i - Electrolysis (copy)

1c (1.55-1.60)

Conductivity of ionic compounds

Conductivity of ionic compounds

• Can conduct electricity in molten state or in solution as they have ions that can move + carry charge

• Can’t conduct in electric state as ions are in fixed positions within lattice and are unable to move

Electrolysis

The breaking down of a substance using electricity

Electric current is passed through electrolyte (molten/dissolved ionic compound), causing it to decompose

Oxidation (loss of electrons) and reduction (gain of electrons) occur (OIL RIG )

Anion

Negative ion

Cation

Positive ion

Cations move towards…

Cathode (negative electrode) and are reduced (gain electrons)

Anions move towards…

Anode (positive electrode) and are oxidised (lose electrons)

As ions gain/lose electrons,…

They form uncharged substances and are discharged from electrolyte. for example gas or plating the rods

Molten ionic compounds

• Can be electrolysed because ions can move freely

• Usually broken up into their elements

Electrolysis of molten lead bromide (PbBr₂)

• +ve Pb²⁺ ions are attracted to -ve cathode
  At cathode, lead ion accepts two electrons and is reduced to lead atom

• -ve Br⁻ are attracted to +ve anode
  At anode, two bromide ions lose one electron each and are oxidisde to bromine molecule

• Electrodes are made from inert (unreactive) material so don’t take part in reaction

Molten electrolysis examples

Electrochemical cell

A circuit, made up of anode, cathode, electrolyte, power source and wires connecting two electrodes

How to set up electrochemical cell for aqueous solution

1. Get two inert electrodes e.g. graphite/platinum electrodes

2. Clean surfaces of two electrons using emery paper (or sandpaper)

3. From this point, be careful not to touch surfaces of electrodes with hands - could transfer grease back onto strips

4. Place both electrodes into beaker filled with electrolyte

5. Connect electrodes to power supply using crocodile clips + wires

   When you turn power supply on, a current will flow through cell

Electrolysis of aqueous solutions

• In aqueous solutions, as well as ions from ionic compund, there are hydrogen ions (H⁺) and hydroxide ions (OH⁻) from water

• At cathode, if H⁺ ions + metal ions are present, hydrogen gas is produced if metal is more reactive than hydrogen (e.g. sodium)

  If metal is less reactive than hydrogen (e.g. copper, silver), a solid layer of pure metal is produced instead

• At anode, if OH⁻ + halide ions (Cl⁻, Br⁻, I⁻) are present, molecules of chlorine, bromine or iodine will form
  If no halide ions present, oxygen will form

Electrolysis of sodium chloride (NaCl)

• Solution of sodium chloride contains four different ions: Na⁺, Cl⁻, OH⁻, H⁺

• Sodium metal is more reactive than hydrogen
  So at cathode, hydrogen gas produced

  2H⁺ + 2e⁻ → H₂

• Chloride ions are present in solution
  So at anode, chlorine gas produced

  2Cl⁻ → Cl₂ + 2e⁻

Aqueous electrolysis examples