Electrolysis and Faraday's laws of Electrolysis Electrolysis is defined as the passage of an electric current through an electrolyte with subsequent migration of positively and negatively charged ions to the negative and positive electrodes. Electrolysis process Chemical reactions in batteries or galvanic cells provide the driving force for electrons to struggle through loads. This is how chemical energy is transformed into electric energy.
Electrolysis in Battery Electrolysis can be carried out in solutions or molten salts (liquid). Because the atoms and ions have to move physically, the medium has to be a fluid. The products, like the reactants in a galvanic cell, can be in a solid, liquid, or gas state Chemical reactions in batteries or galvanic cells provide the driving force for electrons to struggle through loads. This is how chemical energy is transformed into electric energy. Electrolysis can be carried out in solutions or molten salts (liquid). Because the atoms and ions have to move physically, the medium has to be a fluid. The products, like the reactants in a galvanic cell, can be in a solid, liquid, or gas state. Electrolysis of Molten Salts Electrolysis is a process by which electrons are forced through a chemical cell, thus causing a chemical reaction. The positive charge usually attracts electrons, and the electrode providing electrons is called cathode, because reduction takes place on it. Reduction always takes place at the cathode, by definition. In the electrolysis of molten salt, NaCl, the cathode and anode reactions are: Cathode (reduction): Na + + e - = Na Anode (oxidation): 2 Cl - = Cl 2 + 2 e
Electrolysis of NaCl Electrolysis of Water Pure water does not conduct electricity, because the numbers of H + and OH - ions are small (10-7 mol/l each). In the presence of an acid, water can be decomposed. Electrolysis of water
A potential of -2.06 V is the standard cell potential for, Pt H 2 O, [H + ] = 1 M O 2 H 2 O [OH - ] = 1 M H 2 Pt And when a potential greater than 2.06 V is applied such that the forward cell has a positive potential, the following reactions take place Faraday s Laws of Electrolysis Faraday s Laws of Electrolysis give the relationship between the amount of material liberated at the electrode and the amount of electric energy that is passed through the electrolyte. Faraday's first law of electrolysis : According to this law, the chemical deposition due to flow of current through an electrolyte is directly proportional to the quantity of electricity (coulombs) passed through it. i.e. mass of chemical deposition.it states that the amount of any substance that is liberated at an electrode during electrolysis is directly proportional to the quantity of electricity passed through the electrolyte. W Q Q = I t W I t Therefore, W = Z I t Where W = Weight of substance deposited or liberated at the electrode Z = is the constant (electrochemical equivalent) I = current strength in ampere t = time in second Faraday's second law of electrolysis : states that, when the same quantity of electricity is passed through several electrolytes, the mass of the substances deposited are proportional to their respective chemical equivalent or equivalent weight. Consider two
cells connected in a series containing copper sulfate and silver nitrate and if the electric current passes through both the cells then, weight of cobalt α Equivalent weight of cobalt and weight of gold deposited α Equivalent weight of gold W Co α E Co W Au α E Au W α I x tx E Co W =Z x I x t x E Co Chemical Equivalent or Equivalent Weight The chemical equivalent or equivalent weight of a substance can be determined by Faraday s laws of electrolysis and it is defined as the weight of that sustenance which will combine with or displace unit weight of hydrogen. The chemical equivalent of hydrogen is, thus, unity. Since valency of a substance is equal to the number of hydrogen atoms, which it can replace or with which it can combine, the chemical equivalent of a substance, therefore may be defined as the ratio of its atomic weight to its valency. The basic unit of electrical charge is called Faraday which is defined as the charge on one mole of electrons. Electrolysis of sodium iodide solution to find out Faraday can be given as follows : Na + (l) + e- Na (l) Reduction 2 I - (l) I 2 (s) + 2e- Oxidation Net reaction 2Na + (l) + 2I - (l) 2Na(l) + I 2 (s) (Redox) The passage of 1 Faraday of charge will produce 1 mole of Sodium metal and 2
Faraday of charge will produce 1 mole of Iodine. The passage of 2 Faraday of charge will give 2 moles of sodium metal and 1 mole of iodine. Example Calculate the electrical current passing through diluted hydrochloric acid solution for on hour which causes to librating of 0.336 l of hydrogen and oxygen on platinum electrodes. Solution 1 Farad(96500 c) causes libration one equivalent ( 8 g of O 2 )at anode and one equivalent(1 g of H 2 )at cathode. 1 Farad causes to libration of 1/4 mole of oxygen and 1/2 mole of hydrogen. Total librated moles=1/4+1/2=3/4 mole Volume of librated gases by one farad at standard conditions is 3/4X22.4 l.mol- 1=16.8 L Electrical charge needed to libration of 0.336 L 0f O 2 and H 2 mixture is: (0.336 L x 96500 C)/(16.8 L)= 1930 C Current(I)=Electrical charge(q)=time(t) I=(1930 A.s -1 )/(3600 s) I=0.536 A