Edexcel IGCSE Chemistry - Electrolysis

what is electrolysis?

the breaking down of a substance using electricity

what is electrolyte?

a molten or dissolved ionic compound that electric current passes through during electrolysis

which two process take place during electrolysis?

• oxidation

• reduction

what are positive ions called?

cations

what are negative ions called?

anion

what is the negative electrode called?

cathode

what is the positive electrode called?

anode

what happens to cations during electrolysis?

they are reduced

what happens to anions in electrolysis?

they are oxidised

what happens when the ions gain or lose electrons?

they form uncharged substances and become discharged from the electrolyte

what equations are used for electrolysis?

ionic half equations

what do ionic half equations show?

show how electrons are transferred

how do you form an ionic half equation?

• put one of the things being oxidised or reduced on one side of the arrow, and the things it gets oxidised or reduced to on th eother

• balance the numbers of atoms just like in a normal equation

• then add electrons to one side to balance up the charges

what are the two types of compounds that can be electrolysed?

• molten ionic compounds

• aqueous ionic compounds

why can molten ionic compounds be electrolysed?

ions can move freely and carry charge

what is a molten compound broken into when it is electrolysed?

its elements - ions first and then elements when they reach the electrodes

what are electrodes made from?

inert (unreactive metal) so they don’t take part in the reaction

if molten lead bromide is electrolysed, what products will there be?

• at the cathode: lead (II) ions become discharged to form lead → silver coating

• at the anode: bromide ions become discharged to form bromine → orange solution

what are the half equations for the electrolysis of molten lead bromide?

how would you set up a circuit for electrolysis (carry electrolysis out)?

1. get two inert electrodes - graphite or platinum

2. clean the surfaces of the electrodes with emery paper

3. don’t touch the electrodes form now on

4. place both electrodes into a beaker filled with your electrolyte - should dip into the solution

5. connect the electrodes to a power supply using crocodile clips and wires

6. when you turn the power supply on, a current will flow through the cell

7. electrolysis will happen

8. after a few minutes record observations

what ions are present in aqueous solution?

• ions from the ionic compound

• hydrogen ions (H⁺)

• hydroxide ions (OH^-)

how do you determine what is produced at the cathode during the electrolysis of an aqueous compound?

• either the metal ion or the hydrogen ion will be reduced at the cathode

• whichever is least reactive will be produced → if the metal is more reactive, hydrogen will be produced and vice versa

how do you determine what is produced at the anode during the electrolysis of an aqueous compound?

• the non-metal ion or the hydroxide ion could be oxidised at the anode

• if the non-metal ion is a halide ion it will be oxidised at the anode, and a halogen will be produced

• if the non-metal ion is not a halide ion (is polyatomic), hydroxide ions will be oxidised and oxygen (and water) will be produced

what is the half equation for the production of oxygen at the anode?

what is the half equation for the production of hydrogen at the cathode?

what products are formed when aqueous sodium chloride is electrolysed?

• cathode: (hydrogen ions →) hydrogen - bubbles

• anode: (chloride ions →) chlorine - green-yellow gas

• left behind: sodium hydroxide solution

what are the half equations for the electrolysis of aqueous sodium chloride?

what products are formed when aqueous dilute sulfuric acid is electrolysed?

• cathode: hydrogen → bubbles (no metal)

• anode: oxygen → bubbles

what are the half equations for the electrolysis of aqueous dilute sulfuric acid?

what products are formed when aqueous copper (II) sulfate is electrolysed?

• cathode: copper → salmon pink deposit

• anode: oxygen → bubbles

what are the half equations for the electrolysis of aqueous copper (II) sulfate?

what is bauxite?

bauxite = Al₂O₃ = aluminium [(III)] oxide

describe the apparatus for the electrolysis of aluminium oxide (bauxite)

electrodes = made of graphite

aluminium oxide has very high melting point

what is cryolite?

a solvent which lowers the boiling point of bauxite

what are the products formed during electrolysis of bauxite?

• cathode: aluminium ions → aluminium

• anode: oxygen ions → oxygen

what are the half equations for the electrolysis of bauxite?

• cathode: Al³⁺+ 3e^-.→ Al (l)

• anode: 2O^2- → O₂ (g) + 4e^-

which apparatus is used for the electrolysis of water (with sulfuric acid added)?

hoffman apparatus