“OIL RIG”
Oxidation and reduction
Oxidation Is Loss of electrons, Reduction Is Gain. Say it once at the start of every redox question and the agents follow.
Chemistry, Class 11 and 12 · Gujarat Board, CBSE, GUJCET, JEE and NEET
Redox Reactions formulas
Redox looks like bookkeeping and it is. Oxidation numbers, then balancing, then the language of oxidising and reducing agents. Every question in this chapter can be answered by assigning oxidation numbers correctly first.
17 formulas, 3 memory hooks, and the 5 mistakes that cost the most marks.
Oxidation numbers
| What | Formula | Symbols and notes |
|---|---|---|
| Free element | 0 | for example H₂, O₂, Na, S₈ |
| Hydrogen | +1, except −1 in metal hydrides | NaH has hydrogen at −1 |
| Oxygen | −2, except −1 in peroxides and +2 in OF₂ | H₂O₂ has oxygen at −1 |
| Group 1 and 2 metals | +1 and +2 | always, in compounds |
| Fluorine | −1 always | the most electronegative element |
| Sum rule | neutral compound sums to 0, an ion sums to its charge | this is how unknown states are found |
The language
| What | Formula | Symbols and notes |
|---|---|---|
| Oxidation | loss of electrons, oxidation number rises | the substance oxidised is the reducing agent |
| Reduction | gain of electrons, oxidation number falls | the substance reduced is the oxidising agent |
| Oxidising agent | itself gets reduced | for example KMnO₄, K₂Cr₂O₇ |
| Reducing agent | itself gets oxidised | for example H₂, CO, metals |
| Disproportionation | one element both oxidised and reduced | for example 2H₂O₂ → 2H₂O + O₂ |
Balancing and equivalents
| What | Formula | Symbols and notes |
|---|---|---|
| Ion electron method | balance atoms, then O with H₂O, then H with H⁺, then charge with electrons | in basic medium add OH⁻ at the end |
| Oxidation number method | equalise the total rise and fall in oxidation number | then balance the rest by inspection |
| n-factor of an acid | number of replaceable H⁺ | H₂SO₄ has n-factor 2 |
| n-factor in redox | electrons gained or lost per formula unit | KMnO₄ in acid has n-factor 5 |
| Equivalent weight | molar mass / n-factor | used in titration calculations |
| Titration | N₁V₁ = N₂V₂ | normality times volume, on both sides |
Taught by Darshit Bhatt, founder and lead faculty at Darshit Sir Institute. Instrumentation and Control engineer, with a master's degree and a chartered degree, teaching in Paldi, Ahmedabad since 2006.
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Memory hooks
How to remember this chapter.
“The agent suffers the opposite”
Naming the agents
The oxidising agent is itself reduced; the reducing agent is itself oxidised. Whatever the agent does to the other, it has the reverse done to itself.
“KMnO₄ is 5 in acid, 3 in neutral”
n-factor of permanganate
In acidic medium manganese goes from +7 to +2, so five electrons. In neutral or faintly alkaline medium it goes to +4, so three.
Where marks go
The mistakes that cost the most.
- Giving oxygen −2 in peroxides. In H₂O₂ and other peroxides it is −1.
- Naming the agents the wrong way round. The oxidising agent is the one reduced.
- Balancing charge before balancing atoms. Atoms first, then oxygen, then hydrogen, then charge.
- Using n-factor 1 for KMnO₄ without checking the medium.
- Forgetting that the oxidation number of an element in its free state is zero, whatever the molecule.
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