Write the relation between (i) Qc and emf of a cell with reaction aA + bB → cC + dD
Answers
Answer:
CALCULATE EMF OR CELL POTENTIAL - DEFINITION
The potential difference between the two electrodes in a galvanic cell is called a 'Cell Potential' or 'EMF' of the cell. It is measured in volts.
Standard EMF of the cell = Standard Reduction Potential of the half cell on the right hand side (Cathode) - Standard Reduction Potential of the half cell on the left hand side (Anode)
For example, EMF of Daniell cell,
E cello =E Cu 2+ ∣Cuo −E Zn 2+ ∣Zo
If we were taking Oxidation Potentials into account,
Standard EMF of the cell = Standard Oxidation Potential of the half cell on the left hand side (Anode)-Standard Oxidation Potential of the half cell on the right hand side (Cathode). For example, EMF of Daniell cell,E cello =E Zn∣Zn 2o −E Cu∣Cu 2+o
NERNST EQUATION FOR ELECTRODE POTENTIAL - DEFINITION
Electrode potential varies rapidly with temperature and concentrations of the species involved. Hence, to define one particular reference for electrode potentials, standard set of conditions have been defined. If potential is measured under these conditions, it is known as 'Standard Potential'.
The Nernst Equation enables the determination of cell potential under non-standard conditions. It relates the measured cell potential to the reaction quotient and allows the accurate determination of equilibrium constants (including solubility constants).
Nernst equation is given as:
E cell =E cello − nFRT logQ i.e. E cell =E cello − n0.0591 logQ at 25 o C
The equation above indicates that the electrical potential of a electrode depends upon the reaction quotient Q of the reaction.
So if we are trying to determine reduction potential of Cu electrode, we will have to consider Cu 2+ +2e − →Cu (s)
We can then Nernst Equation as:E Cu 2+ /Cu =E Cu 2+
/Cuo − 20.0592 log( [Cu 2+ ]1 ) at 25 o C
This way we can determine the E Cu 2+ /Cu for Copper electrode at any temperature and concentration, if we know the standard reduction potential of Copper electrode i.e. E Cu 2+ /Cuo
NERNST EQUATION FOR EMF OF A CELL - DEFINITION
Cell potential varies rapidly with temperature and concentrations of the species involved as electrode potential is dependent on these factors. Hence, to define one particular reference for electrode potentials, standard set of conditions have been defined. If potential is measured under these conditions, it is known as 'Standard Potential'.
The Nernst Equation enables the determination of cell potential under non-standard conditions. It relates the measured cell potential to the reaction quotient and allows the accurate determination of equilibrium constants (including solubility constants).
Nernst equation is given as:
E
cell
=E
cell
o
−
nF
RT
logQ
i.e. E
cell
=E
cell
o
−
n
0.0591
logQ at 25
o
C
The equation above indicates that the electrical potential of a cell depends upon the reaction quotient Q of the reaction.
For a cell:
Zn∣Zn
2+
∣∣H
+
∣H
2
∣Pt
We have a net chemical reaction of Zn
(s)
+2H
+
→Zn
2+
+H
2(g)
If the concentrations of the ions are not 1.0 M, and the H
2
pressure is not 1.0 atm, then the cell potential may be calculated using the Nernst equation:
E
cell
=E
cell
o
−
2
0.0591
log(
[H
+
]
2
P(H
2
)[Zn
2+
)