Problem 3 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

\(5 \mathrm{~g}\) of zinc is treated separately with an excess of (A) dilute hydrochloric acid and (B) aqueous sodium hydroxide. The ratio of the volumes of \(\mathrm{H}_{2}\) evolved in these two reactions is: (a) \(1: 2\) (b) \(1: 1\) (c) \(1: 4\) (d) \(2: 1\)

Step-by-Step Solution

Answer

The ratio is (b) \(1:1\).

Step 1: Understand the Reactions

When zinc reacts with dilute hydrochloric acid, the reaction is: \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \). When zinc reacts with aqueous sodium hydroxide, it forms a complex compound and hydrogen gas: \( \text{Zn} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow \text{Na}_2\text{[Zn(OH)}_4\text{]} + \text{H}_2 \). Note both reactions produce hydrogen gas.

Step 2: Calculate Moles of Zinc

The molar mass of zinc, \( \text{Zn} \), is approximately \( 65.38 \text{ g/mol} \). The number of moles of zinc in \( 5 \text{ g} \) is \( \frac{5}{65.38} \approx 0.076 \text{ mol} \).

Step 3: Determine Hydrogen Produced in Reaction A

From the reaction with \( \text{HCl} \): \( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \). 1 mole of zinc produces 1 mole of hydrogen. So, \( 0.076 \text{ mol} \) of zinc will produce 0.076 mol of \( \text{H}_2 \).

Step 4: Determine Hydrogen Produced in Reaction B

From the reaction with \( \text{NaOH} \): \( \text{Zn} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow \text{Na}_2\text{[Zn(OH)}_4\text{]} + \text{H}_2 \). Again, 1 mole of zinc produces 1 mole of hydrogen, so \( 0.076 \text{ mol} \) of zinc produces 0.076 mol of \( \text{H}_2 \).

Step 5: Compare the Volumes of Hydrogen Gas

Since both reactions produce \( 0.076 \text{ mol} \) of \( \text{H}_2 \) and each mole of gas is at the same conditions, the volumes are proportional to their amounts. Thus, the ratio of volumes is \( 1:1 \).

Key Concepts

Zinc Reaction with Acids

Zinc reacts quite actively with acids, such as dilute hydrochloric acid, to produce a salt and hydrogen gas. Specifically for hydrochloric acid, the reaction can be represented by the equation:\( \text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2 \).

• This reaction shows that one mole of zinc reacts with two moles of hydrochloric acid.
• The products of the reaction are zinc chloride (\( \text{ZnCl}_2 \)) and hydrogen gas (\( \text{H}_2 \)).

When any acid reacts with zinc, it typically releases hydrogen gas. This is a commonly used method in laboratories to generate small amounts of hydrogen gas for demonstrations or experiments. The reaction is well balanced in that zinc can neutralize the acidic content while releasing hydrogen gas efficiently. Consider that only react enough zinc to safely manage the evolving hydrogen gas, especially in a confined space.

Zinc Reaction with Bases

Unlike acids, not all metals react with bases, but zinc is special in that it does. In the case of zinc's reaction with bases like sodium hydroxide and water, the reaction results in complex formation along with the release of hydrogen gas:\( \text{Zn} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow \text{Na}_2[\text{Zn(OH)}_4] + \text{H}_2 \).

• Here, zinc reacts with sodium hydroxide (\( \text{NaOH} \)) and water (\( \text{H}_2\text{O} \)).
• The products are a sodium zincate complex (\( \text{Na}_2[\text{Zn(OH)}_4] \)) and hydrogen gas.

This type of reaction is known as an amphoteric reaction since zinc oxide (a form zinc often takes in such reactions) has amphoteric properties, meaning it can react with both acids and bases. When zinc reacts with a base, a more complex product is created as compared to simple salt formation seen in acid reactions.

Hydrogen Gas Production

Hydrogen gas is a clear, colorless gas often associated with reactions involving metals and acids or bases. In the scenarios we discussed, both reactions of zinc with hydrochloric acid and sodium hydroxide produce hydrogen gas. For both reactions, the stoichiometry dictates that:

• One mole of zinc ( \(74.38 \text{ g/mol}\)) will yield one mole of hydrogen gas (\(\text{H}_2\)).
• Thus, when 0.076 moles of zinc react, it results in the production of 0.076 moles of \(\text{H}_2\).

Since the number of moles of hydrogen gas produced in both reactions is the same, the volumes of hydrogen gas produced will also be the same under identical conditions (assuming temperatures and pressures are constant). This is why the ratio of the hydrogen gas volumes evolved in zinc's reaction with hydrochloric acid versus sodium hydroxide is 1:1.Producing hydrogen gas through such chemical reactions is a fundamental technique and is pivotal for students to understand basic reaction types and gas stoichiometry. It also illustrates the behavior of diatomic gases under various chemical processes in along simple reaction environments.