Redox Flashcards
Oxidation number rule no.1
ON of any element is 0
Eg. Zn = 0
Oxidation number rule no.2
ON of monoatomic ion equals charge
Eg. Mg2+
Oxidation number rule no.3
ON of oxygen is -2
Exceptions: In H2O2 ON is -1, in F2O ON is +2
Oxidation number rule no.4
ON of hydrogen is +1
Exception: When bonded to metal ON is -1
Oxidation number rule no.5
ON of halides in ionic compounds is -1 (fluorine is always -1)
Eg. NaCl
Exception: Different in covalent compounds
Eg. HClO3
Deciding oxidation/reduction
ON increases => oxidation
ON decreases => reduction
Disproportionation reaction
A reaction where one element is both oxidised and reduced
Oxidising agent
A substance that oxidises another species (OA is reduced)
If asked to explain which species is the OA: X because it gains electrons from Y
Reducing agent
A substance that reduces another species (RA is oxidised)
If asked to explain which species is the RA: Y because it loses electrons to X
Constructing half equations in acidic conditions
- Balance elements other than O
- Balance O by adding water
- Balance H by adding H+
- Balance charge by adding electrons
Writing redox equations with half equations
Balance electrons
multiply halfs to same no.
Add half equations
Ensure electrons cancel
Writing redox equations using oxidation state changes
- Assign ON to identify atoms that change
- Balance species that contain elements that change ON
- Balance any remaining atoms
Constructing half equations in alkaline conditions
- Construct for acidic conditions
- Add OH- to both sides to cancel H+
- H+ + OH- -> H2O
- Cancel H2O
Predicting products in redox reactions
Use molar ratios & changes in oxidation state
Manganate titrations
(MnO4)- : Fe2+ ratio
1 : 5
Colour change: purple -> colourless
No indicator needed
∵colour change associated
with different oxidation states
of manganate
End point = pale pink
First permanent colour change
on first drop excess (MnO4)-
