What happens at the anode in a concentrated aqueous solution of a halide?

If the solution is concentrated and contains a halide ion (Chloride, Bromide, Iodide), the halogen gas (Cl2, Br2, I2) is discharged. If it is a dilute solution, or contains sulfate/nitrate ions, Oxygen gas is discharged from the OH- ions.

O-Level Chemistry: Electrolysis Rules & Predictions

Q: What does PANIC stand for in electrolysis?

PANIC stands for Positive is Anode, Negative Is Cathode. This is the most reliable way to remember the polarity of the electrodes in an electrolytic cell.

Q: What is the difference between molten and aqueous electrolysis?

A molten compound (liquid state, no water) only contains the ions of that compound. An aqueous compound is dissolved in water, meaning water's ions (H+ and OH-) are also present and compete to be discharged at the electrodes.

Q: What is discharged at the cathode in an aqueous solution?

The metal ion AND the Hydrogen ion (H+) race to the cathode. The LEAST reactive element wins. If the metal is highly reactive (like Sodium or Potassium), Hydrogen gas is produced. If the metal is unreactive (like Copper or Silver), the metal is produced.

A diagram of an electrolytic cell showing ion migration towards the anode and cathode.

How do you memorize the electrodes in Chemistry?

Remember the acronym PANIC: Positive is Anode, Negative Is Cathode. Since opposites attract, positive ions (cations) move to the Negative Cathode, and negative ions (anions) move to the Positive Anode.

1. The PANIC Mnemonic and Ion Migration
2. Molten Electrolysis (The Easy Mode)
3. Aqueous Solutions: The Cathode Race
4. Aqueous Solutions: The Anode Concentration Trap
5. Frequently Asked Questions

Electrolysis is the process of using electricity to break down an ionic compound. It's heavily tested in CAIE Paper 2 and Paper 4, and it operates on a strict set of rules. Once you learn the "competition" rules for aqueous solutions, you can predict the products of any chemical cell. This guide from our Ultimate O-Level Chemistry Guide breaks down the framework.

1. The PANIC Mnemonic and Ion Migration

Before predicting products, you must know where the ions are going.

P.A.N.I.C.

Positive is Anode, Negative Is Cathode.

Cations (+): Metal ions and Hydrogen ions are positive. Opposites attract, so they migrate to the Cathode (-). There, they gain electrons (Reduction).
Anions (-): Non-metal ions are negative. They migrate to the Anode (+). There, they lose electrons (Oxidation).

💡 Tutor's Tip

Remember OIL RIG for what happens at the electrodes: Oxidation Is Loss (of electrons), Reduction Is Gain. Reduction always occurs at the cathode, and Oxidation always occurs at the anode.

2. Molten Electrolysis (The Easy Mode)

When an ionic compound is molten (melted to a liquid, symbol 'l'), it contains NO WATER. There is no competition. The compound just splits in half.

Example: Electrolysis of molten Lead(II) Bromide (PbBr2).
Ions present: Pb2+ and Br-
Cathode product: Lead metal (Pb)
Anode product: Bromine gas (Br2)

3. Aqueous Solutions: The Cathode Race

Aqueous solutions (symbol 'aq') are dissolved in water. Water slightly ionises into H+ and OH- ions. This means there are now FOUR ions in the electrolyte competing to be discharged.

At the Cathode: The "Lazy" Rule

Both the positive Metal ion and the positive Hydrogen (H+) ion race to the cathode. The LEAST reactive element wins and gets discharged.

If the metal is ABOVE hydrogen in the reactivity series (e.g., Na, Mg, Al): Hydrogen gas is produced.
If the metal is BELOW hydrogen (e.g., Cu, Ag, Au): The metal is deposited.

4. Aqueous Solutions: The Anode Concentration Trap

At the Anode, the negative Non-metal ion and the OH- ion race for discharge. The rule here depends on concentration and halides.

The Anode Checklist:

Are there Halide ions present? (Cl-, Br-, I-).
If YES, is the solution concentrated? If yes, the Halogen gas is produced (Cl2, Br2, etc).
If NO (e.g., dilute solution, or contains sulfate/nitrate ions), the OH- ion wins. OH- ions discharge to produce Oxygen gas and Water.

📋 From the Desk of Dr. Aisha Rahman

The most frequently tested aqueous solution in CAIE Chemistry is Concentrated Aqueous Sodium Chloride (Brine). Students see 'Sodium' and 'Chlorine' and write those as the products. This is wrong! Na is highly reactive, so H+ wins at the cathode (Hydrogen gas). It is concentrated, so Cl- wins at the anode (Chlorine gas). The remaining Na+ and OH- ions stay in the solution, forming Sodium Hydroxide. You get 3 marks for knowing that!

Frequently Asked Questions

What does PANIC stand for in electrolysis?▼

Positive is Anode, Negative Is Cathode. It tells you the electrical polarity of the electrodes.

What is the difference between molten and aqueous electrolysis?▼

Molten means melted with pure heat (only 2 ions present). Aqueous means dissolved in water (4 ions present, including H+ and OH-). The products are usually completely different.

What is discharged at the cathode in an aqueous solution?▼

The least reactive positive ion. If the metal is highly reactive (e.g. Potassium), Hydrogen gas is discharged. If the metal is unreactive (e.g. Copper), Copper metal is discharged.

What happens at the anode in a concentrated halide solution?▼

The halogen gas (like Chlorine or Bromine) is produced. If it were a dilute solution, Oxygen gas would be produced from the OH- ions instead.

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Electrolysis Blueprint: Predicting Anode and Cathode Products