Lesson 1 · Electrochemistry

⚡ Electrolysis

Electricity can split compounds that heat alone cannot. Electrolysis is how aluminium is made, copper is purified and objects are plated with silver.

🔋 What happens at the electrodes

Electrolysis uses direct current to decompose an ionic compound that is molten or dissolved, so the ions are free to move. Positive ions (cations) move to the cathode (negative electrode) and gain electrons: reduction. Negative ions (anions) move to the anode (positive electrode) and lose electrons: oxidation.

molten lead(II) bromide, PbBr₂ cathode: Pb²⁺ + 2e⁻ → Pb (grey lead) anode: 2Br⁻ → Br₂ + 2e⁻ (brown bromine) PANIC: Positive Anode, Negative Is Cathode

🖼️ See it

Remember PANIC: Positive Anode, Negative Is Cathode.

In solutions, water adds H⁺ and OH⁻ ions: metals less reactive than hydrogen are deposited, and a halide ion is discharged in preference to hydroxide.

🧂 Aqueous solutions

Water also supplies H⁺ and OH⁻ ions, so there is a choice at each electrode. At the cathode, hydrogen forms unless the metal is less reactive than hydrogen (such as copper or silver). At the anode, a halogen forms if a halide ion (Cl⁻, Br⁻, I⁻) is present; otherwise oxygen forms.

concentrated NaCl(aq): cathode H₂, anode Cl₂ (NaOH is left in solution) CuSO₄(aq): cathode Cu, anode O₂ Electroplating: the object to plate is the cathode the plating metal is the anode

Charge: Cu²⁺ + 2e⁻ → Cu. Two moles of electrons deposit one mole of copper atoms.

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Electrodes

Positive ions are attracted to the:

Opposites attract.

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Oxidation

What happens at the anode?

OIL RIG: oxidation is loss.

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Why melt?

A solid ionic compound does not conduct electricity because:

Ions are locked in the lattice.

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Brine

What forms at the cathode in concentrated NaCl solution?

Compare with hydrogen.

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Plating

To silver-plate a spoon, the spoon should be the:

Metal ions go to the cathode.

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The half-equation Cu²⁺ + 2e⁻ → Cu.

How many moles of electrons are needed to deposit 1 mole of copper?

Enter an exact answer, without rounding.

Look at the coefficient of e⁻.

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Cu²⁺ + 2e⁻ → Cu.

How many moles of electrons are needed to deposit 0.5 mole of copper?

Enter an exact answer, without rounding.

Multiply by 2.

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Aqueous copper(II) sulfate is electrolysed. 0.05 mol of copper is deposited (Ar of Cu = 64).

What mass of copper is this, in g?

Enter an exact answer, without rounding.

Mass = moles × Ar.

📝 Lesson quiz · 5 questions

Question 1 of 5

In electrolysis, the electrolyte is:

Question 2 of 5

What forms at the anode when molten lead(II) bromide is electrolysed?

Question 3 of 5

Metals are deposited at the:

Question 4 of 5

In aqueous copper(II) sulfate with inert electrodes, the product at the anode is:

Question 5 of 5

Aluminium is extracted by electrolysis because:

Final Challenge

🏆 8-Question Challenge Quiz

Questions from every topic in this course. Pick an answer for each one, then hit Check My Answers.

Q1 · Electrolysis

During electrolysis, reduction occurs at the:

Q2 · Electrolysis

In electrolysis, positive ions move to the:

Q3 · Electrolysis

Electrolysis of molten lead(II) bromide produces at the cathode:

Q4 · Electrolysis

Molten ionic compounds conduct electricity because:

Q5 · Electrolysis

At the anode, negative ions:

Q6 · Electrolysis

Electrolysis of concentrated sodium chloride solution gives at the cathode:

Q7 · Electrolysis

Electrolysis is used in industry to:

Q8 · Electrolysis

In copper plating, the object to be plated is the: