When two terminals of a battery are connected by a conductor, an electric current flows through the conductor. One terminal continuously sends electrons into the conductor, while the other continuously receives electrons from it. In this way, the battery behaves like an electric pump. The property of a cell (or generator) that makes the charge to move in a particular direction is called electromotive force or emf. Just like a force which moves a mass, an emf moves a mass, an emf moves a charge in a circuit. The emf of a source is not a force but work done in moving a unit charge round the circuit. The emf of a cell is the energy is supplied by the cell to move unit charge round a circuit join to it. It is the potential difference between the terminals of the cell in an open circuit. It does not depend on the size of a cell but depends on the chemical used in it. It is denoted by E. its unit is volt in SI-units.
CONSTRUCTION OF A CELL
In a cell, electrodes are dipped in their salt solutions and can be linked through salt solutions or salt bridge. There is a certain potential creates on each electrode due to reactions accompanied on electrodes. For example, when a metal rod is dipped in a solution of its own ions, there will be some positive negative charge generates on rod with respect to the solution. It can be either positive or negative charge.
When metal ions and their solution are in contact with each other, a definite potential difference is developed between the metal and the solution which is known as electrode potential. There are three possibilities in this solution.
Metal ions strike to electrode surface and deflected back to solution without nay change.
Metal ions gain electrons and convert in metal atom, hence metal ions are reduced.
Mn+ + ne- → M
Metal atoms may lose electrons and change to metal ions, hence metal atoms get oxidized.
M(s) → Mn+ + ne-
If metal ions have higher tendency to get reduced, they will gain electrons from metal rod and get reduced. This reaction generates positive charge on metal rod with respect to the solution and finally equilibrium will be maintained. If metal ions have higher tendency to get oxidized, electrons will accumulate on metal road and develop negative charge on metal rod and ultimately equilibrium will be maintained.
In both cases, there is a separation of charges between the metal rod and solutions and a potential difference exist between them. This potential difference between metal rod and its ion in solution is known as electrode potential also defined as tendency of an electrode to lose or gain electrons when it is in contact with the solution of its own ions. The potential of that electrode where oxidation takes place is called as oxidation potential and potential of electrode associated with reduction is called as reduction potential.
If the electrode is suspended in a solution of one molar concentration and the temperature is kept at 298 K, the electrode potential is called as standard electrode potential and represented as E°. If cell contains gaseous reactants, 1 atmosphere pressure and 298 K temperature will be standard conditions. The difference between the electrode potential of the two half cells is known as cell potential or cell voltage. It is called as electromotive force (emf or EMF) of the cell if no current is drawn from the cell.
The EMF of cell depends on the nature of the reactants involve in reaction, concentration of the solution taken in cell and reaction temperature. T
Effect of Concentration on Cell EMF
The EMF of a redox reaction in a voltaic cell is determined not only by the type of redox reaction, but also the concentrations of the reactants and products (i.e. the reducing agent and oxidizing agent)
The EMF of the cell will fall as the reactants are used up and products increase in concentration
At equilibrium concentrations of reactants and products, the EMF = 0. Electrons flow spontaneously in a redox reaction because the system is attempting to achieve equilibrium. When equilibrium is achieved, net electron flow is zero.
The Nernst Equation
Walther Hermann Nernst (1864 – 1941) was a German chemist who came up with an equation that related the EMF of a redox reaction on the concentration of reactants and product
Consider the reaction
Where ln =natural log
A common form of the equation does a way with the natural log and puts the equation in the form of log10:
Yet another simplification occurs if the temperature of the reaction is known. For example, 298K is a common reference temperature for redox reactions (this is room temperature). At 298K the value of 2.303*RT/F = 0.0592 V mole:
This is the Nernst Equation. It allows us to do the following for redox reactions:
1) If we know the value of E0 and we measure the EMF of the cell (under non-standard conditions) we can determine the value of Q (and therefore the concentrations of reactants and products).
2) If we know the value of E0 and the concentrations of reactants and products, then we can determine the resulting EMF of the cell.
this is how emf of cell depends upon the concentration of reactants and products formed in the cell.
Thus the more concentrated reactants would create more emf for the battery
Equilibrium Constants for Redox Reactions
what happens when the redox reaction achieves equilibrium concentrations of reactants and products?
From thermodynamics, DG = 0
If the redox reaction is at equilibrium, there is no longer any net reduction or oxidation reaction that drives an electromotive force (the cell is “dead”)
or, rearranging to solve for K:
What does this equation tell us?
This says that the equilibrium constant is directly related to the standard EMF for the redox reaction (conversely, the standard EMF for the reaction is directly related to the value of the equilibrium constant)
A redox reaction where the equilibrium lies far to the right, will have a large value for K, and a large value for the standard EMF
If we can calculate the standard EMF for a reaction (by comparing the relative difference between the two half-reactions) we can derive the equilibrium constant for the reaction.