Chapter 9, Problem 70A

Interpretation Introduction

Interpretation:

The statement that the limiting reactant is always the reactant that is present in the smallest amount with respect to mass needs to be classified as true or false.

Concept Introduction:

• A limiting reactant is one that is fully consumed during the course of the reaction.
• It determines the amount of product formed in the reaction

Answer to Problem 70A

False.

Explanation of Solution

The statement that the limiting reactant is always the reactant that is present in the smallest amount with respect to mass needs is False. This is because limiting reagents are deduced based on the molar amounts of each reactant that is present and by comparison to the stoichiometric coefficients of the reactants in the balanced equation. Therefore, the limiting reactant is always the reactant that is present in the smallest amount with respect to moles.

Calculations:

The given reaction is:

  ${\text{N}}_{\text{2}}{\text{ + 3H}}_{\text{2}}\to {\text{ 2NH}}_{\text{3}}$

Consider the following initial amounts of N₂ and H₂

N₂ = 2 g

H₂ = 1 g

Step 1: Calculate the moles of N₂ as follows:

Mass of N₂ = 2 g

Molar mass of N₂ = 14 g/mol

  $\begin{array}{l}{\text{Moles of N}}_2=\frac{{\text{Mass of N}}_2}{{\text{Molar mass N}}_2}\\ =\frac{2\text{ g}}{14\text{ g/mol}}=0.1428\text{ moles}\end{array}$

Step 2: Calculate the moles of H₂ as follows:

Mass of H₂ = 1 g

Molar mass of H₂ = 2 g/mol

  $\begin{array}{l}{\text{Moles of H}}_2=\frac{{\text{Mass of H}}_2}{{\text{Molar mass H}}_2}\\ =\frac{1\text{ g}}{2\text{ g/mol}}=0.5\text{ moles}\end{array}$

Step 3: The limiting reagent is deduced as follows:

Based on the reaction stoichiometry:

1 mole of N₂ reacts with 3 moles of H₂

Therefore, 0.1428 moles of N2 will react with: $0.1428\times 3=0.4284{\text{ moles H}}_{\text{2}}$

Since H₂ is present in excess, N₂ will be the limiting reagent and will determine the amount of product formed.

If the given statement were true, then H2 should be the limiting reagent since it is present in smaller amount by mass. However, as shown above this is not the case.

Conclusion

Therefore, the limiting reactant is always the reactant that is present in the smallest amount with respect to moles.