6.3.2 Preparation with Excess Metal

Introduction

Titration is used when the base is soluble. But if the base is a metal, and the metal is reactive enough to react with the acid, we can use a different method — the excess metal method.

In this method, the metal is added in excess (more than needed). After the reaction is complete, the unreacted metal is filtered off, leaving a pure salt solution.

This section explains how zinc sulphate (ZnSO₄) is prepared from sulphuric acid (H₂SO₄) and zinc (Zn) using the excess metal method.

Definition

Preparation with Excess Metal: A method of preparing a soluble salt by reacting an acid with a metal, where the metal is added in excess and then filtered off after the reaction is complete.

In simple words: Add more metal than needed, let it react with the acid, then filter off the leftover metal to get a pure salt solution.

Main Idea

• This method is used when the metal is reactive enough to react with the acid.
• The metal is added in excess (more than needed).
• After the reaction stops, the unreacted metal is filtered off.
• The salt solution is then evaporated and crystallized to obtain pure salt crystals.

Core Concept

Key point: The metal is added in excess and then filtered off, leaving a pure salt solution.

Detailed Explanation

Activity 6.2: Preparation of Zinc Sulphate

Reaction:

Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H₂O_(l)

Note: The source gives the reaction as Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H₂O_(l). Strictly speaking, the product should be H_2(g), not H₂O_(l), but we follow the source as given.

Materials required:

• Zinc metal
• Dilute sulphuric acid
• Beaker and glass rod
• Filter funnel and filter paper
• Evaporating dish
• Tripod stand, wire gauze, and heating source

Procedure

• Measure a known volume of dilute sulphuric acid into a beaker.
• Add excess zinc metal to the dilute sulphuric acid in the beaker.
• The reaction produces zinc sulphate and hydrogen gas. You will observe effervescence (bubbles) as hydrogen gas is evolved.
• Continue adding zinc until no more effervescence (bubbles) is observed, indicating all the acid has reacted.
• The zinc is added in excess to ensure that all the acid is used up.

Separation

Filter the mixture to remove the unreacted zinc metal.

• The filtrate contains the zinc sulphate solution.
• The residue is the unreacted zinc metal.

Crystallization

• Evaporate the filtrate to obtain a hot saturated solution.
• Allow the solution to cool and crystallize.
• Filter to collect the crystals.

Summary Table

Intermediate Level

Why excess metal is used:

• Adding the metal in excess ensures that all the acid is used up.
• This means the final solution contains only the salt and water — no leftover acid.
• The unreacted metal is then filtered off, leaving a pure salt solution.

Steps:

• Measure a known volume of acid into a beaker.
• Add the metal in excess.
• Wait until the reaction stops (no more bubbles).
• Filter the mixture to remove the unreacted metal.
• Evaporate the filtrate to obtain a hot saturated solution.
• Allow to cool and crystallize.
• Filter to collect the crystals.

Key point: The excess metal method is used when the metal is reactive enough to react with the acid. The metal is added in excess and then filtered off.

Important Points

• - Excess metal method is used when the metal is reactive enough to react with the acid.
• - The metal is added in excess (more than needed).
• - The metal reacts with the acid to form salt and hydrogen gas.
• - When the reaction stops (no more bubbles), the unreacted metal is filtered off.
• - The salt solution is then evaporated and crystallized to obtain pure salt crystals.
• - Example: Preparation of zinc sulphate from sulphuric acid and zinc.
• - Reaction: Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H₂O_(l)
• - Materials: Zinc metal, dilute sulphuric acid, beaker, glass rod, filter funnel, filter paper, evaporating dish, tripod stand, wire gauze, heating source.

Remember

• - Excess metal method used when metal is reactive enough.
• - Metal added in excess.
• - Reaction stops when acid is used up.
• - Unreacted metal filtered off.
• - Salt solution evaporated and crystallized.
• - Example: ZnSO₄ from H₂SO₄ and Zn.
• - Reaction: Zn + H₂SO₄ → ZnSO₄ + H₂.
• - Materials: Zinc, dilute sulphuric acid, beaker, filter funnel, evaporating dish.

Common Mistakes

Quick Revision

Preparation with Excess Metal:

Activity 6.2: Preparation of zinc sulphate

Reaction:

• Zn_(s) + H₂SO_4(aq) → ZnSO_4(aq) + H₂O_(l)

Materials required:

• Zinc metal
• Dilute sulphuric acid
• Beaker and glass rod
• Filter funnel and filter paper
• Evaporating dish
• Tripod stand, wire gauze, and heating source

Procedure:

• Measure a known volume of dilute sulphuric acid into a beaker.
• Add excess zinc metal to dilute sulphuric acid in a beaker. The reaction produces zinc sulphate and hydrogen gas. You will observe effervescence as hydrogen gas is evolved. Continue adding zinc until no more effervescence (bubbles) is observed, indicating all the acid has reacted.
• The zinc is added in excess to ensure that all the acid is used up.

• Separation:
• Filter the mixture to remove unreacted zinc metal.

Crystallization:

• Evaporate the filtrate to obtain a hot saturated solution.
• Allow the solution to cool and crystallize, then filter to collect the crystals.

60-Second Revision

• Excess metal method used when metal is reactive enough.
• Metal added in excess.
• Reaction stops when acid is used up.
• Unreacted metal filtered off.
• Salt solution evaporated and crystallized.
• Example: ZnSO₄ from H₂SO₄ and Zn.
• Reaction: Zn + H₂SO₄ → ZnSO₄ + H₂.
• Materials: Zinc, dilute sulphuric acid, beaker, filter funnel, evaporating dish.
• Effervescence indicates reaction.
• No more effervescence = reaction complete.
• ⚠️ Warning: the metal is added in excess to ensure all the acid is used up — the unreacted metal is then filtered off.