Electrolysis

electrolysis

breaks down a substance using elecricity

During electrolysis

an electric current is passed through an electrolyte,the ions move towards the electrode where they react and the compound decomposes

electrolyte

a molten or dissolved ionic compound

positive ions in the electrolyte will

move towards the the cathode and gain electrond (reduced)

Negative ions in the electrolyte will

move toward the anode and lose electrons (oxidised)

cathode

Negative electrode

anode

Positive electrode

Ions travelling to the elecrodes create

a flow of charge through the electrolyte

as ions gain or lose electrons they form

The uncharged element and are discharged from the electrolyte

An ionic solid can't be electrolysed because

The ions are in fixed positions and can't move

molten ionic compounds can be electrolysed because

The ions can move freely and conduct electricity

positive metal ions are

reduced to the element at the cathode

Negative non-metal ions are

oxidised to the element at the anode

half equations show

The reactions at the electrodes

when you combine half equations

the number of electrons is the same for each half equation

when electrolysis happens in aqueous solution

the less reative element (out of hydrogen or the metal) is produced at the cathode, oxygen is given out unless there is a halogen at the anode

In aqueous solutions

as well as the ions from the compound there will be hydrogen ions (H+) and hydroxide ions (OH-) from the water

Half equation for hydrogen

2H+ + 2e- \= H2

half equation at anode (no halogen)

4OH- \= O2 + 2H2O + 4e-

Aluminium is

extracted from the ore bauxite by electrolysis

Bauxite contains

Aluminium oxide

Alumminium oxide has a very high melting temperature so is

mixed with cryolite to lower the melting point

In electrolosis of alumminium

the positive Al+3 ions are attracted to the cathode where they pick up three electrons and turn into neutral aluminium atoms that sink to the bottom. the negative O2- ions are attracted to the anode where they loose two electrons, the neutral oxygen atoms combine to form O2 molecules

carbon anodes have to be replaced because

they gradually burn away as the oxygen reacts with them forming carbon dioxide

products of electrolysis of sodium chloride solution (brine)

chlorine and hydrogen gas given off and sodium hydroxide solution

Investigating electrolysis of a solution practical

1. set up the electrolysis apparatus: fill the cell with sodium chloride solution and ensure tubes are filled
2. Clamp the electrolysis cell into place and turn on battery
3. Time using stopwatch for 5 minutes to observe the reaction

positive chloride test

bleaches damp litmus paper

hydrogen positive test

Squeaky pop

oxygen positive test

relights glowing splint

to test volume of hydrogen produced

observe the amount of bubbles produced at the cathode