Section A
Q1.
(a)
(i) Does the Frenkel disorder in AgBr create vacancies of Ag^ + , vacancies of Br^- or both ? Explain. The ionic radii are 1.15 Å for Ag^ + and 1.96 Å for Br^- .
(ii) Calculate the temperature at which the fraction of Frenkel defects in a crystal of AgBr exceeds 1 part per billion. The enthalpy of Frenkel defect formation, \Delta Hf, has the value of 1.16 eV/defect, and the entropic prefactor, A, has a value of 3.091.
(b) Does the enthalpy of formation of compounds containing a certain element change, if the enthalpy of formation of the element under standard state conditions is set equal to 100 kJ mol^{-1} rather than to zero ? If it changes, how will it change for the compound AnBm, if the formation enthalpy of element A is set equal to 100 kJ mol^{-1} ?
(c) Draw an energy level diagram (with energy on the y-axis and internuclear distance r, on the x-axis) plotting a C - F and C - I bond. You should include numbers on y-axis. No numbers are needed on x-axis, but relative distances should be correct. Bond dissociation energies are 238 kJ/mol and 484 kJ/mol for C - I and C - F, respectively.
(d) Explain the application of phase rule in desilverization of lead.
(e) Calculate the equilibrium constant of the cell reaction 2Ag+ + Zn \longrightarrow 2Ag + Zn2+ occurring in the zinc-silver cell at 25°C when [Zn2+] = 0.10 M and [Ag^ + ] = 10 M. The e.m.f. of the cell is found to be 1.62 V.
