What is 'boiling point' ? Drive a relation between `DeltaH and DeltaU` for a chemical reaction Draw neat labelled diagram of calomel electrode. Resistance and conductivity of a cell containning `0*001" M KCl"` solution at 298 K are 1500 `Omega and 1*46xx10^(-4)S.cm^(-1)` respectively. What is the cell constant ?

Explanation

Boiling point : The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the atmospheric pressure. Since at any temperature, the vapour pressure of a solution (containning non-volatile solute) is always higher than that of the pure solvent, the boiling point of a solution is always higher than that of the pure solvent.
Relation between `DeltaH and DeltaU` for a chemical reaction :
If a process is carried out at constant pressure, the work of expansion is given by
`W=PDeltaV` . . . .(i)
where `DeltaV` is the increase in volume and P is the constant pressure.
According to first law of thermodynamics
`q=DeltaU+W` . . . .(ii)
where q is the heat absorbed by the system, `DeltaU` is the increase in internal energy of the system and W is the work done by the system.
Under condition of constant pressure,
Putting `W=PDeltaV` and heat absorbed `q_(P)`, we
`q_(P)=DeltaU+PDeltaV` . . . . (iii) Suppose when the system absorbs `q_(P)` calories of heat, its internal energy increases from `U_(1)` to `U_(2)` and volume increases from `V_(1)` to `V_(2)`. Then `DeltaU=U_(2)-U_(1)andDeltaV=v_(2)-V_(1)`.
`:.` Equation (iii) becomes
`q_(P)=(U_(2)-U_(1))+(V_(2)-V_(1))`
`=(U_(2)+PV_(2))-(U_(1)-PV_(1))` . . . (iv)
The defining equation of enthalpy (H) is H=U+PV
If `H_(2)` is the enthalpy of the system in the final state and `H_(1)` is the enthalpy in the initial state, then `H_(2)=U_(2)+PV_(2)andH_(1)=U_(1)+PV_(1)` putting these value in equation (iv) we get
`q_(P)=H_(2)-H_(1)=DeltaH` . . . .(v)
Further putting the value of `q_(P)` from equation (v) to equation (iii) we get
`DeltaH=DeltaU+PDeltaV` . . . (vi)
`PDeltaV` can be replaced by `DeltanRT` (where `Deltan=c,R=` universal gas constant and T is temperature in absolute scale)
Then equation (vi) becomes
`DeltaH=DeltaU+DeltanRT` . . .(vii)
Equations (vi) and (vii) give relation between `DeltaH andDeltaU`
Diagram of calomel electrode

Given, Resistance (R) `=1500Omega`,
Conductivety `(K)=1*46xx10^(-4)Scm^(-1)`
`R=(1)/(K)*(l)/(a)`
where l is the distance the electrodes of the cell and a is the surface area of electrode
or, `(l)/(a)=RK`
`=1500xx1*46xx10^(-4)OmegaxxOmega^(-1)xxcm^(-1)`
`=0*219cm^(-1)`
`:.` Cell constant (l/a) `=0*219cm^(-1)`