C4: Electrochemistry

  • Conductivity:

    • Metals and graphite conduct electricity (have free electrons)

  • Electrolysis:

    • The decomposition of of an ionic compound, when molten/aqueous, by the passage of an electric current

  • Electrolysis and the movement of electrons in the external circuit

    • Electrolytes: liquids that conduct electricity

    • When electrolytes conduct electricity→ ionic compound is decomposed

    • loss/gain of electrons at the electrodes

    • movement of ions in the electrolyte

  • Electrolytic cells

    • The apparatus electrolysis is carried out:

      • Current supplied by battery

      • Inert electrodes (often graphite) carry current into/out of electrolyte

      • Electrons flow from negative terminal → cathode → anode → positive terminal

    • Positive ions (metal ions/H+ ions) → cathode (negative) - cations

    • Negative ions (non-metal ions) → anode (positive) - anions

    • Charge is transferred:

      • Cathode: electrons move from electrode to cations

      • Anode: electrons move from anion to electrode

      • Electrolyte: charged is transferred by the movement of ions present. Positive ions → negative cathode/ negative ions → positive anode

Products of electrolysis

  • Molten ionic compounds

    • Metal formed at cathode

    • Non-metal formed at anode

  • Solutions/aqueous solutions

    • Negative OH– ions and non-metal ions are attracted to the anode

    • If halide ions (Cl–, Br–, I–) are present, the halogen is produced at the anode

      • If concentrated halide → halide forms

      • If dilute halide → oxygen forms

    • If there are no halide ions but OH– ions are present, oxygen is produced at the anode

    • H+ and metal ions are attracted to the cathode but only one will gain electrons

    • If the metal is above hydrogen in the reactivity series:

      • The ions of the metal remain in the solution

      • hydrogen will be produced and bubbling will be seen at the cathode

    • If the metal is below hydrogen in the reactivity series:

      • The less reactive metal ions are discharged

      • So, the metal is produced and this will be seen plating onto the cathode

  • Electrolysis of aqueous sodium chloride: using inert electrodes

    • Product at the negative electrode:

      • The H+ ions are discharged at the cathode as they are less reactive than sodium ions

      • The H+ ions gain electrons to form hydrogen gas 

    Product at the positive electrode: using inert electrodes

    • The Cl– ions are discharged at the anode 

    • They lose electrons and chlorine gas forms

    • The Na+ and OH– ions remain behind and form the NaOH solution

  • Electrolysis of dilute sulfuric acid 

    • negative electrode:

      • H+ ions are attracted to the cathode 

      • The H+ ions gain electrons to form hydrogen gas 

    • positive electrode:

      • OH- ions are attracted to the anode

      • They lose electrons and form oxygen gas and water

* Reactivity series:

  • Please - potassium(k)

  • Send - sodium(Na)

  • Cats - calcium(Ca)

  • Monkeys - magnesium(Mg)

  • And - Aluminum(Al)

  • Zeebras - zinc(Zn)

  • In - iron(Fe)

  • Happy - [hydrogen](H)

  • Cages of - copper(Cu)

  • Silver - silver(Ag)

  • Gold - gold(Au)

  • Platinum - Platinum(Pt)

  • Electroplating

    • A process where the surface of one metal is coated with a layer of a different metal

    • Cathode: being electroplated

    • Anode: made from pure metal that will plate the object

    • electrolyte: aqueous solution of soluble salt of pure metal

  • Metal purifying: process of seperating pure metal from impurities

    • Cathode: sheet of pure metal

    • anode: lump of impure metal

    • electrolyte: aqueous solution

    • metal we want to purify transfers to cathode from impure anode

  1. Hydrogen-oxygen fuel cells

  • A fuel cell is an electrochemical cell in which a fuel donates electrons at one electrode and oxygen gains electrons at the other electrode

    • H2 → 2H+ + 2e– 

    • O2 + 4e– → 2O2– 

  • The hydrogen-oxygen fuel cell produces electricity by combining both elements, releasing energy and water

  • The overall equation for the reaction within a hydrogen fuel cell is:

hydrogen   +   oxygen   →   water

2H2 + O2 → 2H2O

  • The diagram below shows the setup of a hydrogen fuel cell 

    • The air entering provides the oxygen

    • The fuel entering is hydrogen

    • The only chemical product made is water