Sulphuric Acid — Part 2: The concentrated acid and tests: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Three powers of the concentrated acid

Part 2 — Concentrated sulphuric acid shows three special properties besides being an acid:

In this lesson you will

Write equations for hot concentrated sulphuric acid oxidising carbon and sulphur
Describe and explain the action of concentrated sulphuric acid on sugar and on blue vitriol
Distinguish dehydration from drying
Use the non-volatile nature of sulphuric acid to prepare HCl and HNO₃, and test for sulphuric acid and sulphates

Table of three properties of concentrated sulphuric acid with what each does and an example
Property	What it does	Example
Oxidising agent	Gives oxygen when hot	Oxidises carbon and sulphur
Dehydrating agent	Removes the elements of water	Chars sugar; whitens blue vitriol
Non-volatile acid	High boiling point	Drives out HCl and HNO₃ from their salts

Concept

Oxidising agent: carbon and sulphur

Hot concentrated sulphuric acid gives oxygen (H₂SO₄ → H₂O + SO₂ + [O]) and is itself reduced to sulphur dioxide:

Table of the oxidation of carbon and sulphur by hot concentrated sulphuric acid
Heated with	Balanced equation	Products
Carbon (charcoal)	C + 2H₂SO₄ → CO₂ + 2H₂O + 2SO₂	CO₂, water, SO₂
Sulphur	S + 2H₂SO₄ → 3SO₂ + 2H₂O	SO₂ and water only

Worked example

Worked example: gas from charcoal

Problem. 1.2 g of charcoal is heated with excess concentrated sulphuric acid. Find the total volume of gas (CO₂ and SO₂) at STP. (C = 12; molar volume 22.4 L)

Step 1. C + 2H₂SO₄ → CO₂ + 2H₂O + 2SO₂: 1 mole C gives 1 mole CO₂ and 2 moles SO₂.

Step 2. Moles of C = 1.2 ÷ 12 = 0.1.

Step 3. Gas = 0.1 + 0.2 = 0.3 mole; volume = 0.3 × 22.4 = 6.72 L (2.24 L CO₂, 4.48 L SO₂).

Table of the moles and volumes of carbon dioxide and sulphur dioxide from 1.2 grams of charcoal
Gas	Moles	Volume at STP
CO₂	0.1	2.24 L
SO₂	0.2	4.48 L
Total	0.3	6.72 L

Cookies