Limiting Reagent: Videos & Practice Problems

In chemical reactions involving multiple reactants, the concept of the limiting reactant is crucial as it is the substance that is entirely consumed first and dictates the maximum amount of product that can be formed, known as the theoretical yield. To identify the limiting reactant, stoichiometry is performed for each reactant to calculate the amount of product each can produce. The reactant yielding the least amount of product is the limiting reactant, while the other is in excess. This process not only determines the theoretical yield but also informs which reactant will be left over after the reaction is complete. Understanding the limiting reactant is essential for predicting reaction outcomes and optimizing the use of materials in chemical processes.

Limiting Reagent Concept

In the study of stoichiometry, understanding the concept of the limiting reagent, also known as the limiting reactant, is crucial. The limiting reagent is the reactant that is entirely consumed during a chemical reaction, thereby determining the maximum amount of product that can be formed. This maximum amount is referred to as the theoretical yield, which represents the ideal scenario where 100% of the reactants convert into products.

When multiple reactants are present in a chemical equation, identifying the limiting reagent becomes essential. The other reactants that remain after the reaction is complete are termed excess reagents. To ascertain which reactant is limiting and which is in excess, one must calculate the potential product yield from each reactant based on their initial amounts. This calculation allows for a clear determination of the limiting reagent, as it will produce the least amount of product.

In summary, the limiting reagent plays a pivotal role in stoichiometry by influencing the theoretical yield of a reaction, while the excess reagent indicates the leftover reactants post-reaction. Understanding these concepts is fundamental for accurately predicting the outcomes of chemical reactions.

Limiting Reagent in Reactions

In stoichiometry, we often work with a single reactant to determine the amount of product formed in a chemical reaction. However, when dealing with multiple reactants, the process becomes more complex. Each reactant will have a specified amount, typically measured in grams, which must be converted to moles using the molar mass. This conversion is essential because stoichiometric calculations rely on the balanced chemical equation, where coefficients indicate the ratio of moles of reactants and products.For each reactant, the steps involve converting grams to moles, using the coefficients from the balanced equation to find the moles of the desired product, and then converting those moles back into the desired units, such as grams, molecules, or ions. This dual approach allows for the calculation of the amount of product that can be formed from each reactant.Through these calculations, it becomes possible to identify the limiting reagent, which is the reactant that will be completely consumed first, thus determining the maximum amount of product that can be produced. The other reactant is referred to as the excess reagent. The theoretical yield, which is the maximum amount of product expected from the reaction, can be calculated based on the limiting reagent.This methodical approach ensures that all reactants are accounted for, providing a comprehensive understanding of the reaction's dynamics and the quantities involved. By practicing these calculations, students can gain confidence in their ability to analyze chemical reactions involving multiple reactants.

Limiting Reagent Example

In a chemical reaction involving Zinc Sulfide (ZnS) and Hydrochloric Acid (HCl), the products formed are Zinc Chloride (ZnCl₂) and Hydrogen Sulfide (H₂S) gas. The balanced chemical equation for this reaction is:

ZnS + 2 HCl → ZnCl₂ + H₂S

To determine the mass of Hydrogen Sulfide produced from 12.11 grams of Zinc Sulfide and 15.92 grams of Hydrochloric Acid, we first convert the masses of the reactants into moles. The molar mass of Zinc Sulfide is calculated by adding the atomic masses of Zinc (65.39 g/mol) and Sulfur (32.07 g/mol), resulting in a total of 97.46 g/mol. Thus, the number of moles of Zinc Sulfide is:

\[ \text{Moles of ZnS} = \frac{12.11 \text{ g}}{97.46 \text{ g/mol}} \approx 0.124 \text{ moles} \]

Next, we calculate the molar mass of Hydrochloric Acid, which is the sum of the atomic masses of Hydrogen (1.008 g/mol) and Chlorine (35.45 g/mol), yielding 36.458 g/mol. The number of moles of Hydrochloric Acid is then:

\[ \text{Moles of HCl} = \frac{15.92 \text{ g}}{36.458 \text{ g/mol}} \approx 0.436 \text{ moles} \]

Using the stoichiometry from the balanced equation, we find the moles of Hydrogen Sulfide produced. According to the equation, 1 mole of ZnS reacts with 2 moles of HCl to produce 1 mole of H₂S. Therefore, we need to determine which reactant is the limiting reagent. The moles of HCl required for the available moles of ZnS can be calculated as:

\[ \text{Required moles of HCl} = 0.124 \text{ moles ZnS} \times 2 = 0.248 \text{ moles HCl} \]

Since we have 0.436 moles of HCl available, Zinc Sulfide is the limiting reagent. Now, we can calculate the moles of Hydrogen Sulfide produced from the moles of Zinc Sulfide:

\[ \text{Moles of H}_2\text{S} = 0.124 \text{ moles ZnS} \times 1 = 0.124 \text{ moles H}_2\text{S} \]

Finally, to find the mass of Hydrogen Sulfide, we multiply the moles of H₂S by its molar mass (34.086 g/mol):

\[ \text{Mass of H}_2\text{S} = 0.124 \text{ moles} \times 34.086 \text{ g/mol} \approx 4.23 \text{ g} \]

Thus, the theoretical yield of Hydrogen Sulfide produced in this reaction is 4.235 grams. This indicates that Zinc Sulfide is the limiting reagent, while Hydrochloric Acid is in excess.

The following reaction shows the mineral ilmenite, FeTiO₃, being reacted with chlorine gas and sand in order to extract titanium (IV) chloride. 
FeTiO₃ (s)  +  3 Cl₂ (g)  +  3 C (s) → 3 CO (g)  +  FeCl₂ (s)  +  TiCl₄ (g) 
Assuming a 100% yield, how many grams of titanium (IV) chloride can be extracted when reacting 18.0 g ilmenite, 30.0 g Cl₂ and 40.0 g C?

• A. 211 g
• B. 26.8 g
• C. 22.5 g
• D. 92.9 g

Answer: (C)

Lithium solid reacts with oxygen gas to create lithium oxide solid.
4 Li (s)  +  O₂ (g)  →  2 Li₂O (s)
If 131 g of Li are allowed to react with 215 g O₂, how many kilograms of the excess reactant would remain?

• A. 4.00 kg
• B. 0.0640 kg
• C. 9.44 kg
• D. 0.0912 kg

Answer: (B)

Here’s what students ask on this topic:

How can one find the limiting reagent?

To find the limiting reagent in a chemical reaction, follow these steps:

1. Write a balanced chemical equation for the reaction.
2. Calculate the molar mass of each reactant.
3. Determine the moles of each reactant using the mass given and their respective molar mass.
4. Use the stoichiometry of the balanced equation to find the mole ratio between the reactants and the products.
5. Calculate the theoretical yield of product from each reactant separately, treating each one as if it were completely consumed. This is done by using the mole ratio from the balanced equation to convert moles of reactant to moles of product.
6. The reactant that produces the least amount of product is the limiting reagent because it will be consumed first, preventing more product from forming.
7. The limiting reagent determines the maximum amount of product that can be formed from the given quantities of reactants.

Remember, once the limiting reagent is used up, the reaction cannot proceed further, even if other reactants are still present.

What is a limiting reagent?

A limiting reagent in chemistry is the reactant that is completely consumed first in a chemical reaction, thus determining the maximum amount of product that can be formed. Think of it as an ingredient in a recipe that runs out first, limiting the amount of the final dish you can make. In a chemical reaction, once the limiting reagent is used up, the reaction stops, even if other reactants are still present in excess. To identify the limiting reagent, you compare the mole ratio of the amount of reactants used to the stoichiometry of the balanced chemical equation. The reactant that provides the smallest yield of product, based on stoichiometry, is the limiting reagent. This concept is crucial in practical chemistry applications, as it helps predict the amounts of products formed and informs the efficient use of reactants.

How do you calculate theoretical yield from the limiting reagent?

To calculate the theoretical yield from the limiting reagent, you need to follow these steps:

1. Identify the limiting reagent: This is the reactant that will be completely consumed first in the chemical reaction and thus limits the amount of product formed.
2. Calculate the moles of the limiting reagent: Use the mass of the limiting reagent and its molar mass to find out how many moles are available for the reaction.
3. Use the stoichiometry of the balanced chemical equation: The balanced equation provides the ratio of moles of reactants to moles of products. Use this ratio to determine how many moles of product can be formed from the moles of the limiting reagent.
4. Calculate the theoretical yield in grams: Multiply the moles of product by its molar mass to convert the theoretical yield to grams.

Remember, the theoretical yield is the maximum amount of product that can be produced in a chemical reaction if everything goes perfectly and no product is lost in the process. It's a calculation based on stoichiometry and doesn't account for practical losses or inefficiencies.