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4.1Electrolysis

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

5 min read 19 parts Lesson 2 of 6 1 quiz

📚 Combined Science 0653 - Chemistry Hub / Topical Revision
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4.1 Electrolysis — COMBINED SCIENCE (CHEMISTRY) notes: Define electrolysis, identify the parts of an electrolytic cell and predict the products from molten compounds, brine and dilute sulfuric acid.Electrolysis — = breaking down of an ionic compound, molten or in aqueous solution…

  • Covalent compounds
  • Solid ionic compounds
  • Electrolyte
  • Memory aid
  • Inert electrodes
  • Binary compounds

Part 1 of 9What 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.

Part 2 of 9Parts 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).

Part 3 of 9Products 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.

Part 4 of 9Electrolysis 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.

Part 5 of 9Electrolysis 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.

Part 6 of 9Electrolysis of dilute sulfuric acid

Ions present
  • H+ and SO42- 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.

Part 7 of 9Tests for the gases made

GasTestResult
HydrogenLighted splintSqueaky pop
OxygenGlowing splintRelights
ChlorineDamp litmus paperTurns red then bleached white

Part 8 of 9Half-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.
ElectrolyteCathode half-equationAnode half-equation
Molten PbBr2Pb2+ + 2e- → Pb2Br- → Br2 + 2e-
Concentrated NaCl(aq)2H+ + 2e- → H22Cl- → Cl2 + 2e-
Dilute H2SO42H+ + 2e- → H24OH- → O2 + 2H2O + 4e-

Part 9 of 9More 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.

Key terms

TermMeaning
ElectrolysisDecomposition of an ionic compound by electric current.
ElectrolyteMolten or dissolved ionic compound that conducts.
ElectrodeRod that carries current into or out of the electrolyte.
AnodePositive electrode.
CathodeNegative electrode.
OxidationLoss of electrons.
ReductionGain of electrons.
Inert electrodeElectrode that does not take part in the reaction.

Avoid these mistakes

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.

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