Stoichiometry and Limiting Reagents (exam 3 for chem)

Stoichiometry and Limiting Reagents

Stoichiometric Ratio

  • Definition: The stoichiometric ratio refers to the ratio between the reactants and products in a chemical reaction, defined by the coefficients in a balanced chemical equation.

  • Key Concept: Coefficients (big numbers in front of chemical formulas) are crucial for determining this ratio; subscripts (numbers within chemical formulas) are not involved in stoichiometric calculations.

Converting Moles to Grams

  • Example Calculation: For ClF3:

    • Moles of ClF3 can be calculated after canceling out moles of F2.

    • To convert from moles to grams, calculate the molar mass of ClF3, resulting in 0.811 grams of ClF3.

Excess vs Limiting Reagents

  • Excess Reagent: The reactant that is not completely consumed during a reaction.

    • Example given: Cl2 is in excess.

  • Limiting Reagent: The reactant that is completely consumed, limiting the amount of product formed.

    • Example given: P4 is the limiting reagent in the reaction with excess Cl2.

Balancing Chemical Equations

  • Always need to balance your chemical equation before performing stoichiometric calculations.

  • Example:

    • Reaction: P4 + Cl2 ⟶ PCl3

    • Balanced Equation:

    • 1 P4 yields 4 PCl3, needing 6 Cl2 (balanced).

Molar Mass Calculations

  • Calculating the molar masses for reactants and products is essential for stoichiometric calculations.

  • Example Molar Masses:

    • Molar mass of P4 = 123.9 g/mol

    • Molar mass of PCl3 = 137.3 g/mol

Stoichiometric Coefficients and Moles

  • Utilize the balanced coefficients from the reaction to set up mole ratios when converting to/from grams:

    • Stoichiometric ratio (e.g., from P4 to PCl3) is crucial for calculating moles.

Percent Yield

  • Definition: The percent yield indicates how efficient a reaction is based on theoretical and actual yields.

  • Formula:

    • Percent Yield = (Experimental Yield / Theoretical Yield) × 100

  • Example: If theoretical yield is 8 sandwiches (from bread) but only 7 were made, the percent yield would be:

    • Percent Yield = (7/8) × 100 = 87.5%

Practical Application in Chemistry Labs

  • Knowledge of determining limiting and excess reagents is important during laboratory work (

    • Example: reaction of copper and acid).

Example of Making a Sandwich

  • Metaphor: Making sandwiches to understand limiting and excess reagents

    • Ingredients: 2 slices of bread and 3 slices of cheese make 1 sandwich.

    • 16 slices of bread can make 8 sandwiches (as 2 slices are needed per sandwich).

    • 30 slices of cheese can make 10 sandwiches (as 3 slices are needed per sandwich).

    • Limiting agent is whatever can make the least amount of sandwiches (bread in this scenario).

Application - Bath Bomb Reaction

  • Start with a known amount of baking soda to calculate potential products.

  • Theoretical products based on balanced equation (noting stoichiometric ratios) can be determined by multiplying available grams by molar mass ratios.

Percent Yield Calculation with Bath Bombs

  • Example:

    • Theoretical yield: 630.8 grams of sodium carbonate produced, but only 612.5 grams were gathered in practice.

    • Percent yield calculation:

    • Percent Yield = (612.5 g / 630.8 g) × 100 = 97.09%.

Example Problem - Magnesium Oxide Reaction

  • Reaction details: 42.5 grams of magnesium with 33.3 grams of oxygen.

  • Determine limiting reagent and calculate theoretical yield. If actual yield is 61.5 grams, percent yield can be calculated as:

    • Percent Yield = (61.5 g / Theoretical Yield) × 100

Reagent Comparison for Leftovers

  • After determining limiting reagent, calculate how much of the excess reagent remains:

    • If the limiting reagent is fully consumed, calculate the moles of the excess used, and subtract from the initial mass to find leftover grams.

Example Problem - Ammonia and Oxygen Reaction

  • Balanced reaction: NH3 + O2 ⟶ Products.

    • Using initial amounts in grams to determine limiting reagent and calculating leftover excess reagent after the reaction is complete.

ICE Tables for Stoichiometry

  • ICE stands for Initial, Change, End.

  • Useful for visualizing reactants and products.

  • Create a table to organize data for each reactant and product's initial, change in quantity, and final amounts after reaction completion.