Question

Consider the redox reaction

Which species acts as the oxidising agent in this reaction ?

• A. Cr2O7^2-
• B. S02
• C. Cr3+
• D. SO4^2-

Answer 1

Answer:

A. Cr2O7^2-

• Redox reactions involve the transfer of electrons between chemical species. Oxidation is the loss of electrons, while reduction is the gain of electrons. An oxidising agent is a substance that accepts electrons from another substance, causing that other substance to be oxidised, and is itself reduced in the process.

Analysing the Given Reaction

The reaction involves:

• Dichromate ion (Cr₂O₇^2-)

• Sulphite ion (SO₃^2-)

• Acidic medium (H⁺)

• Chromium(III) ions (Cr³⁺)

• Sulphate ions (SO₄^2-)

• Water (H₂O)

Determining Oxidation States

To identify the oxidising agent, we need to determine the changes in oxidation states of the elements involved:

• In Cr₂O₇^2-: Oxygen usually has an oxidation state of -2. Let the oxidation state of Chromium be 'x'. So, 2x + 7(-2) = -2. This gives 2x - 14 = -2, so 2x = +12, and x = +6.

• In SO₃^2-: Oxygen has an oxidation state of -2. Let the oxidation state of Sulfur be 'y'. So, y + 3(-2) = -2. This gives y - 6 = -2, so y = +4.

• In Cr³⁺: The oxidation state of Chromium is directly given as +3.

• In SO₄^2-: Oxygen has an oxidation state of -2. Let the oxidation state of Sulfur be 'z'. So, z + 4(-2) = -2. This gives z - 8 = -2, so z = +6.

Identifying Oxidation and Reduction

• Chromium (Cr): Its oxidation state changes from +6 in Cr₂O₇^2- to +3 in Cr³⁺. This decrease in oxidation state signifies reduction (gain of electrons).

• Sulfur (S): Its oxidation state changes from +4 in SO₃^2- to +6 in SO₄^2-. This increase in oxidation state signifies oxidation (loss of electrons).