Unit 8: Why do electrons matter (Redox & Electrochemistry)

Key Definitions:

Oxidation
- Gain of oxygen
- Loss of hydrogen
- Loss of electrons
Reduction
- Loss of oxygen
- Gain of hydrogen
- Gain of electrons
Reducing agent
- Substance that donates electrons
- Causes another substance to be reduced
- The reducing agent is oxidised
Oxidising agent
- Substance that accepts electrons
- Causes another substance to be oxidised
- The oxidising agent is reduced

OIL RIG

Oxidation Is Loss (of electrons)

Reduction Is Gain (of electrons)

Oxidation Number/ Oxidation State:

The oxidation state is a number assigned to an atom that represents its degree of electron control in a compound.
- The oxidation state is a number assigned to an atom that represents how the bonding electrons are distributed between atoms based on electronegativity.
It helps us track electron transfer during reactions.

An element:

is oxidised if its oxidation number increases
is reduced if its oxidation number decreases

Rules for Oxidation States:

Free elements = 0

Example:

Mg, O₂, N₂, Ar → oxidation state 0
Simple ions = charge on the ion

Examples:

Mg²⁺ → +2

O²⁻ → −2

N³⁻ → −3
Total oxidation state in a neutral compound = 0
Total oxidation state in a polyatomic ion = charge of the ion
Common oxidation states follow periodic trends

Group 1 metals → +1

Group 2 metals → +2

Aluminium → +3
Some elements have variable oxidation states

This is common for:
- transition metals
- some non-metals

Example:

Mn in MnO₂

Oxygen oxidation state = −2

There are two oxygen atoms:

Total = −4

The compound is neutral:

Mn + (−4) = 0

Mn = +4

Cr in K₂Cr₂O₇

Known oxidation states:

K = +1

O = −2

Calculation:

2(+1) + 2Cr + 7(−2) = 0

2 + 2Cr − 14 = 0

2Cr = +12

Cr = +6

Common Oxidation States:

Element	Usual Oxidation State	Exceptions	Explanation
Li, Na, K	+1	None	Group 1 metals lose one electron
Mg, Ca	+2	None	Group 2 metals lose two electrons
F	−1	None	Most electronegative element
O	−2	−1 in peroxides (H₂O₂); +2 in OF₂	Depends on bonding partner
H	+1	−1 in metal hydrides (NaH)	Hydrogen is more electronegative than metals
Cl	−1	Positive when bonded to O or F	O and F attract electrons more strongly

redox

A redox reaction is a reaction where oxidation and reduction occur simultaneously.

Electrons lost by one substance are gained by another
Electrons are conserved

Redox reactions can be represented using half-equations that show electron transfer.