Find the change in entropy for the condensation of 20.5 g benzene (C_6 H_6) Delt

Question

Find the change in entropy for the condensation of 20.5 g benzene (C_6 H_6) Delta H_fus = 9.95 kJ/mol Delta H_vap = 30.8 kJ/mol T_m.p. = 5.5 degree C T_b.p. = 80.1 degree C What is the weight percent of ammonium cyanide in an aqueous solution that is 1.53 m, if the solution has a density of 1.26 g/mL? How is ion solubility affected as temperature increases? How is gas solubility affected? Under standard conditions, the lattice energy of LiF is - 1037 kJ/mol, while the lattice energy of KBr is -689 kJ/mol. If Delta_solution H is 4.73 kJ/mol for LiF and 19.87 kJ/mol for KBr, which has the greater Delta_hydration H and why?

Explanation / Answer

14) benzene(g) -----> benzene(l)

      For this phase change the change in entropy is Delta s = delta H vaporisation / boiling temperature

                                                                                                          = 30.8x1000J/ 353.1K = 87.227J / mol

For one mol of benzene = 78g the change in entropy is 87.227J

Hence for 20.5 g of benzene = (20.5 x 87.227) / 78 = 22.925 J

15) molality of ammonium cyanide solution = 1.53 m

      density of solution                                         = 1.26g/mL

We know molaity = number of moles/ weight of solvent(kg)

If we take weight of solvent as 1000g(1Kg) then m = weight of solute / molar mass

Thus weight of solute = molaity x molar mass = 1.53 x 44= 67.32 g

Since weight of solvent is assumed 1000g, weight percent = [67.32/1000] x100 = 6.732%

Alternatively taking the concentration as molarity(M)

weight of solute is calculated as above as 67.32g assuming volume of solution =1000mL.

since density = 1.26g/mL, then weight of solution = 1260g

weight of solvent = weight of solution - weight of solute = 1260 - 67.32 = 1192.68 g

Then weight percentage =[ 67.32x 100] / 1192.68 = 5.644%

16) The ionic solubility depends on the hydration energy and lattice energy.

    If the lattice energy is more than hydration energy , the heat of solution is positive, that is dissolution is endothermic and with increase in tempeature, solubility of ionic solid increases.Eg. glucose ammonium nitrate.

If the lattice energy is less than hydration energy, heat of solution is negative, that is dissolution is exothermic and with increase in temperature, solubility of the salt decreases. eg. potasium dichromate, calcium sulphate, cerium sulphate.

In case of gases , the dissolution is always exothermic process, hence with increase in temperautre always the solubility decreases.

17) LiF has much higher hydration energy. When ionic solid is dissolving in water lattice energy needs to be supplied to break crystals, hence is positive and hydration energy is liberated thus negative. enthaply of solution of LiF is small positive value compared to KBr inspite of its huge lattice energy, show it has high hydration energy also.

The reason for high hydration energy of LiF is the small size of both Li+ and F- ions.

Usually the hydration energy of negative ions is inconsequential due to their large sizes, but fluoride ion being ver y small, has considerable hydration energy.

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