Electrolysis — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

Section 1 of 19What electrolysis is

Electrolysis

• = breaking down of an ionic compound, molten or in aqueous solution, by an electric current.

Covalent compounds

• Covalent compounds cannot undergo electrolysis because they do not conduct electricity.

Solid ionic compounds

• Solid ionic compounds cannot conduct because the ions are fixed in place.
• When molten or in solution the ions are free to move and carry charge.

Section 2 of 19Parts of an electrolytic cell

Parts of an electrolytic cell

Electrode

• Electrode a rod, usually metal or graphite, that carries current into or out of the electrolyte.

Electrolyte

• Electrolyte the molten or dissolved ionic compound that conducts electricity.

Anode

• Anode the positive electrode, which attracts anions (negative ions).

Cathode

• Cathode the negative electrode, which attracts cations (positive ions).

Memory aid

• Positive Anode, Negative Is Cathode (PANIC).

Section 3 of 19Products at each electrode

Cathode

• Cathode metals or hydrogen are formed because they form positive ions.
• If the metal is less reactive than hydrogen, such as copper or silver, the metal forms.
• If the metal is more reactive than hydrogen, such as sodium or magnesium, hydrogen forms in solution.

Anode

• Anode non-metals other than hydrogen are formed because they form negative ions.

Inert electrodes

• Inert electrodes graphite or platinum are used so that they do not react.

Section 4 of 19Electrolysis of molten lead(II) bromide

1

Heat lead(II) bromide in a beaker until it is molten.

2

Place two graphite electrodes in it and connect them to a power supply.

3

Bromide ions move to the anode and lose electrons, forming brown bromine gas.

4

Lead ions move to the cathode and gain electrons, forming grey lead metal.

• Binary compounds
  • molten binary ionic compounds always give the metal at the cathode and the non-metal at the anode.
  • Molten potassium chloride gives potassium and chlorine.

Section 5 of 19Electrolysis of concentrated aqueous sodium chloride (brine)

Ions present

• Na⁺ and Cl^- from the salt and H⁺ and OH^- from water.

Cathode

• H⁺ ions are discharged because they are less reactive than sodium, forming hydrogen gas.

Anode

• Cl^- ions are discharged, forming chlorine gas.

Left in solution

• Na⁺ and OH^- ions, which make sodium hydroxide solution.

Uses

• Uses chlorine makes bleach, hydrogen makes margarine and sodium hydroxide makes soap and detergents.

Section 6 of 19Electrolysis of dilute sulfuric acid

Ions present

• H⁺ and SO₄^2- from the acid plus H⁺ and OH^- from water.

Cathode

• H⁺ ions gain electrons to form hydrogen gas.

Anode

• OH^- ions lose electrons to form oxygen gas and water.

Volumes

• Volumes twice as much hydrogen is made as oxygen, as seen in a Hoffman voltameter.

Section 7 of 19Tests for the gases made

Gas	Test	Result
Hydrogen	Lighted splint	Squeaky pop
Oxygen	Glowing splint	Relights
Chlorine	Damp litmus paper	Turns red then bleached white

Section 8 of 19Half-equations and charge flow

1Oxidation

• = loss of electrons and occurs at the anode (AN OX).

2Reduction

• = gain of electrons and occurs at the cathode (RED CAT).

3Electrons

• Electrons flow through the external circuit from the anode to the cathode and not through the solution.

4Ions

• Ions carry charge through the electrolyte.

Electrolyte	Cathode half-equation	Anode half-equation
Molten PbBr2	Pb2+ + 2e- → Pb	2Br- → Br2 + 2e-
Concentrated NaCl(aq)	2H+ + 2e- → H2	2Cl- → Cl2 + 2e-
Dilute H2SO4	2H+ + 2e- → H2	4OH- → O2 + 2H2O + 4e-

Section 9 of 19More from the specification

Halide

• A negative ion formed when a halogen gains one electron.

Sulfate

• Salt of sulfuric acid.

Reactivity series

• A list of metals in order of how readily they react.

Section 10 of 19Principles of electrolysis

Principles of electrolysis

Electrolysis

Electrolysis the breakdown of a molten or dissolved ionic compound by an electric current.

Electrolyte

• Electrolyte the ionic compound, molten or in solution, that conducts the electricity.

Solid ionic compounds do not conduct

• Solid ionic compounds do not conduct their ions are fixed in place;
  • molten or dissolved, ions are free to carry charge.

Covalent compounds

• Covalent compounds do not conduct, so they cannot be electrolysed.

Anode

Anode the positive electrode, which attracts anions; cathode = the negative electrode, which attracts cations.

Molten binary compounds

• Molten binary compounds always give the metal at the cathode and the non-metal at the anode.

Inert electrodes

• Inert electrodes graphite or platinum, so they do not react with the electrolyte.

Section 11 of 19Ions in aqueous solutions

Water

• Water aqueous solutions always contain water, which dissociates slightly: H₂O → H⁺ + OH^-.

Competition

• Competition ions from the compound and ions from water are both present, so only one ion at each electrode is discharged.

Concentration

• Concentration concentrated and dilute solutions of the same compound can give different products.

Section 12 of 19Products at the cathode

Products at the cathode

Ions attracted

• H⁺ and metal ions, but only one type gains electrons.

Metal above hydrogen in the reactivity series

• Metal above hydrogen in the reactivity series hydrogen gas is produced, because the metal ions stay in solution.

Metal below hydrogen (copper, silver)

• Metal below hydrogen (copper, silver) the metal is produced and plates onto the cathode.

Half-equation for hydrogen

• 2H⁺ + 2e^- → H₂.

Half-equation for a metal

• Half-equation for a metal for example Al³⁺ + 3e^- → Al.

Section 13 of 19Products at the anode

Products at the anode

Ions attracted

• OH^- and other negative ions, but only one type is discharged.

Halide ions present (concentrated)

• Halide ions present (concentrated) the halogen is produced at the anode.

No halide ions, or a dilute halide solution

• OH^- is discharged and oxygen is produced: 4OH^- → 2H₂O + O₂ + 4e^-.

Example

• Example concentrated barium chloride gives chlorine at the anode; dilute barium chloride gives oxygen.

Other ion

• Other ion the remaining negative ion stays in solution.

Section 14 of 19Common aqueous electrolytes

Aqueous solution	Anode	Cathode	Notes
Concentrated sodium chloride (brine)	Chlorine	Hydrogen	Sodium hydroxide solution remains
Dilute sodium chloride	Oxygen	Hydrogen
Copper(II) sulfate (graphite electrodes)	Oxygen	Copper	Blue colour fades
Dilute sulfuric acid	Oxygen	Hydrogen	Twice as much hydrogen as oxygen by volume

Section 15 of 19Copper(II) sulfate with graphite electrodes

Ions present

• Cu²⁺ and SO₄^2- from the salt, and H⁺ and OH^- from water.

Cathode

• Cathode copper is below hydrogen, so Cu²⁺ gains electrons and is reduced: Cu²⁺ + 2e^- → Cu.

Anode

• OH^- loses electrons more readily than SO₄^2-, so oxygen forms.

Observations

• Observations a pink-brown layer of copper on the cathode, bubbles of colourless gas at the anode, and the blue colour fading.

Section 16 of 19Copper(II) sulfate with copper electrodes

Anode

• Anode copper atoms are oxidised and dissolve: Cu → Cu²⁺ + 2e^-, so the anode loses mass.

Cathode

• Cu²⁺ is reduced to copper, so the cathode gains mass.

Equal masses

• Equal masses the mass gained by the cathode equals the mass lost by the anode.

Solution

• Solution the Cu²⁺ concentration and the blue colour stay constant.

Uses

• Uses purifying copper and electroplating.

Section 17 of 19Charge transfer, oxidation and reduction

Electrons

• Electrons carry charge in the external circuit, flowing from the negative terminal to the cathode and from the anode to the positive terminal.

Ions

• Ions carry charge in the electrolyte; cations move to the cathode and anions to the anode.

Oxidation

Oxidation loss of electrons; reduction = gain of electrons.

At the anode

• At the anode negative ions lose electrons, so oxidation occurs.

At the cathode

• At the cathode positive ions gain electrons, so reduction occurs.

Memory aids

• OIL RIG and RED CAT / AN OX (reduction at the cathode, oxidation at the anode); PANIC (positive anode, negative is cathode).
  • (oxidation is loss, reduction is gain)

Section 18 of 19Key terms

Term	Meaning
Electrolysis	Decomposition of an ionic compound by electric current.
Electrolyte	Molten or dissolved ionic compound that conducts.
Electrode	Rod that carries current into or out of the electrolyte.
Anode	Positive electrode.
Cathode	Negative electrode.
Oxidation	Loss of electrons.
Reduction	Gain of electrons.
Inert electrode	Electrode that does not take part in the reaction.

Term	Meaning
Electrolysis	Decomposition of an ionic compound using electricity.
Electrode	Conductor through which current enters or leaves the electrolyte.
Anode	Positive electrode; oxidation occurs here.
Cathode	Negative electrode; reduction occurs here.
Discharged	Having lost or gained electrons at an electrode.
Half-equation	Equation showing the change at one electrode.

Section 19 of 19Exam technique

1

Saying sodium forms from aqueous sodium chloride — hydrogen forms because sodium is more reactive than hydrogen.

2

Swapping the anode and cathode charges — the anode is positive.

3

Saying electrons travel through the electrolyte — ions carry the charge there.

4

Using a reactive electrode and forgetting it can take part in the reaction.

5

Saying oxidation is gain of electrons — it is loss.

1

Saying that sodium is produced from aqueous sodium chloride — hydrogen is produced.

2

Swapping anode and cathode charges — the anode is positive.

3

Writing a half-equation without electrons.

4

Thinking oxidation is gain of electrons — it is loss.

5

Forgetting that water contributes H⁺ and OH⁻ ions.