Problem 73 - Chemistry: The Central Science in SI Units, Global Edition

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 which compound is the limiting reactant on its number of initial moles, not on its initial mass in grams?

Step-by-Step Solution

Answer

(a) The limiting reactant is the reactant that runs out first in a chemical reaction, causing the reaction to stop and not produce more products. The excess reactant is the reactant present in a greater quantity than necessary for the stoichiometric balance and remains partially unused when the reaction is completed. (b) The limiting reactant determines the amount of product formed in a reaction because once it is consumed, the reaction stops, and no more product can be formed. The reaction requires the presence of all reactants in their stoichiometric quantities for it to occur. (c) The choice of the limiting reactant should be based on the number of initial moles because the stoichiometry of a reaction depends on the mole ratios of the reactants, not on their masses. Using initial moles enables calculations of mole ratios directly from the balanced chemical equation and helps determine which reactant will run out first, leading to the correct identification of the limiting reactant. The initial mass in grams may not provide accurate information about the reactant's stoichiometry within the reaction, while the number of initial moles does.

Step 1: a) Limiting and Excess Reactants Definitions

The limiting reactant is the reactant that runs out first in a chemical reaction. Due to this, the reaction stops and cannot produce more products. On the other hand, the excess reactant is the reactant present in a greater quantity than necessary for the stoichiometric balance. It remains partially unused when the reaction is completed.

Step 2: b) Role of Limiting Reactant in Product Formation

The limiting reactant determines the amount of product formed in a reaction because once it is consumed, the reaction stops and no more product can be formed. This is due to the fact that all reactants must be present in their stoichiometric quantities for the reaction to occur. If one of the reactants runs out, the reaction will cease to occur, and the product generation will be limited by the amount of the limiting reactant.

Step 3: c) Choosing the Limiting Reactant Based on Initial Moles

The choice of the limiting reactant should be based on the number of initial moles because the stoichiometry of a reaction depends on the mole ratios of the reactants, not on their masses. Using the initial moles of a substance allows you to calculate the mole ratios directly from the balanced chemical equation and helps in determining which reactant will run out first, leading to the correct identification of the limiting reactant. The initial mass in grams may not provide accurate information about the reactant's stoichiometry within the reaction, while the number of initial moles does.

Key Concepts

Limiting Reactant

In a chemical reaction, the limiting reactant is the substance that is entirely consumed first, causing the reaction to stop. It's like running out of one ingredient when baking cookies - you can't keep making more without it. This reactant controls the amount of product formed because no additional product can be created once the limiting reactant is depleted. That’s why the term 'limiting' is used; it limits the extent of the reaction. Understanding which reactant is the limiting reactant is crucial for predicting the quantities of products formed in a reaction.

• Think of it as the bottleneck in a process.
• Once the limiting reactant is finished, the reaction halts.
• Finding the limiting reactant helps in waste reduction and cost-saving by understanding the precise amounts needed.

Excess Reactant

Opposite to the limiting reactant, the excess reactant is present in more than enough quantities to completely react with the limiting reactant. You'll always end up with some of it left over after the reaction has finished. This is like having too much frosting for your batch of cookies - no matter how much you ice each cookie, you will have frosting leftover.

• The excess reactant remains partly unused.
• This reactant is not essential for determining the amount of product formed.
• Managing excess reactant is key to efficiently conducting reactions and minimizing waste.

Chemical Reactions

Chemical reactions are processes where substances, known as reactants, are transformed into different substances, known as products. To fully understand chemical reactions, it's important to consider stoichiometry - the quantitative relationship between reactants and products in a chemical equation.
Choosing the limiting reactant is based on the initial moles, not mass, because chemical reactions depend on the ratios in moles as per their balanced equations. Grams do not give a direct comparison of availability unless converted to moles. Doing so ensures each reactant is accurately assessed according to the reaction equation, closely linked to its stoichiometric coefficients.

• Chemical equations need to be balanced to reflect real-world conservation of mass and energy.
• The amount of each reactant affects how much product can be formed.
• Calculating in moles helps in assessing the exact reactant requirement.