactants in a Practical Scenario

  • Sandwich Example:

    • Ingredients: 2 pieces of bread and 1 piece of cheese can make 1 sandwich.

    • In another scenario, if you have 3 pieces of bread and 4 pieces of cheese, the limiting reactant (which runs out first) will determine how many sandwiches can be made.

    • Moral: The total quantity of product (sandwiches) cannot be determined by considering the excess reactants alone.

Determining the Limiting Reactant

Methodology

  1. Identify the balanced chemical equation that represents the reaction in question.

  2. Convert to moles for comparison.

  3. Assess how many moles of product each reactant can produce, based on stoichiometry.

  4. The reactant that produces the lesser amount of product is the limiting reactant.

Urea Synthesis Example

  • Chemical Reaction: 2NH3 (g) + CO2 (g) \rightarrow (NH2)2CO (aq) + H_2O (l)

    • Given 637.2 g of NH₃ and 1142 g of CO₂

Steps to Determine the Limiting Reagent

  • Convert grams to moles:

    • n = \frac{mass}{molar\ mass}

    • For NH₃:
      637.2\ g \times \frac{1\ mol}{17.03\ g} = 37.41\ mol\ NH_3

    • For CO₂:
      1142\ g \times \frac{1\ mol}{44.01\ g} = 25.95\ mol\ CO_2

    • Determine moles of product (Urea) produced by both:

    • From NH₃:
      37.41\ mol\ NH3 \times \frac{1\ mol\ (NH2)2CO}{2\ mol\ NH3} = 18.71\ mol\ (NH2)2CO

    • From CO₂:
      25.95\ mol\ CO2 \times \frac{1\ mol\ (NH2)2CO}{1\ mol\ CO2} = 25.95\ mol\ (NH2)2CO

    • Conclusively, NH₃ is the limiting reagent because it produces fewer moles of urea.

Remaining Amounts

  • Calculate the excess of CO₂:

    • 1\ mol\ CO2 : 1\ mol\ (NH2)_2CO

    • Mass of CO₂ reacted to produce 18.71 moles of urea:
      18.71\ mol\ (NH2)2CO \times \frac{1\ mol\ CO2}{1\ mol\ (NH2)2CO} \times 44.01\ g/mol = 823.4\ g\ CO2

    • Remaining mass of CO₂:
      1142\ g - 823.4\ g = 319\ g

Reaction Yield

Theoretical vs Actual Yield

  • Theoretical Yield: This is the maximum amount of product that could be formed from the limiting reactant based on stoichiometric calculations. It assumes that every molecule of the limiting reactant reacts completely.

  • Actual Yield: This refers to the mass of product that is actually obtained from the reaction. The actual yield is often less than the theoretical yield due to various factors including incomplete reactions and side reactions.

Percent Yield Calculation

  • Formula:
    \text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100\%

Sample Problem: Aspirin Production

  • Reaction: Acetylsalicylic acid (aspirin) production from salicylic acid and acetic anhydride.

    • Calculate if 105.6 g of aspirin are produced when theoretical yield is 136.7 g:

    • Percent Yield:
      \text{Percent Yield} = \left( \frac{105.6\ g}{136.7\ g} \right) \times 100\% = 77.25\%

Types of Chemical Reactions

Definitions

  1. Combination (Synthesis) Reaction: Involves two or more reactants combining to form a single product.

  2. Decomposition Reaction: A single reactant breaks down to form two or more products.

  3. Combustion Reaction: A substance combusts or burns in the presence of oxygen, usually producing heat and light.

Examples to Identify Reaction Types

  • Given multiple reaction equations, classify them as:

    • a) Combination

    • b) Combustion

    • c) Decomposition

Conclusion

  • Understanding limiting reactants and production yields is crucial for effectively predicting the outcomes of chemical reactions and optimizing reactant usage in practical applications.