Problem 67 - Chemistry: The Central Science

Question

(a) Define the terms limiting reactant and excess reactant. (b) Why are the amounts of products formed in a reaction determined only by the amount of the limiting reactant? (c) Why should you base your choice of what compound is the limiting reactant on its number of initial moles, not on its initial mass in grams?

Step-by-Step Solution

Answer

(a) A limiting reactant is a substance that is completely consumed in a reaction and determines the maximum amount of product that can be formed. An excess reactant is a substance that is not completely consumed in a reaction and is left over after the reaction is complete. (b) The amounts of products formed in a reaction are determined only by the amount of the limiting reactant because the reaction can only proceed until the limiting reactant is completely consumed. Once the limiting reactant is used up, the reaction stops, and no more products can be formed, even if there is still an excess reactant present. Therefore, the limiting reactant determines the maximum amount of product that can be formed. (c) You should base your choice of the limiting reactant on its number of initial moles and not on its initial mass in grams because the number of moles determines how many individual particles or entities of a substance are present in a reaction, whereas the mass in grams does not provide this information. The reaction proceeds based on the number of particles or entities of each reactant available, so it is essential to compare their amounts in terms of moles to determine which reactant would be used up first and thus be the limiting reactant.

Step 1: (a) Define the terms limiting reactant and excess reactant.

A limiting reactant is a substance that is completely consumed in a reaction and determines the maximum amount of product that can be formed. An excess reactant is a substance that is not completely consumed in a reaction and is left over after the reaction is complete.

Step 2: (b) Why are the amounts of products formed in a reaction determined only by the amount of the limiting reactant?

The amounts of products formed in a reaction are determined only by the amount of the limiting reactant because the reaction can only proceed until the limiting reactant is completely consumed. Once the limiting reactant is used up, the reaction stops, and no more products can be formed, even if there is still an excess reactant present. Therefore, the limiting reactant determines the maximum amount of product that can be formed.

Step 3: (c) Why should you base your choice of what compound is the limiting reactant on its number of initial moles, not on its initial mass in grams?

You should base your choice of the limiting reactant on its number of initial moles and not on its initial mass in grams because the number of moles determines how many individual particles or entities of a substance are present in a reaction, whereas the mass in grams does not provide this information. The reaction proceeds based on the number of particles or entities of each reactant available, so it is essential to compare their amounts in terms of moles to determine which reactant would be used up first and thus be the limiting reactant.

Key Concepts

Excess Reactant

In a chemical reaction, determining the role of each reactant is crucial to predict the outcome of the reaction. An excess reactant is one that remains after the reaction has completed because it was not fully used. This happens when there's more of this reactant available than what is needed to completely react with the limiting reactant. The excess reactant does not influence the amount of product formed. Instead, it is the leftover substance in a reaction vessel once all the limiting reactant is consumed.

This concept is important because:

• It helps in calculating the waste generated in reactions, which is essential for cost management and environmental impact assessments.
• Understanding which reactant is excess aids in planning industrial reactions where reduction of waste is a goal.
• It allows chemists to improve the efficiency of reactions by adjusting amounts of reactants based on their roles.

To assess which reactant is in excess, compare the stoichiometric amounts required with the available amounts of reactants.

Reaction Stoichiometry

Reaction stoichiometry involves using balanced chemical equations to calculate the relationships between reactants and products during a chemical reaction. The coefficients in a balanced equation tell us how many moles of each substance are involved in the reaction. This allows chemists to predict amounts of products formed from given quantities of reactants.

Stoichiometry helps to:

• Ensure reactions are balanced, meaning the mass and atoms of each element are conserved throughout the reaction.
• Calculate how much of each product can be produced from a specific amount of reactants.
• Determine the limiting reactant, which dictates the maximum amount of product possible.

For example, in a reaction between hydrogen and oxygen to form water ( 2H₂ + O₂ → 2H₂O ), the stoichiometry tells us two moles of hydrogen will react with one mole of oxygen to produce two moles of water. Understanding this helps to precisely control reaction conditions and optimize yield.

Mole Concept

The mole concept is foundational to understanding chemical reactions and stoichiometry. A mole is a unit that represents a specific number of particles, typically atoms or molecules, and is equal to Avogadro's number ( 6.022 imes 10^{23} ). This number allows chemists to count particles in a sample by weighing it.

Some key points about the mole concept include:

• It helps convert between mass and number of particles, which is critical in stoichiometry.
• Using moles rather than grams aligns with the way chemical reactions progress, as they occur at the particle or molecular level.
• Mole ratios, obtained from balanced chemical equations, are used to determine how much of a reactant is necessary to completely react with another.

By comparing the initial moles of each reactant in a chemical reaction, it becomes easier to determine which will be the limiting reactant, as it is the reactant that will completely be used up first, stopping the reaction.