A Level Chemistry Chapter 20 Electrochemistry Detailed Summary Note including the topics - electrolysis, Faraday's Law, electrode potential, and standard cell potential
electrolysis is the breaking down of a compound into its elements
using an electric current
-electrolyte the compound that is broken down during electrolysis
%
(either molten ionic compound / conc. aqueous solution of ions)
-electrodes metal of graphite rods that conducts electricity
-power supply direct current (flows from anode to cathode)
electrolysis of molten electrolytes
-cations move to cathode where they get reduced
if metal is formed, a layer of metal is deposited on a cathode or
it forms a molten layer in the cell
-anions move to anode where they get oxidised
electrolysis of aqueous solutions
-solutions have more than one cation & anion due to presence of H O
-water contributes H and OH so the ions discharged during
electrolysis depends on relative electron potential and conc. of ions
relative electron potential (E)
describes how easily an ion is discharged during electrolysis
- cation with the most positive E will be discharged at cathode
bcs it is the most easily reduced ion
- anion with the most negative E will be discharged at the anode
bcs it is the most easily oxidised ion
concentration of ions
- ions that are present in higher concentrations are more likely
to be discharged
Faraday's Law
Q(charge in C) I(current in A) x t(time in s) =>
Q Ixt
-
-the amount/quantity of electricity can also be expressed by the
faraday (F) unit
-one F is the amount of electric charge carried by 1 mole of e or
1 mole of singly charged ions (1F 96,500 Cmol ) I
-
=
F(Faraday's constant) L(Avogadro's constant) x e(charge on e )
-
-
-
4
F Lxe
-
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date
Faraday's constant can be used to calculate
1) the mass of a substance deposited at an electrode
eg. calculating the amount of Mg deposited when a current of
21
2.20A flows through the molten magnesium bromine for 15 mins
-write the half-equation to find the no. of moles of e
-
Mg (aq) 2e Mg(s)
2+
I 7
(2 moles of e ) 2t
-determine the no. of Coulombs required to deposit 1 mol of Mg
I
96,500 x 2 193,000 Cmol
-
-calculate the charge transferred during electrolysis
Q Ixt-
2.20 x (60x50)
=
1980C
-
-calculate the mass of Mg deposited by using ratio
2+
2) the volume of gas liberated at an electrode
eg. calculating the volume of O produced at room temperature
2
when concentrated aqueous sulfuric acid is electrolysed for 35
mins using a current of 0.75A
-write the half-equation
4OH (aq) O (g) 2H O(l) 4e (4 moles of e )
-
-
1 +
-
-determine the no. of C required to form one mole of O z
96,500 x 4 386,000 Cmol
-
I
-
-calculate the charge transferred during electrolysis
Q Ixt
-
0.75 x (60x35)
=
1,575 C
-
-calculate the volume of O liberated by using ratio
z
(use the relationship that 1 mol of gas occupies 24.0dm at rtp) 1
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