Oxidation-Reduction Reactions

25min Part 1 / Ch2 / Lesson 6

Prerequisites: 1-2-3

Objectives

Oxidation and reduction always occur together (if one substance is oxidized, another is reduced).

Oxidation-reduction relationship (electron perspective)

Rules for assigning oxidation numbers:

Atoms in elements: 0
Monatomic ions: equal to the charge ( $\mathrm{Na^+}$ → +1, $\mathrm{Cl^-}$ → −1)
H in compounds: usually +1
O in compounds: usually −2
In a compound: oxidation numbers sum to 0
In a polyatomic ion: oxidation numbers sum to the ion’s charge

Example: Oxidation number of S in $\mathrm{H_2SO_4}$

$(+1) \times 2 + x + (-2) \times 4 = 0$ → $x = +6$

Oxidizing agents (get reduced)	Reducing agents (get oxidized)
$\mathrm{KMnO_4}$ (potassium permanganate)	$\mathrm{H_2}$ (hydrogen)
$\mathrm{H_2O_2}$ (hydrogen peroxide)*	Metals (Na, Zn, Fe, etc.)
$\mathrm{Cl_2}$ (chlorine)	$\mathrm{H_2S}$ (hydrogen sulfide)
$\mathrm{HNO_3}$ (dilute nitric acid)	$\mathrm{H_2C_2O_4}$ (oxalic acid)

Common oxidizing and reducing agents

* $\mathrm{H_2O_2}$ can act as either an oxidizing or reducing agent depending on the reaction partner.

Common Misconception

“Oxidizing agents get oxidized” → Oxidizing agents oxidize other substances and are themselves reduced. Don’t be misled by the name.

Q1 Being oxidized means electrons are:

Q2 What is the oxidation number of S in H₂SO₄?

Q3 In 2Mg + O₂ → 2MgO, which is oxidized?

Q4 What happens to a reducing agent after the reaction?

Q1. In the following reaction, identify which atom is oxidized and which is reduced. Show the change in oxidation numbers.

$\mathrm{Cu + 2AgNO_3 \rightarrow Cu(NO_3)_2 + 2Ag}$

Solution

Cu: $0 \rightarrow +2$ (oxidation number increases → oxidized → reducing agent)
Ag: $+1 \rightarrow 0$ (oxidation number decreases → reduced → oxidizing agent)

Cu loses 2 electrons; each Ag⁺ gains 1 electron.

Q2. Find the oxidation number of Mn in $\mathrm{MnO_4^-}$ .

Solution

$x + (-2) \times 4 = -1$ $x - 8 = -1$ $x = +7$