Problem 74 - Chemistry Matter and Change

Question

Explain why the statement, “The limiting reactant is the reactant with the lowest mass” is incorrect.

Step-by-Step Solution

Answer

The statement "The limiting reactant is the reactant with the lowest mass" is incorrect because the limiting reactant depends on the molar ratios in accordance with the stoichiometric coefficients of the balanced chemical equation, not the mass of reactants. For example, in a reaction between 4 moles of hydrogen gas (8 g) and 3 moles of oxygen gas (96 g) to form water, hydrogen gas is the limiting reactant despite having a lower mass than oxygen gas.

Step 1: Understand the concept of limiting reactant

The limiting reactant is the reactant that is used up first in a chemical reaction and thus determines the amount of product that can be formed. It is important to note that the limiting reactant is not necessarily the reactant with the lowest mass.

Step 2: Provide an example with two reactants

Let's consider the following balanced chemical reaction: \[ 2 H_{2} + O_{2} \rightarrow 2 H_{2}O \] The reaction states that two moles of hydrogen gas (H₂) react with one mole of oxygen gas (O₂) to produce two moles of water (H₂O).

Step 3: Prepare different amounts of reactants in terms of moles

Suppose we have 4 moles of hydrogen gas (H₂) and 3 moles of oxygen gas (O₂). Molar mass of hydrogen gas (H₂) = 2 g/mol Molar mass of oxygen gas (O₂) = 32 g/mol Now, calculate the mass of each reactant: Mass of 4 moles of H₂ = 4 moles * 2 g/mol = 8 g Mass of 3 moles of O₂ = 3 moles * 32 g/mol = 96 g

Step 4: Identify the limiting reactant

To identify the limiting reactant, we will compare the molar ratios of the reactants with the stoichiometry of the balanced chemical equation. For hydrogen gas (H₂): (4 moles H₂) / 2 = 2 For oxygen gas (O₂): (3 moles O₂) / 1 = 3 Since the molar ratio for hydrogen gas (H₂) is smaller than that of oxygen gas (O₂), hydrogen gas is the limiting reactant.

Step 5: Conclusion

In our example, hydrogen gas (H₂) is the limiting reactant with a mass of 8 g, while oxygen gas (O₂) has a mass of 96 g. This clearly proves that the statement “The limiting reactant is the reactant with the lowest mass” is incorrect. The limiting reactant is not necessarily related to the mass of the reactant but depends on the molar ratios in accordance with the stoichiometric coefficients of the balanced chemical equation.

Key Concepts

Chemical Stoichiometry

Chemical stoichiometry involves the calculation of reactants and products in a chemical reaction. It is a fundamental concept in chemistry that helps us understand the proportions in which substances react. When analyzing a chemical reaction, stoichiometry allows us to predict how much of each product will form from given quantities of reactants. This is crucial for determining the limiting reactant, which is the substance that gets completely consumed first, limiting the amount of product formed.
To use stoichiometry effectively, one must know the balanced chemical equation for the reaction. This equation provides the mole ratio of reactants to products, which is essential for calculations.

• Balanced equations ensure that the number of atoms for each element is the same on both sides of the equation.
• Stoichiometric coefficients indicate the number of moles of a substance involved in the reaction.

Therefore, understanding chemical stoichiometry is key to mastering chemical reactions and anticipating the results of different scenarios in the lab or industrial processes.

Molar Mass

Molar mass is a critical concept when working with chemical stoichiometry. It is defined as the mass of one mole of a substance and is expressed in grams per mole (g/mol). Each element's molar mass can be found on the periodic table, and it plays a vital role in converting between mass and moles, central to stoichiometric calculations.

Here's how molar mass works in practice:

• The molar mass of a compound is the sum of the molar masses of the individual elements in its formula.
• For instance, water (H₂O) has a molar mass where hydrogen's molar mass is 1 g/mol and oxygen's is 16 g/mol. Thus, the molar mass of H₂O is 18 g/mol.

When solving stoichiometric problems, one often needs to convert grams of a substance to moles using its molar mass, making it a foundational tool in chemistry. Knowing the molar mass allows chemists to bridge the gap between the microscopic world of atoms and molecules and the macroscopic world of grams and liters.

Balanced Chemical Equation

A balanced chemical equation is an essential feature of any chemical reaction. It represents the chemical change that transpires when substances react, ensuring that the law of conservation of mass is upheld. This law states that mass is neither created nor destroyed in a chemical reaction. Thus, a balanced equation ensures that the same number of atoms for each element is present on both sides of the reaction.

Key aspects of a balanced chemical equation include:

• The reactants are on the left side, while the resulting products are on the right.
• Coefficients are used to balance the equation, reflecting the number of moles of each substance involved.

A properly balanced equation is necessary for accurate stoichiometric calculations, as it reflects the precise proportion of reactants and products. For example, in the equation \(2 H_{2} + O_{2} \rightarrow 2 H_{2}O\), two moles of hydrogen gas react with one mole of oxygen gas to produce two moles of water, capturing the stoichiometric relationship that guides further calculations.