Indian Forest Service (Mains), Chemistry optional · 2021 · Paper I
Q1(c). C–F and C–I potential-energy curves
Question
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 the y-axis. No numbers are needed on the 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.
Answer in brief
The C–F well is deeper and lies at a shorter equilibrium distance; the C–I well is shallower and lies farther to the right.
Key points
- Set the energy of the separated atoms to zero; mark the C–F minimum at −484 and the C–I minimum at −238 kJ mol⁻¹.
- Each minimum is the equilibrium separation, where attractive and repulsive forces balance: dV/dr = 0.
- At very short r, strong repulsion raises the energy rapidly; at large r, both curves approach zero from below.
- The deeper C–F well corresponds to its greater dissociation energy; the smaller F atom also permits a shorter bond.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverspotential energy curvebond dissociation energyC–F bondC–I bondbond length
All parts of Question 1
- 1(a) Frenkel disorder in silver bromide8 marks
- 1(b) Changing the enthalpy reference8 marks
- 1(c) C–F and C–I potential-energy curves8 marks
- 1(d) Desilverisation of lead8 marks
- 1(e) Equilibrium constant of a Zn–Ag cell8 marks