3.3Reaction Types
Redox & Electrochemistry — EST 2 Chemistry Revision Notes & Topical Questions
5 min read 8 parts Lesson 3 of 4 1 quiz
Lesson summary
Redox & Electrochemistry EST Chemistry notes: Assign oxidation numbers and identify redox changes.OIL RIGOxidation Is Loss, Reduction Is Gain (of electrons).AN OX, RED CATanode = oxidation, cathode = reduction.E°cell = E°cathode - E°anode (using reduction potentials).What the sym…
- Zn-Cu cell
- Electroplating
- Applications
- Redox & Electrochemistry EST Chemistry: what to do next
- Oxidizing agent
- Reducing agent
Section 1 of 8Redox & Electrochemistry
1Redox
A redox reaction transfers electrons. Oxidation and reduction always happen together: electrons lost by one species are gained by another.
2Oxidation numbers
Oxidation numbers track electron transfer. In KMnO4: K = +1, O = -2 × 4 = -8, so Mn = +7. An increase means oxidation; a decrease means reduction.
3Activity series
The activity series ranks metals by how easily they are oxidized. A metal displaces the ion of any metal below it: Zn + Cu2+ → Zn2+ + Cu occurs, but Cu + Zn2+ does not.
4Galvanic cell
A galvanic cell separates the two half-reactions so electrons travel through an external wire. It has two electrodes, two electrolyte solutions and a salt bridge (or porous barrier).
5
The more positive the reduction potential, the stronger the oxidizing agent. The half-cell with the higher E° becomes the cathode.
6Electrolytic cell
An electrolytic cell uses electricity to force a reaction. Electrolysis of molten NaCl gives Na at the cathode and Cl2 at the anode; electrolysis of water gives H2 (cathode) and O2 (anode) in a 2:1 volume ratio.
7Electroplating
Electroplating makes the object to be plated the cathode, where metal ions are reduced onto its surface; the anode is often the plating metal.
8Applications
Applications batteries (dry cells, lead-acid, lithium-ion), corrosion (rusting of iron is oxidation by O2 and water), and prevention by galvanizing (zinc as sacrificial anode).
Section 2 of 8Process & sequence
1
Assign oxidation numbers to every atom.
2
Identify which element is oxidized and which is reduced.
3
Split the reaction into two half-reactions.
4
Balance atoms other than O and H, then O with H2O and H with H+.
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Balance charge by adding electrons to each half-reaction.
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Multiply so electrons lost equal electrons gained, then add and cancel.
7
Check that atoms and charges both balance.
How a galvanic cell workswrite these steps in this order; each step is a marking point.
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Oxidation at anode releases electrons
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Electrons travel through external wire
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Reduction at cathode uses electrons
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Salt bridge ions balance charge
5
Measured voltage = E°cell
Section 3 of 8Compare
| Feature | Galvanic | Electrolytic |
|---|---|---|
| Spontaneous? | Yes (E°cell > 0) | No (needs power) |
| Energy change | Chemical → electrical | Electrical → chemical |
| Anode sign | Negative | Positive |
| Cathode sign | Positive | Negative |
| Example | Zn-Cu (Daniell) cell, batteries | Electroplating, electrolysis of NaCl(l) |
In both types, oxidation is at the anode and reduction at the cathode.
| Species | Oxidation number |
|---|---|
| Free element (Na, O2) | 0 |
| Monatomic ion | Equal to its charge |
| Oxygen in compounds | -2 (peroxides -1) |
| Hydrogen | +1 with nonmetals, -1 in metal hydrides |
| Fluorine | Always -1 |
| Sum in a polyatomic ion | Equals the ion's charge |
Section 4 of 8Worked examples
+0.46 V
Calculate E°cell for a cell made from Ag+/Ag (E° = +0.80 V) and Cu2+/Cu (E° = +0.34 V).
- Ag has the higher E°, so it is the cathode (reduced); Cu is the anode (oxidized).
- E°cell = 0.80 - 0.34 = +0.46 V, spontaneous.
- OverallCu + 2Ag+ → Cu2+ + 2Ag.
CO is the reducing agent
Identify the oxidizing and reducing agents: Fe2O3 + 3CO → 2Fe + 3CO2.
- Fe goes from +3 to 0 (reduced), so Fe2O3 is the oxidizing agent.
- C goes from +2 to +4 (oxidized), so CO is the reducing agent.
Section 5 of 8Keep going
Redox & Electrochemistry EST Chemistry: what to do next
These Redox & Electrochemistry EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Redox & Electrochemistry EST Chemistry understanding with the practice questions below, then reread any Redox & Electrochemistry EST Chemistry point you were unsure about before moving on.
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Previous lesson: 3.2 Oxidation and reduction
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Next lesson: 3.4 Precipitation and other reactions
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Course: EST 2 Chemistry — all units
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Official syllabus: OpenStax free science textbooks
Section 6 of 8Key terms
Section 7 of 8Command terms
Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.
Section 8 of 8Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Oxidation · Reduction · Oxidation number · Oxidizing agent · Reducing agent · Half-reaction · Galvanic (voltaic) cell · Electrolytic cell · Anode · Standard reduction potential (E°)
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.
Marked practiceQuizzes for this lesson
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