Electrolysis

electrolysis is the process of using electrocity to break down a compound. electrolysis takes place in an electrolytic cell.electrolytic cell consist from battery, electrode and electrolyte.

electrolyte is a molten ionic compound or an aqueous solution that conduct electricity. electrolyte has mobile ion that allow electricity to flow.

electrodes is the solid that conduct electricity and connect the battery to the electrolyte. example carbon, platinum and copper.

solid ionic compounds can't conduct electricity because the ions is solid compounds are held in fixed positions in a lattice and because they are unable to move to conduct electricity.

when electrolysis the external circuit during electrolysis, electrons flow from the negative terminal to the positive terminal of the battery.
>> Cations (+) move towards the cathode (-)
>> Anions (-) move towards the anode (+)

~ at the cathode
 cations receive electrons from the cathode and reduction occurs at the cathode.

~at the anode
anions give up electrons at the anode and oxidation occurs here.

the process of gaining or losing electrons at the electrodes is called discharge. when ions are discharged at the electrodes, they form atoms or molecules.

• electrolysis of molten sodium chloride

NaCl is binary compound, a binary compound is a compound containing only two elements. when solid sodium chloride is heated strongly, it melts at 801 celcius, it forms mobile Na+ ions and Cl- ions.
 Cathode = Na+ + e- -> Na
 Anode   = 2Cl-        -> Cl2 + 2e-
answer = 2NaCl (l) -> 2Na (l) + Cl2

inert electrodes such as carbon electrodes are used to prevent reactions from occuring between chlorine and the electrode.

• electrolysis of dilute sodium chloride

NaCl (aq)
cathode = Na+ & H+
anode    = Cl- & OH-

electrolysis of dilute NaCl is essentially the electrolysis of water. since water is being removed , the concentration of NaCl solution increases gradually.

• Molten NaCl

cathode = Na+ ions discharged
anode = Cl- discharged

• Dilute NaCl

cathode =H+ ions discharged
anode = OH- discharged
 in the electrolysis of dilute NaCl solution, H+ ions are discharged in preference to Na+ ions

•  electrolysis of concentrated NaCl

being concentrated,Cl- ions are discharged as chlorine gas. equal volumes of hydrogen gas chlorine gas are produced. the resulting solution becomes alkaline because there are more OH- ions than H+ ions left the solution.
concentrated NaCl = brine


Electrolysis Purification 

to purify copper,we perform electrolysis of aqueous copper (II) sulphate, but using copper electrodes. in this case, the electrodes are called reactive electrodes.

Purification of copper

anode= impure copper
cathode= a thin sheet of pure copper
during electrolysis, the impure copper anode dissolves. impurity such as silver and platinum fall to the bottom of the cell, they are called anode slime.

electrolysis - copper plating 

anode= pure copper
cathode = the metal object to be copper-plated
electrolyte= aqueous copper (II) sulphate

oxidation

a substance is oxidised if one the following happens after reaction

• it gains oxygen
• it loses hyfrogen
• it loses electrons
• it increase its oxidation state

Oxidation state

change an atom of an element would have if it existed as an ion in a compound (even if it actually covalently bended)

Rules

1. the oxidation state of a free element is zero
2. the oxidation state of a simple ion is the same as the change on the ion