# AHL: Electrolytic cells and standard electrode potentials

> IB Chemistry HL · R3: What are the mechanisms of chemical change?
> Source: https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-electrolytic-cells-and-standard/

This subtopic covers the structure and operation of electrolytic cells, contrasts them with galvanic cells, and explains how to use standard electrode potentials to predict electrolysis products and calculate minimum required voltage for HL IB Chemistry.

**Prerequisites:** Redox reactions and half-equations; [Galvanic cells and standard electrode potential measurement](https://www.owlsprep.com/study/ib-chemistry-hl-u6-galvanic-cells-standard-potentials/)

## Learning objectives

- Distinguish between the structure and operation of electrolytic and galvanic cells
- Use standard electrode potentials to predict which species will react at each electrode
- Predict products of electrolysis for molten and aqueous electrolytes
- Calculate the minimum external voltage required for electrolysis to occur

## Key Differences Between Cell Types

Both electrolytic and galvanic cells are electrochemical cells that involve redox reactions, but they have opposite purposes and electrode polarity. Galvanic cells generate electricity from spontaneous reactions, while electrolytic cells use electricity to drive non-spontaneous reactions.

**Anode and Cathode Consistency** — Across *all* electrochemical cells, oxidation always occurs at the anode, and reduction always occurs at the cathode. Only the polarity of the electrodes changes between cell types.

*Example:* In electrolytic cells, the anode is positive (to attract anions) and the cathode is negative (to attract cations), opposite to galvanic cells.

| Property | Electrolytic Cell | Galvanic Cell |
| --- | --- | --- |
| Redox spontaneity | Non-spontaneous | Spontaneous |
| Energy flow | External input required | Energy output produced |
| Anode polarity | Positive (+) | Negative (-) |
| Cathode polarity | Negative (-) | Positive (+) |

> **Memory Hook**
>
> OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain of electrons. AnOX RedCat: Oxidation at Anode, Reduction at Cathode. These rules never change, regardless of cell type.

> **Exam tip:** IB examiners often test whether you can correctly state the polarity of electrodes in electrolytic cells, don't mix it up with galvanic cells.

## Predicting Reactions with Standard E°

Standard electrode potential ($E^\circ$) measures the tendency of a half-reaction to undergo reduction. For electrolysis, we use these values to determine which species will react at each electrode when multiple options are present.

1. At the cathode (reduction): The species with the *most positive* $E^\circ$ will be reduced first
2. At the anode (oxidation): The species with the *most negative* $E^\circ$ will be oxidized first

**Worked example:** Given two possible cathode half-reactions for electrolysis of aqueous copper(II) sulfate: $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \quad E^\circ = +0.34\ V$ and $2H_2O(l) + 2e^- \rightarrow H_2(g) + 2OH^-(aq) \quad E^\circ = -0.83\ V$. Predict which species is reduced.

1. Recall that the species with the most positive $E^\circ$ has the highest tendency to reduce.
2. Compare the two values:
3. $$+0.34\ V > -0.83\ V$$
4. Conclusion: Copper(II) ions are reduced to solid copper metal at the cathode.

**Check your understanding**

Check your understanding of prediction rules

1. Which species oxidizes first at the anode, given $E^\circ_{Cl_2/2Cl^-} = +1.36\ V$ and $E^\circ_{O_2/H_2O} = +1.23\ V$?

   - Chloride ions
   - Water

   *Why:* Correct! Oxidizing the chloride gives a reversed E° of -1.36 V, which is more negative than water's -1.23 V, so chloride oxidizes first.

## Products of Electrolysis

The products of electrolysis depend on whether the electrolyte is molten or aqueous. Molten electrolytes only have ions from the salt, so products are straightforward. Aqueous electrolytes also have water, which can be oxidized or reduced, leading to different products.

**Worked example:** Predict the products of electrolysis of molten magnesium chloride.

1. 1. Identify all species present: Only molten $Mg^{2+}$ and $Cl^-$ (no water).
2. 2. Assign reactions: $Mg^{2+}$ migrates to negative cathode for reduction; $Cl^-$ migrates to positive anode for oxidation.
3. 3. Write half-equations:
4. $$\text{Cathode: } Mg^{2+}(l) + 2e^- \rightarrow Mg(l)$$
5. $$\text{Anode: } 2Cl^-(l) \rightarrow Cl_2(g) + 2e^-$$
6. Products are liquid magnesium metal and chlorine gas.

> **Overpotential Exception**
>
> For aqueous solutions containing chloride, bromide, or iodide ions, overpotential causes the halide ion to oxidize at the anode instead of water, even if $E^\circ$ values predict water oxidation. This is a common exam trap.

## Calculating Minimum Required Voltage

For electrolysis, the overall reaction is non-spontaneous, so $E^\circ_{cell}$ will be negative. The external power supply must provide a voltage greater than the absolute value of this negative $E^\circ_{cell}$ to drive the reaction. The calculation of $E^\circ_{cell}$ follows the same rule as for galvanic cells:

$$E^\circ_{cell} = E^\circ_{cathode\ (reduction)} - E^\circ_{anode\ (reduction)}$$

**Worked example:** Calculate the minimum voltage required to electrolyze molten potassium iodide, given $E^\circ_{K^+/K} = -2.93\ V$ and $E^\circ_{I_2/2I^-} = +0.54\ V$.

1. 1. Identify cathode and anode reactions: $K^+$ reduces at cathode, $I^-$ oxidizes at anode.
2. 2. Substitute into the $E^\circ_{cell}$ formula:
3. $$E^\circ_{cell} = (-2.93\ V) - (+0.54\ V) = -3.47\ V$$
4. 3. Minimum voltage equals the absolute value of $E^\circ_{cell}$:
5. $$V_{min} = |-3.47\ V| = 3.47\ V$$

*Calculator:* forbidden

## Common pitfalls

- **Wrong:** Swapping anode and cathode polarity for electrolytic cells
  - Why it fails: Students memorize polarity from galvanic cells and forget it reverses
  - Correct: Anode = oxidation (always), so anode is positive in electrolytic cells, cathode negative
- **Wrong:** Predicting group 1/2 metal is reduced in aqueous solution
  - Why it fails: Students forget water has a more favorable reduction potential than group 1/2 metal ions
  - Correct: For aqueous group 1/2 metal salts, water reduces to hydrogen at the cathode, not the metal ion
- **Wrong:** Reporting positive $E^\circ_{cell}$ for electrolytic reactions
  - Why it fails: Students confuse calculation rules with spontaneous galvanic cells
  - Correct: Non-spontaneous electrolytic reactions have negative $E^\circ_{cell}$, minimum voltage is its absolute value
- **Wrong:** Relying only on $E^\circ$ for aqueous halide oxidation
  - Why it fails: Students forget overpotential changes the predicted product for chloride, bromide and iodide
  - Correct: Aqueous halides (Cl⁻, Br⁻, I⁻) oxidize to the halogen at the anode, not water
- **Wrong:** Treating molten and aqueous electrolytes the same
  - Why it fails: Students forget to add water's possible half-reactions for aqueous electrolytes
  - Correct: Always list all species present (including water) before predicting electrolysis products

## Cheatsheet

| Concept | Key Rule for Electrolytic Cells |
| --- | --- |
| Electrode role | Anode = oxidation, Cathode = reduction (always) |
| Electrode polarity | Anode = +, Cathode = - |
| Reduction preference | Most positive $E^\circ$ reduces first |
| Oxidation preference | Most negative $E^\circ$ oxidizes first |
| Minimum voltage | $V_{min} = \|E^\circ_{cell}\|$, $E^\circ_{cell} < 0$ |
| Aqueous halide anode | Halide oxidizes, not water (overpotential) |
| Molten salt product | Metal at cathode, non-metal at anode |

## What's next

This subtopic is a core AHL electrochemistry topic that is regularly tested across both Paper 1 and Paper 2 of IB Chemistry HL exams. Mastery of prediction rules and voltage calculations will prepare you for both qualitative and quantitative questions on electrolysis. Next, you can explore Faraday's laws of electrolysis, which let you calculate the mass of product formed from current and time, or review galvanic cells to reinforce your understanding of the differences between the two electrochemical cell types.

- [AHL: Catalysis and reaction mechanisms](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-catalysis-and-reaction-mechanisms/)
- [AHL: Spectroscopic identification of organic compounds](https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-spectroscopic-identification-of-organic/)
- [Practical and investigative skills](https://www.owlsprep.com/study/ib-chemistry-hl-u7-overview/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/ib-chemistry-hl-u6-ahl-electrolytic-cells-and-standard/