Problem 137 - Chemistry and Chemical Reactivity

Question

Let us explore a reaction with a limiting reactant. Here, zinc metal is added to a flask containing aqueous HCl, and \(\mathrm{H}_{2}\) gas is a product. $$\mathrm{Zn}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{ZnCl}_{2}(\mathrm{aq})+\mathrm{H}_{2}(\mathrm{g})$$ The three flasks each contain 0.100 mol of HCl. Zinc is added to each flask in the following quantities. When the reactants are combined, the \(\mathrm{H}_{2}\) inflates the balloon attached to the flask. The results are as follows: Flask 1: Balloon inflates completely, but some \(\mathrm{Zn}\) remains when inflation ceases. Flask 2: Balloon inflates completely. No Zn remains. Flask 3: Balloon does not inflate completely. No Zn remains. Explain these results. Perform calculations that support your explanation.

Step-by-Step Solution

Answer

Flask 1 had excess Zn, Flask 2 had exact stoichiometric amounts, and Flask 3 had insufficient Zn.

Step 1: Identify the Balanced Chemical Equation

First, we need to understand the chemical reaction given. The balanced chemical equation is \( \mathrm{Zn}(\mathrm{s}) + 2 \mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{ZnCl}_{2}(\mathrm{aq}) + \mathrm{H}_{2}(\mathrm{g}) \). This indicates that 1 mole of zinc reacts with 2 moles of hydrochloric acid to produce 1 mole of hydrogen gas.

Step 2: Determine Mols Needed for Reaction

Given that each flask contains 0.100 mol of \( \mathrm{HCl} \), we calculate how much \( \mathrm{Zn} \) is required. Since the balanced equation tells us that 2 mol of \( \mathrm{HCl} \) react with 1 mol of \( \mathrm{Zn} \), the moles of \( \mathrm{Zn} \) needed are \( \frac{0.100}{2} = 0.050 \) mol for the complete reaction.

Step 3: Analyze Flask 1 Results

In Flask 1, the balloon inflates completely but zinc remains, indicating that \( \mathrm{HCl} \) is the limiting reactant, not all zinc was consumed. This tells us more zinc than necessary was provided, but only 0.050 mol could react with 0.100 mol of \( \mathrm{HCl} \).

Step 4: Analyze Flask 2 Results

In Flask 2, the balloon inflates completely and no zinc remains. This means \( \mathrm{Zn} \) is the limiting reactant, exactly 0.050 mol of \( \mathrm{Zn} \) was used, producing a complete reaction using all available \( \mathrm{HCl} \).

Step 5: Analyze Flask 3 Results

In Flask 3, the balloon does not inflate completely and no zinc remains. This means that \( \mathrm{Zn} \) was again the limiting reactant and less than 0.050 mol of \( \mathrm{Zn} \) was added, meaning not all \( \mathrm{HCl} \) reacted, resulting in an incomplete reaction.

Step 6: Conclusion from Calculations

Flask 1 had excess zinc, Flask 2 had the perfect stoichiometric amount of materials, and Flask 3 had insufficient zinc. The complete inflation or leftover zinc reflects which reactant was limiting in each case.

Key Concepts

Chemical Reaction Stoichiometry

In the world of chemistry, stoichiometry is the major key to understanding chemical reactions. It essentially acts as a recipe for the reaction, indicating the required proportions of reactants and estimating the products formed. Here, zinc and hydrochloric acid react to create zinc chloride and hydrogen gas. For stoichiometry:

• The chemical equation must first be balanced.
• The balanced equation will provide the molar ratios of reactants and products.

This allows us to calculate how much of each reactant is necessary for the reaction and predict the quantities of products formed. Stoichiometry is crucial, as it ensures that we use reactants efficiently and understand which reactant will be consumed first, known as the limiting reactant.

Balanced Chemical Equation

A balanced chemical equation is a fundamental aspect of understanding any reaction. It indicates that the number of atoms for each element is the same on both sides of the reaction equation. This balance ensures that mass is conserved throughout the reaction. In our exercise, the reaction between zinc and hydrochloric acid is represented by:\[ \text{Zn(s)} + 2 \text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)} \]

• This indicates one mole of zinc reacts with two moles of hydrochloric acid.
• It produces one mole of zinc chloride and one mole of hydrogen gas.

Balancing equations is essential because it reflects the stoichiometric relationships that help us determine reactant and product amounts.

Reaction Products Analysis

When analyzing the products of a chemical reaction, it is helpful to look at the limiting and excess reactants. The limiting reactant is the one that determines the maximum amount of product that can be formed. In our exercise:

• Flask 1 shows hydrochloric acid as the limiting reactant since extra zinc remains unreacted.
• In Flask 2, zinc is the limiting reactant, fully consumed without any excess reactant.
• In Flask 3, again zinc limits the reaction but is present in insufficient quantity to use up all the hydrochloric acid.

Understanding which reactants limit the reaction helps predict and verify product amounts and reveals inefficiencies in reactant use.

Stoichiometric Calculations

Stoichiometric calculations allow chemists to quantify aspects of a reaction. To determine the limiting reactant, you calculate how much product each reactant can produce and identify the reactant that makes the least amount of product. Here's the breakdown for our scenario:

• Each flask has 0.100 mol of \( \text{HCl} \). According to the balanced equation, 0.050 mol of \( \text{Zn} \) is needed for complete reaction.
• Flask 1 used more than this amount, suggesting \( \text{HCl} \) was the limitation.
• Flask 2 had exactly 0.050 mol of \( \text{Zn} \), aligning perfectly with the stoichiometric ratio.
• Flask 3 had less than 0.050 mol, meaning \( \text{Zn} \) was insufficient to react fully with the \( \text{HCl} \).

These calculations ensure accurate predictions and inform us which products are formed during the chemical reaction.