Long Answer Type(i) Write the equation which relates the number of phases, components and degree of freedom of a system in equilibrium. Â Â
(ii) How many phases and components are there in a system consisting of:
(1) a mixture of O2Â and N2Â gases
(2) N2Â gas containing H2O vapour
(3) a mixture of ice and water
(4) a mixture of powdered iron and sulphur
(5) a lump of solid iron (II) sulphide
Give a suitable explanation, in brief, for your answer.
Consider the reaction,
A + B → C + D  Â
The initial rates for different initial concentrations of the reactants are given below :
Initial concentration (mol 1– 1Initial rate (mol 1 –1 s– 1)
| Â |
A |
B |
 |
|
(i) |
1.0 |
1.0 |
2.0 x 10 – 3 |
|
(ii) |
2.0 |
1.0 |
4.0 x 10– 3 |
|
(iii) |
4.0 |
1.0 |
8.0 x 10– 3 |
|
(iv) |
1.0 |
2.0 |
2.0 x 10– 3 |
|
(v) |
1.0 |
4.0 |
2.0 x 10’3 |
(a) What are the orders with respect to A and B ?
(b) What is the overall order ?
(c) Write the rate law equation.
(d) Calculate the rate constant.
(e) Suggest a possible mechanism.
Explain the principle involved in the following observations : Â
(i) Cu2+Â is precipitated as CuS in the acidic medium while Ni2+Â is precipitated as NiS in a basic medium with H2S.
(ii) A solution of a weak acid and its salt with strong base has a definite pH value which does not change appreciably on dilution.
(iii) A solution of sodium chloride is neutral while that of ferric chloride is acidic in nature.
(i) The Cu2+ ions belong to group II and can be precipitated in the presence of a very low concentration of S2–ions. Whereas Ni2+ ions belong to group IV and are precipitated in the presence of high concentration of S2–ions. Hence, to check the precipitation of Ni2+ ions in group II, HCl is added which checks the dissociation of H2S into S2– ions.
On the other hand, to precipitate Ni2+ ions medium is made basic so as to facilitate the dissociation of H2S into S2–ions.
(ii) A solution of a weak acid and its salt with a strong base is called an acidic buffer. Hence, it has a tendency to maintain its pH constant. That is why it has a definite pH value.
(iii) Sodium chloride is a salt of a strong acid and a strong base. It does not undergo hydrolysis when dissolved in water. The Na+Â ions furnished by NaCl has no tendency to take up OH-Â ions furnished by water and same in the case with Cl-Â ions furnished by NaCl. Therefore, the equilibrium of H+Â and OH-Â remains almost undisturbed and the solution remains neutral.
In the case of FeCl3, the Fe3+ ions furnished by FeCl3 on dissolving in water take up the OH- ions furnished by water. Similarly, the Cl– ions also take up H+ ions of water and form HCl which makes the solution acidic.
(i) Why could fluorine not be prepared for a long time from HF and metal fluorides either by electrolysis or by any chemical reaction ?Â
(ii) Describe the modern method for the preparation of fluorine by giving :
(a) Materials used in the construction of the cell.
(b) Electrolyte used.
(c) Electrode reactions.
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