Limiting Reagents and Limiting Reactant Calculations

Limiting Reagents

Introduction to Limiting Reagents

In chemical reactions, the reactants may not always be present in the exact stoichiometric proportions required by the balanced equation. The limiting reagent (or limiting reactant) is the substance that is completely consumed first, thus determining the maximum amount of product that can be formed.

• Excess Reagent: The reactant that remains after the reaction is complete.

• Limiting Reagent: The reactant that is entirely used up during the reaction.

Example: If you have 3 molecules of hydrogen and 1 molecule of oxygen, and the reaction requires a 2:1 ratio, oxygen will be the limiting reagent.

Visual Representation of Limiting Reagents

Diagrams often show reactant particles combining, with the limiting reagent being the one that runs out first, leaving excess of the other reactant.

Limiting Reactant

Definition and Identification

The limiting reactant is the reactant that is completely consumed in a chemical reaction, thus limiting the amount of product formed. It can be identified using the mole ratio from the balanced chemical equation.

• Balanced Equation Example: $\mathrm{Fe} + \mathrm{S} \rightarrow \mathrm{FeS}$

• Mole Ratio: 1:1 (Fe:S)

• Identification: Compare the available moles of each reactant to the required stoichiometric ratio.

Example: If you have 3 moles of Fe and 1 mole of S, S is the limiting reactant because it will be used up first.

Identifying the Limiting Reagent

Methods for Determining the Limiting Reagent

To identify the limiting reagent, compare the amount of each reactant available to the amount required by the stoichiometric ratio in the balanced equation.

• Method 1: Calculate the number of moles of each reactant and compare to the stoichiometric ratio.

• Method 2: Calculate the amount of one reactant required to react completely with the other. If there is insufficient quantity, that reactant is limiting.

Example: When 9.87 moles of aluminum are added to hydrochloric acid, calculate the moles of HCl required for complete reaction. If there is not enough HCl, it is the limiting reagent.

Calculating Mass with Limiting Reagent

Mass Calculations in Limiting Reagent Problems

To determine the mass of product formed, use the amount of the limiting reagent and the stoichiometry of the reaction.

• Step 1: Identify the limiting reagent.

• Step 2: Use the balanced equation to determine the mole ratio between the limiting reagent and the product.

• Step 3: Calculate the mass of product formed using molar masses.

• Example: Given 100 g of sulfur and 100 g of iron, calculate the mass of FeS produced:

  • Balanced equation: $\mathrm{Fe} + \mathrm{S} \rightarrow \mathrm{FeS}$

• Calculate moles of Fe and S:

• Moles of Fe: $\frac{100\,\mathrm{g}}{55.85\,\mathrm{g/mol}} \approx 1.79\,\mathrm{mol}$

• Moles of S: $\frac{100\,\mathrm{g}}{32.07\,\mathrm{g/mol}} \approx 3.12\,\mathrm{mol}$

• Fe is the limiting reagent (less moles available).

• Mass of FeS produced: $1.79\,\mathrm{mol} \times 87.91\,\mathrm{g/mol} \approx 157\,\mathrm{g}$

Summary Table: Limiting Reagent Identification

Step	Description	Example
1	Write balanced chemical equation	$\mathrm{Fe} + \mathrm{S} \rightarrow \mathrm{FeS}$
2	Calculate moles of each reactant	Fe: 1.79 mol, S: 3.12 mol
3	Compare mole ratio to stoichiometry	1:1 ratio; Fe is limiting
4	Calculate mass of product from limiting reagent	FeS: 157 g

Additional info:

• Limiting reagent problems are fundamental in both general and organic chemistry, as they determine the theoretical yield of a reaction.

• Understanding limiting reagents is essential for efficient use of chemicals and for predicting reaction outcomes.