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3.2.4Chemical Changes

Electrolysis and Redox

5 min read 6 parts Lesson 16 of 17 1 quiz

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Section 1 of 6Redox in terms of electrons

Oxidation
  • Oxidation Oxidation is the loss of electrons by a species.
Reduction
  • Reduction Reduction is the gain of electrons by a species.
Together
  • Together They always happen at the same time in a redox reaction, because electrons lost by one species are gained by another.
Memory aid
  • Memory aid OIL RIG: Oxidation Is Loss, Reduction Is Gain.

Section 2 of 6Where each process happens

1Ions reach electrodes
  • Ions reach electrodes Positive ions travel to the cathode and negative ions travel to the anode.
2Cathode
  • Cathode Positive ions gain electrons from the electrode, so they are reduced.
3Anode
  • Anode Negative ions lose electrons to the electrode, so they are oxidised.
4Neutral products
  • Neutral products The uncharged atoms or molecules are discharged as the products.

Section 3 of 6Half equations

ElectrolyteCathode (reduction)Anode (oxidation)
Molten lead(II) bromidePb2+ + 2e− → Pb2Br− → Br2 + 2e−
Aqueous sodium chloride2H+ + 2e− → H22Cl− → Cl2 + 2e−
Dilute sulfuric acid2H+ + 2e− → H22H2O → O2 + 4H+ + 4e−
Aqueous copper(II) chlorideCu2+ + 2e− → Cu2Cl− → Cl2 + 2e−

Section 4 of 6Writing a half equation

1
Write the ion that arrives at the electrode on the left, and the product on the right.
2
Balance the atoms, remembering that gases such as H2, Cl2 and Br2 are diatomic.
3
Add electrons to the left (cathode) or right (anode) so that the total charge on each side is equal.
4
Label the process: reduction at the cathode, oxidation at the anode.

Section 5 of 6Key terms

TermMeaning
OxidationThe loss of electrons.
ReductionThe gain of electrons.
Half equationAn equation showing only the oxidation or only the reduction, including electrons.
DischargeThe conversion of an ion into an uncharged atom or molecule at an electrode.
RedoxA reaction in which oxidation and reduction occur together.

Section 6 of 6Exam technique

1
Putting electrons on the wrong sidepositive ions gain electrons (left side), so reduction half equations show e− on the left.
2
Writing Cl or Br instead of Cl2 or Br2 as the product, which breaks the balance.
3
Forgetting that the charge must balance on both sides, not just the atoms.
4
Saying oxidation means gaining oxygen only; at this level the electron definition is used for electrolysis.
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.