Redox Reactions

🟢 Lite — Quick Review (1h–1d)

Rapid summary for last-minute revision before your exam.

Redox Reactions — Key Facts Oxidation: loss of electrons (OIL — Oxidation Is Loss); reduction: gain of electrons (RIG — Reduction Is Gain) Oxidation number rules: free element = 0; monatomic ion = charge; O = -2 (except peroxides = -1); H = +1 (except metal hydrides = -1) Balancing redox: half-reaction method (separate into oxidation and reduction half-reactions, balance atoms and charge, then add) ⚡ Exam tip: In a galvanic cell, anode is negative (oxidation); cathode is positive (reduction) — opposite of electrolytic cell

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🟡 Standard — Regular Study (2d–2mo)

Standard content for students with a few days to months.

Redox Reactions — JAMB Chemistry Study Guide Common redox reactions:

• Combustion: $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$ (C goes from -4 to +4, O goes from 0 to -2)
• Displacement: $Zn + CuSO_4 \rightarrow ZnSO_4 + Cu$ (Zn loses 2e, Cu²⁺ gains 2e)
• Breathing: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$ (cellular respiration)

Oxidising agents: $KMnO_4$ (in acidic medium: $MnO_4^- \rightarrow Mn^{2+}$), $K_2Cr_2O_7$ (orange to green: $Cr_2O_7^{2-} \rightarrow Cr^{3+}$), $HNO_3$, halogen elements Reducing agents: $NaBH_4$, $LiAlH_4$, metals ($Zn$, $Mg$, $Fe$), $SO_2$, $SnCl_2$

Disproportionation: same element is both oxidised and reduced (e.g., $2Cl_2 + 2H_2O \rightarrow HCl + HOCl$ where Cl is both +1 and -1).

Titration calculations: use mole ratios from balanced redox equations; normality $N = n \times M$ where $n$ = number of electrons transferred per formula unit.

Common student mistakes: forgetting to balance electrons when combining half-reactions; assigning wrong oxidation states to compounds like $Fe_3O_4$ (contains both Fe²⁺ and Fe³⁺); confusing oxidising agent with the substance being reduced.

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🔴 Extended — Deep Study (3mo+)

Comprehensive coverage for students on a longer study timeline.

Redox Reactions — Comprehensive Chemistry Notes

Oxidation number method for balancing: Example: $Fe^{2+} + Cr_2O_7^{2-} \rightarrow Fe^{3+} + Cr^{3+}$ (acidic medium) Step 1: Assign oxidation numbers $Fe^{2+} \rightarrow Fe^{3+}$: Fe goes from +2 to +3 (oxidation, loses 1 e⁻) $Cr_2O_7^{2-} \rightarrow Cr^{3+}$: Cr goes from +6 to +3 (reduction, gains 3 e⁻ per Cr)

Step 2: Balance electrons Multiply Fe oxidation by 6: $6Fe^{2+} \rightarrow 6Fe^{3+} + 6e^-$ Multiply Cr reduction by 2: $Cr_2O_7^{2-} + 6e^- \rightarrow 2Cr^{3+}$ (in acidic medium, add $7H_2O$ on right, $14H^+$ on left)

Step 3: Add and simplify $6Fe^{2+} + Cr_2O_7^{2-} + 14H^+ \rightarrow 6Fe^{3+} + 2Cr^{3+} + 7H_2O$

Standard electrode potentials: $E°{cell} = E°{cathode} - E°{anode} = E°{reduction} - E°{oxidation}$ Positive $E°{cell}$ means spontaneous reaction (galvanic cell). $\Delta G° = -nFE°_{cell}$ where $F = 96,485 C/mol$

Competitive reactions: When multiple oxidising agents are present, the strongest one (most positive reduction potential) reacts first. Same for reducing agents.

JAMB exam patterns:

• 2022 JAMB: Calculate oxidation number of Cr in $K_2Cr_2O_7$
• 2021 JAMB: Which element is oxidised in the reaction $2Na + Cl_2 \rightarrow 2NaCl$?
• 2020 JAMB: Balance the redox equation in acidic medium: $MnO_4^- + Fe^{2+} \rightarrow Mn^{2+} + Fe^{3+}$
• 2019 JAMB: What mass of copper is deposited when 0.5 F of electricity is passed through $CuSO_4$ solution?

Common oxidation states of elements:

Element	Common oxidation states
O	-2, -1 (peroxide), -1/2 (superoxide)
H	+1, -1 (metal hydrides)
Fe	+2, +3
Mn	+2, +4, +7
Cr	+3, +6
S	-2, +4, +6
N	-3, +3, +5
Cl	-1, +1, +3, +5, +7

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📊 JAMB Exam Essentials

Detail	Value
Questions	180 MCQs (UTME)
Subjects	4 subjects (language + 3 for course)
Time	2 hours
Marking	+1 per correct answer
Score	400 max (used for university admission)
Registration	January – February each year

🎯 High-Yield Topics for JAMB

• Use of English (Grammar + Comprehension) — 60 marks
• Biology for Science students — 40 marks
• Chemistry (Organic + Physical) — 40 marks
• Physics (Mechanics + Optics) — 35 marks
• Mathematics (Algebra + Geometry) — 40 marks

📝 Previous Year Question Patterns

• Q: “The process of photosynthesis requires…” [2024 Biology]
• Q: “The electronic configuration of Fe is…” [2024 Chemistry]
• Q: “Find the value of x if 2x + 5 = 15…” [2024 Mathematics]

💡 Pro Tips

• Use of English carries the most weight — master grammar rules and comprehension strategies
• JAMB syllabus is your Bible — questions come directly from it. Download and use it.
• Past questions are highly predictive — repeat patterns appear every year
• For Science students, Biology and Chemistry are high-scoring if you study NCERT-level content

🔗 Official Resources

• JAMB Official
• JAMB Syllabus

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