Chemistry Class Notes. 1 May 2013
CELLS
In chemistry, a cell is a device in which redox reaction takes place.
Electrodes are metal wire/rod/strip on which redox reaction takes place
Anodes are the electrode on which oxidation takes place
-negative
-electrons leave here
Cathodes are the electrode on which reduction takes place
-positive
-electrons come here
2 types of cell
1.electrochemical-where electrical energy is produced at the expense of
chemical change
2.electrolytic cell- involves use of electrical energy for chemical change
ELECTRODES
Whenever any metal is dipped in its ionic solution there exists and maintains equilibrium
between it and its ionic state. This is an electrode.
Whenever you join any two electrodes together, one will gain electrons, and one will
lose electrons. A.K.A. redox reactions will take place.
The electrons flow –ve to +ve.
The electricity flows the opposite way, +ve to –ve.
In all redox reactions anodes will have an excess of –ve ions, cathodes will have an
excess of positive ions.
After a while the reaction will stop, the solutions will repel the electrons because of their
charge.
SALT BRIDGE
Can only be made of KCl, KNO3, NH4NO3, usually we use KNO3, the ions and cations
have a very close ionic speed. Has to be semi-solid or aqueous.
The salt bridge carries the excess ions to the other side.
2 function
1. To complete circuit
a. It makes a full circuit, now two bridges, aka a circle
2. To neutralize the excess charge, to maintain electrical neutrality in
solution
Agar-agar, means it’s semi-solid, jelly-like.
3 May 2013
Reactivity series, same as electrochemical series
ELECTRO CHEMICAL CELLS
Anode, negative, typically on the left
Cathode, positive, right
Any two metals dipped in their ionic solution connected by a wire will conduct
electricity
What is neutrality?
Excess positive on the left hand side, in the anode
Excess negative, electrons, on the right, cathode
Salt can conduct electricity in Molten state, jelly
Emf
Emf= Electro-motive force = Eright-Eleft (Electro potential of the right – electrode
potential of the left)
Es given in data booklet
Negative, or 0, means NO reaction will happen
It has to be positive for a reaction to take place
Example
Iron rod in CuSO4, copper sulfate is blue
Fe -> fe 3+ = 3e-, oxidizing, left
Cu2+ +2e- -> Cu (s) , reducing, right
Emf = 0.34 – -0.45
=0.79 V
A reaction takes place
ELECTROCHEMICAL SERIES
From it you can predictl what will replace what
Whatever is going to replace should be stronger, closer to the top of the reactivity series
A stronger metal will replace a weaker metal
Whatever metal is costlier wil not react, gold, platinum, they’re nice because they don’t
react
SPM
Semi permeable membrane
Only allows solvent to pass through it
Water can get into an egg, but the egg part can’t get out
ELECTROLYTIC CELL
Opposite to electro chemical, negative on the right, positive on the right
Used to purify costly metals, the one on the left is impure and it dissolves, and the right
one is more pure
The impure metal that dissolves is called anode mud
Two copper rods in copper sulfate solution
Cu -> cu2+ + 2e-, oxidation
Cu 2+ 2e- -> cu , reduction
Electrochemical
1. Produces electricity
2. Anode is negative, cathode is
positive
3. Salt bridge needed
4.
Electrolytic
1. Uses electricity
2. Anode is positive
3. No Salt bridge required
NaCl:
Na -> Na+ +eNa + H20 -> NaOh + H2
Don’t use aq NaCL, it makes h gas, it is inefficient, use molten