Electrode Potential (Introduction)

Electrode Potential:
Electrode potential is simply the potential of a metal to gain electrons and undergo reduction and is measured in volts with the help of a voltmeter. Thus, it can also be called reduction potential.The proper definition is given below:

''It is the potential of an electrode measured with a voltmeter when the electrode in question is connected with a standard hydrogen electrode or SHE through a salt bridge.''

The equation given above is the standard way of writing electrode reactions i.e. in the form of reduction equations. It however, should be noted that the reaction can proceed in either direction depending on the environment as shown by the sign of equilibrium.

Standard Electrode Potential:

Standard electrode potential can simply be taken as electrode potential measured under standard conditions. Well of course, you will need a better definition:
  "Potential of a cell when an electrode is connected to a standard hydrogen electrode under standard       conditions.''
                                                                        OR
  '' It is the voltage of a half-cell measured under standard conditions with a standard hydrogen                  electrode as the other half-cell."
Similarly, Standard Cell Potential can be defined as the difference of the electrode potential values two electrodes connected through a salt bridge under standard conditions, which forms the voltage of that cell.

Standard Hydrogen Electrode forms the basis of thermodynamic scale of oxidation of oxidation-reduction potentials. Its standard potential has been declared to be zero at all temperatures to form the basis for comparison with all other electrode reactions.  Platinum is used as an electrode because it provides an inert connection to the H2(g)/H+(aq) system and adsorbs hydrogen gas at its surface to facilitate the formation of equilibrium quickly.

Note: The standard conditions are necessary to maintain in the measurement of standard electrode potential because electrode potential varies with conditions such as temperature. The standard conditions are:
1. Concentration of 1 mol/dm^3.
2. Temperature of 25 degree centigrade of 298 K.
3. Pressure of 1 atmospheric pressure.

To measure Standard Electrode Potential connect the electrode in question to an SHE under standard conditions as shown by the diagram above. If the system in question concerns ions of same element in different oxidation states like that of Fe2+/Fe3+ then a platinum electrode is used in equilibrium with the aqueous solution of the ions. The reading is then taken from the voltmeter. Here it should be noted that the electrode potential of the hydrogen electrode is taken to be 0.0V for comparison with other electrodes. Therefore, the value of voltmeter here is the standard electrode potential of the electrode in question. 

If the voltmeter gives negative value then that indicates that SHE is behaving as a cathode and if the voltmeter gives positive value then that means SHE is behaving as an anode. Remember that oxidation occurs at anode and reduction at cathode. You can remember this with the help of this short phrase, AN OX and RED CAT.

However, if we know the electrode potentials of the two electrodes connected in a cell we can calculate the Eo value of the cell with the help of the formula given above in the diagram. Since, the electrode potential values are the measure of reduction potentials the electrode with higher electrode potential undergoes reduction and the one with lower electrode potential value is the anode. Cell potential can then be found by subtracting electrode potential of the anode from that of cathode.

The direction of electron flow is always from where the electrons are being produced towards where they are consumed i.e. from anode to cathode.

 Salt Bridge?

It consists of a U-tube filled with concentrated solution of potassium chloride along with gelatin or agar. It is used to electrically connect the two half cells and to provide electrical neutrality by allowing migration of ions. However, the salt used in salt bridge must not react with chemicals in the half cells.

  

                                                                                                                   -Psycho Killer

Faraday Constant and determination of Avogadro Constant

                                     

Faraday Constant:

"Faraday constant is the charge in coulombs on one mole of electrons." That is, 1 Faraday is equal to elementary charge times number of electrons in one mole of electrons. This leads us to the relationship given below:

                                                 F=Le

 where, F= Faraday,

             L= Avogadro constant and

             e= elementary charge i.e magnitude of charge on an electron.

The unit used for Faraday Constant is C/mol or some books put it as just C.

This expression gives us the value of 96320 C for Faraday constant.

Faraday constant can also be derived by dividing the Avogadro constant by the number of electrons in one coulomb i.e precisely equal to 6.25x10^18. This leads us to a value of  96320 C. 

Determination of Avogadro Constant by Electrical Method:

The apparatus is set up as shown above in the diagram.

The rheostat is used to maintain a constant low current for a particular time.  Initial mass of the cathode is noted. After the electrolysis, the change in mass of the cathode is noted. The charge passed into the cell is found by the product of current and time in seconds according to the equation:

                                    Q=IxT

where, Q=charge passed,

           I= current passed, and 

           T= time in seconds.

Values:

Change in mass of the cathode=M

Current in the circuit=I

Time for which the current is passed=T

Calculations:

Charge passed=IT

Charge required to deposit 1 mole of Copper=(ITx63.5)/M

Since, 

Cu(aq) + 2e-  =   Cu(s)

2 Faraday are required to deposit 1 mole of copper. So, two Farday = (ITx63.5)/M.

Dividing (ITx63.5)/M by two will also give you the value of Faraday constant.

Now, since F=Le

L=F/e 

Insert the value of F found and the value of  e which is 1.6x10^-19.

This will give you the value of L which is 6.02x10^23

                                                                                   

                                                                                                            -Psycho Killer

Entropy Explained

                                                                        Entropy
 Entropy maybe explained as a measure of number of specific ways in which a thermodynamic system may be arranged. It is commonly understood as the measure of disorder of a system. And according to Second Law of Thermodynamics entropy of a system must increase.
   
          Second law of thermodynamics:
         In any cyclic process, the entropy will either increase or remain the same.

Note:  Systems that are not isolated may decrease in entropy provided that they increase the entropy               of their entropy by at least the same amount.
      Entropy=klnW
                where k=Boltzman Constant=1.38x10^-23
             and W=number of ways a change can occur. It is governed by laws of chance and probability.
                    
The total entropy can be given as the sum of entropy of the system and the entropy of the surroundings. 
                                   ΔS(surroundings)(total)=ΔS(system)+ΔS(surroundings)
Factors affecting Entropy:
1. Pressure:
    The entropy of a system decreases with an increase in pressure. If we increase the pressure on the system, the volume decreases. The energies of the particles are in a smaller space, so they are less spread out. The entropy decreases. The effect is greatest for gases because a change in pressure causes a large change in volume. The volume change for a liquid is small and even smaller for a solid. So, pressure changes have lesser effects on liquids and solids.
2. Temperature:
    As temperature increases, entropy increases. The warmer a body is, the more intensely and randomly the atoms oscillate, spin and swirl, the greater the agitation, the worse the atomic disorder. It should be noted here that randomness of type, orientation and motion cause the total disorder. 
 3. Energy:
     Entropy increases with energy. The more energy or quanta there are to share, the greater the number of ways of sharing them. This can be also related to the temperature as increasing the temperature increases the number of energy quanta available and so increases the entropy.
4. Number of molecules:
    The greater the number of molecules involved, the greater the ways of sharing them and this increases the W and so increases the entropy.

Some important points to remember:
1. Entropy increases in the order solid, liquid and gas.
2. Ions and molecules in solution generally have higher entropies than solids.
3. Substances with larger, complex molecules have higher entropies than those with smaller ones,
4. If a large molecule breaks down into smaller ones, entropy increases since there are more ways of
arranging several small molecules than few large molecules i,e W increases increasing the entropy.
5. For a reaction to occur the total entropy must be positive. If it is negative the reaction will not
    occur. For example, conversion of CO2 to C and O2.

Feasibility of reactions:
A reaction will only occur if there is a total increase in the entropy. Reactions with total negative entropy do not occur. However, by changing some conditions the reactions can be made to happen but they do not happen spontaneously. But the total entropy should increase. There are reactions in which the entropy tends to decrease, but there is an increase in the entropy of the surroundings leading to total increase in entropy.
                                         ΔS(total)=ΔS(system)+ΔS(surroundings)
So, positive ΔS(surroundings) outweighs the negative entropy of the system and thus, Second law of Thermodynamics still holds.ΔS(surroundings) can be given by: -ΔH/T
                                         where ΔH is the enthalpy change of the reaction 
                                           and T is the temperature of the body. 
                                 
                                                                                                    
 But there are reactions in which kinetic stability plays its role and they need high activation energy to occur. These reactions once started can proceed spontaneously but need high activation energy in the first place.
The total entropy of a reaction may be found with the help of the formula given below because it is a state function.
               ΔS=ΣS(products)-ΣS(reactions)



Effects of increasing Entropy:

Increase in entropy will cause:
1. bodies to become warmer
2. thermal expansion
3. substance will finally melt.

Entropy is involved in every reaction that occurs and plays a major part in each and there are many interesting phenomenons related to entropy. This was only a short review of the topic. Feel free to search more on it internet and in books. You might also be interested in the HEAT DEATH theory of the end of the universe. 
                                                                                      -Psycho Killer