Limiting Reagent and Mole-to-Mole Comparisons in Chemical Reactions

Limiting Reagent and Theoretical Yield

Introduction to Limiting Reagent

The limiting reagent is the reactant that is completely consumed in a chemical reaction and determines the maximum amount of product that can be formed. The theoretical yield is the calculated maximum amount of product possible, based on the limiting reagent.

• Theoretical Yield: The maximum amount of product that can be formed from a chemical reaction, assuming complete conversion of the limiting reagent.

• Limiting Reagent: The reactant that runs out first, thus limiting the amount of product formed.

• To identify the limiting reagent, compare the mole ratios of reactants used in the balanced chemical equation.

Mole-to-Mole Comparison

Using Balanced Equations for Calculations

Mole-to-mole comparisons are essential for determining how much of one substance will react with or produce another in a chemical reaction. This process uses the coefficients from the balanced chemical equation.

• Start with the grams of the given reactant.

• Convert grams to moles of the given reactant using molar mass.

• Use the coefficient ratio from the balanced equation to convert moles of the given reactant to moles of the unknown product or reactant.

• If needed, convert moles of the unknown to grams using its molar mass.

Step-by-Step Process

1. Convert the given quantities into moles of the given reactant. If any compound is said to be in excess, just use the moles of the limiting reagent.

2. Do a mole-to-mole comparison to convert moles of the given reactant into moles of the unknown product or reactant.

3. If necessary, convert the moles of unknown into the final desired units (e.g., grams).

Example Calculation

Example: Chromium (III) oxide reacts with hydrogen sulfide (H2S) gas to form chromium (III) sulfide and water:

$\text{Cr}_2\text{O}_3 (s) + 3\text{H}_2\text{S} (g) \rightarrow \text{Cr}_2\text{S}_3 (s) + 3\text{H}_2\text{O} (g)$

• Given: 12.0 g H2S

• Find: Mass of Cr2S3 formed

• Answer: 12.80 g

Practice Problems

Practice 1: Acrylonitrile Synthesis

Acrylonitrile (C3H3N) is produced by the following reaction:

$2\text{C}_3\text{H}_6 (g) + 2\text{NH}_3 (g) + 3\text{O}_2 (g) \rightarrow 2\text{C}_3\text{H}_3\text{N} (g) + 6\text{H}_2\text{O} (g)$

• If 12.0 g C3H6, 10.0 g NH3, and 5.0 g O2 react, what mass of acrylonitrile can be produced, assuming 100% yield?

Practice 2: Aluminum and Iron(III) Oxide Reaction

The reaction between solid aluminum and iron (III) chloride can generate high temperatures and is used in welding:

$2\text{Al} + \text{Fe}_2\text{O}_3 \rightarrow \text{Al}_2\text{O}_3 + 2\text{Fe}$

• If 150 g of Al are reacted with 432 g of Fe2O3, what is the mass of the excess reactant remaining?

Summary Table: Steps for Limiting Reagent Problems

Step	Description
1	Convert all reactant masses to moles
2	Use mole ratios from the balanced equation to determine which reactant is limiting
3	Calculate the amount of product formed from the limiting reagent
4	If required, determine the amount of excess reactant remaining

Additional info: The notes and questions are directly relevant to GOB Chemistry, focusing on stoichiometry, limiting reagents, and yield calculations, which are foundational topics in introductory chemistry courses.