Problem 18 - Chemistry in 30 days

Question

When the same amount of zinc is treated separately with excess of sulphuric acid and excess of sodium hydroxide, the ratio of volumes of hydrogen evolved is (a) \(1: 1\) (b) \(1: 2\) (c) \(2: 1\) (d) \(9: 4\)

Step-by-Step Solution

Answer

The correct answer is (a) \(1:1\).

Step 1: Understand the Chemical Reactions

Zinc reacts with sulfuric acid according to the reaction \( \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 \). It also reacts with sodium hydroxide: \( \text{Zn} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow \text{Na}_2\text{Zn(OH)}_4 + \text{H}_2 \). Both reactions produce hydrogen gas.

Step 2: Balancing the Moles

When excess sulfuric acid is used, 1 mole of Zn produces 1 mole of \( \text{H}_2 \). In the reaction with sodium hydroxide, 1 mole of Zn also produces 1 mole of \( \text{H}_2 \). Hence, both reactions produce the same amount of hydrogen gas per mole of zinc used.

Step 3: Calculate the Volume of Hydrogen

Assuming ideal conditions (STP), the volume of gas produced is directly proportional to the moles of hydrogen. Therefore, for the same amount of zinc, because both reactions produce the same moles of hydrogen gas, they will produce the same volume of \( \text{H}_2 \).

Step 4: Determine the Ratio of Volumes

Since both reactions produce equivalent moles of hydrogen gas from the same amount of zinc, the volume ratio of hydrogen is \(1:1\). This means the volumes of hydrogen evolved are equal in both reactions.

Key Concepts

Zinc and Sulfuric Acid Reaction

In this chemical reaction, zinc (\( \text{Zn} \)) reacts with sulfuric acid (\( \text{H}_2\text{SO}_4 \)) to produce zinc sulfate (\( \text{ZnSO}_4 \)) and hydrogen gas (\( \text{H}_2 \)). The process can be represented by the equation:\[ \text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2 \]This reaction is quite straightforward. Zinc displaces hydrogen from sulfuric acid due to its higher reactivity. Here, zinc acts as a reducing agent, and the hydrogen ions (\( \text{H}^+ \)) from sulfuric acid are reduced to form hydrogen gas. This is a type of single-replacement reaction, commonly found in metal and acid reactions. From this reaction, we can see that one mole of zinc produces one mole of hydrogen gas. It's important to emphasize that 1 mole of a gas at standard temperature and pressure (STP) occupies 22.4 liters. This relationship is crucial for understanding how volumes are calculated in chemical reactions.

Zinc and Sodium Hydroxide Reaction

Unlike the reaction with sulfuric acid, zinc's interaction with sodium hydroxide (\( \text{NaOH} \)) is a bit different as it involves more complex products:\[ \text{Zn} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow \text{Na}_2\text{Zn(OH)}_4 + \text{H}_2 \]In this scenario, zinc reacts with sodium hydroxide and water to form sodium zincate (\( \text{Na}_2\text{Zn(OH)}_4 \)) along with hydrogen gas. This is also a type of displacement reaction where zinc displaces the hydrogen from water after forming sodium zincate.Key points to remember:

• 1 mole of zinc produces 1 mole of hydrogen gas in this reaction as well.
• The formation of sodium zincate is peculiar as it represents zinc's amphoteric nature, which means it can react with both acids and bases.

Even though this reaction may seem more complicated than the one with sulfuric acid, the outcome concerning hydrogen gas is the same: equivalent moles of hydrogen gas are produced.

Hydrogen Gas Production

Hydrogen gas (\( \text{H}_2 \)) is a common product in both reactions—whether zinc is mixed with sulfuric acid or sodium hydroxide. Here's why hydrogen gas is significant:

• It is a diatomic molecule, meaning it usually exists as two hydrogen atoms bonded together.
• It's the lightest and most abundant element in the universe, often used in industry and as a clean fuel source.

In both the zinc and sulfuric acid reaction, and zinc and sodium hydroxide reaction, the conditions assume an excess of reactants and are carried out under ideal conditions (STP). Under these circumstances, we calculate the volume of hydrogen gas based simply on the moles of hydrogen produced.Since both reactions produce the same number of moles of hydrogen gas from the same amount of zinc, the volume of hydrogen gas evolved in each case is identical. Thus, the ratio of hydrogen volumes produced by the two reactions is 1:1, affirming the equivalency in hydrogen production under those given conditions.