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7.1.1Practical Skills

Electrolysis

5 min read 7 parts Lesson 2 of 6 1 quiz

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Section 1 of 7The electrolysis apparatus

Electrolyte
  • Electrolyte The ionic compound that is broken down, either molten or dissolved in water.
  • Its ions are free to move, so they can carry charge through the liquid.
Electrodes
  • Electrodes Two rods, often made of inert graphite or platinum, connect the electrolyte to a direct-current supply.
  • The cathode is the negative electrode and the anode is the positive electrode.
Why a solid will not work
  • Why a solid will not work In a solid ionic compound the ions are held in a lattice and cannot move, so no current flows.

Section 2 of 7What happens during electrolysis

1Ions move
  • Ions move Positive ions (cations) are attracted to the cathode and negative ions (anions) to the anode.
2Electron transfer
  • Electron transfer At the cathode, positive ions gain electrons. At the anode, negative ions lose electrons.
3Elements formed
  • Elements formed The ions are discharged and become uncharged atoms or molecules.
4Compound decomposed
  • Compound decomposed The electrolyte is broken down into its elements, such as a metal and a non-metal.

Section 3 of 7Predicting products from a molten compound

Molten electrolyteProduct at cathode (–)Product at anode (+)
Lead(II) bromide, PbBr2LeadBromine
Sodium chloride, NaClSodiumChlorine
Zinc chloride, ZnCl2ZincChlorine
Aluminium oxide, Al2O3 (in molten cryolite)AluminiumOxygen

Section 4 of 7Solutions in water

Two competing ions at each electrode
  • Two competing ions at each electrode Water itself contains a few H+ and OH– ions, so each electrode has a choice of ions to discharge.
  • At the cathode, hydrogen is produced if the metal is more reactive than hydrogen; a metal that is less reactive than hydrogen,
    • such as copper, is deposited instead.
At the anode
  • At the anode Oxygen is produced from hydroxide ions unless a halide ion (chloride, bromide or iodide) is present, in which case the halogen is formed.
  • For example, concentrated sodium chloride solution gives hydrogen at the cathode and chlorine at the anode, leaving sodium hydroxide solution.

Section 5 of 7Practical points and uses

1
Set up the electrolyte in a beaker with two inert electrodes connected to a power supply.
2
Switch on and observe each electrode, noting any gas bubbles or metal deposits.
3
Test the gases collected: chlorine bleaches damp litmus paper, hydrogen gives a squeaky pop with a lighted splint, and oxygen relights a glowing splint.
4
Take care with toxic gases such as chlorine and bromine by working in a fume cupboard or with a small quantity.
5
Industrial uses include extracting reactive metals such as aluminium and producing chlorine and sodium hydroxide.

Section 6 of 7Key terms

TermMeaning
ElectrolysisThe decomposition of an ionic compound, molten or in solution, using an electric current.
ElectrolyteA molten or dissolved ionic compound that conducts electricity because its ions can move.
CathodeThe negative electrode, where positive ions gain electrons.
AnodeThe positive electrode, where negative ions lose electrons.
Inert electrodeAn electrode, such as graphite, that does not react with the electrolyte.

Section 7 of 7Exam technique

1
Saying electrons flow through the electrolyteelectrons travel in the wires, while ions carry the current in the liquid.
2
Mixing up the electrodespositive ions go to the negative cathode.
3
Stating that a solid ionic compound conducts: its ions are fixed in place, so it does not.
4
Forgetting that water contributes hydrogen and hydroxide ions in a solution, which changes the products.
5
Writing sodium as the cathode product from sodium chloride solution: hydrogen forms instead because sodium is more reactive.
كيمياء كهربية — المقارنة الحاسمة قواعد الإشارات ونصف التفاعل

قواعد الأنود والكاثود وسريان الإلكترونات

  • الأنود دائماً: قطب الأكسدة (فقد إلكترونات)؛ إشارته سالبة في الجلفانية، وموجبة في الإلكتروليتية.
  • الكاثود دائماً: قطب الاختزال (اكتساب إلكترونات)؛ إشارته موجبة في الجلفانية، وسالبة في الإلكتروليتية.
  • سريان الإلكترونات: دائماً في الدائرة الخارجية عبر السلك من الأنود إلى الكاثود.
  • القنطرة الملحية: تتدفق الأنيونات (-) نحو نصف خلية الأنود، والكاتيونات (+) نحو نصف خلية الكاثود لمعادلة الشحنات الزائدة.