Friday, August 29, 2008

Chemical Equilibrium - Past JEE Questions

State True or False

1. If equilibrium constant for the reaction A2 + B2 ↔ 2AB, is K, then for the backward reaction AB ↔ ½ A2 + ½ B2, the equilibrium constant is 1/K.

(JEE 1984)

Answer: False

Reason: For A2 + B2 ↔ 2AB,
K = [AB] ²/[A2][B2]

For AB ↔ ½ A2 + ½ B2 equilibrium constant is

= [A2] 1/2[B2] 1/2/[AB]

This is not equal to 1/K.

2. Solubility of sodium hydroxide increases with increase in temperature.
(JEE 1985).

Answer: False

The dissolution of NaOH in water is an exothermic process. According to Le Chatelier principle, solubility decreases with increase in temperature. Increase in temperature favours backward reaction.



3. The rate of exothermic reaction increases with increasing temperature.
(JEE 1990).

Answer: True
Reason: In general, rate of a reaction increases with increase in temperature which is due to increase in the rate constant of the reaction. As per the Arrhenius equation relating k and T, as temperature increases k increases. (Note: At equilibrium if the temperature is increased, for an exothermic reaction the equilibrium shifts leftward).


Fill in blanks


4. For a given reversible reaction, at a fixed temperature equilibrium constants Kp and Kc are related by ____________________ .
(JEE 1994)

Answer : Kp = Kc (RT) Δ vg



5. A ten fold increase in pressure on the reaction

N2(g) + 3H2(g) ↔ 2NH3(g) at equilibrium results in _____________ in Kp.

(JEE 1996)

Answer: no change

Reason: Kp of an equilibrium reaction is independent of pressure of the system.

6. For a gaseous reaction 2B → A, the equilibrium constant Kp is _____________ to/than Kc.
(JEE 1997)

Answer: smaller




MCQs with multiple answers

7. When NaNO3 is heated in a closed vessel, oxygen is liberated and NaNO2 is left behind. At equilibrium

a. addition of NaNO2 favours reverse reaction.
b. addition of NaNO3 favours forward reaction.
c. increasing temperature favours forward reaction.
d. increasing temperature favours reverse reaction.

(1986)

Answer: ( c), (d),

Reason:

2NaNO3(s) ↔ 2NaNO2(s) + O2 (g)

Kp = Pressure of O2

Changing the quantities of NaNO3 and NaNO3 does not affect equilibrium position as Kp or Pressure of O2 remains constant.

Increasing temperature favours the forward reaction as this is endothermic direction.
Increasing pressure shifts the reaction in the backward direction as Pressure of O2 has to be kept constant.

8. For the reaction

PCl5 (g) ↔ PCl3 (g) + Cl2 (g)

The forward reaction at constant temperature is favoured by

a. introducing an inert gas at constant volume.
b. introducing chlorine gas at constant volume.
c. introducing an inert gas at constant pressure.
d. introducing PCl5 at constant volume.
e. increasing volume of the container
(1991)

Answer: c, d, and e.

Reason: Introducing an inert gas at constant pressure would increase the volume of the system. So the equilibrium shifts towards the side with larger number of gaseous molecules, i.e. the reaction proceeds in the forward direction.

Increasing volume produces the same result.

Introducing PCl5 at constant volume increases the concentration of the reactant and thus equilibrium is shifted in the forward direction.

Chemical Kinetics - Order of Reaction and Reactants - Model Question

JEE Question 2007 Paper II

Consider a reaction aG+bH--> Products. When concentration of both the reactants G and H is doubled, the rate increases by eight times. However, when concentration of G is doubled keeping the concentration of H fixed, the rate is doubled. The overall order of the reaction is
(A) 0
(B) 1
(C) 2
(D) 3

Solution: D

This is a question of chapter chemical kinetics.
Reason: When G alone is doubled, rate is doubled. That is the exponent of [G] is one in rate law. When both are doubled, rate increases by 8 times, telling us that exponent of [H] is 2 in rate law. So overall order of reaction is sum of the two exponents that is 3.
--------------------

Sunday, August 17, 2008

Inorganic Chemistry Past JEE Fill Blanks Questions

Fill in the blanks

1. The increase in solubility of iodine in an aqueous solution of potassium iodide is due to the formation of ___________ . (1982)

2. Galvanization of iron denotes coating with ___________ .(1983)

3. If metal ions of group III are precipitated by NH4Cl and NH4OH without prior oxidation by conc. HNO3 ___________ in not completely precipitated. (1984)

4. Sodium dissolved in liquid ammonia conducts electricity because _________. (1985)

5. Hydrogen gas is liberated by the action of aluminum with concentrated solution of ___________. (1987)

6. Silver chloride is sparingly soluble in water because its lattice energy is greater than __________ energy. (1987)

7. _______ phosphorus is reactive because of its highly strained tetrahedral structure.

8. In extractive metallurgy of zinc, partial fusion of ZnO with coke is called _______ and reduction of the ore to the molten metal is called ____________ .
(smelting, calcining, roasting, sintering) (1988)

9. The salts _________ and _______________ are isostructural.
(FeSO4.7H2O; CuSO4.5H2O; MnSO4.4H2O; ZnSO4.7H2O)

10. ______________ acid gives hypo ____________ ion.
(hydrobromic, hypobromous, perbromic, bromide, bromite, perbromate) (1988)

11. Sulphur acts as ___________ agent in vulcanization of rubber. (1989)

12. The basicity of phosphorus acid (H3PO3) is ____________ . (1990)

13. In P4O10, the number of oxygen atoms bonded to each phosphorus atom is _________ . (1992)

14. The lead chamber process involves oxidation of SO2 by atmospheric oxygen under the influence of __________ as catalyst. (1992)

15. Ca2+ has a smaller ionic radius than K+ because it has __________- . (1993)

16. The formula of the deep red liquid formed on warming dichromate with KCl in concentrated sulphuric acid is ______________ . (1993)

17. The two types of bonds present in B2H6 are covalent and __________ . (194).

18. The type of magnetism exhibited by [Mn(H2O)6] 2+ is ________________ . (1994)

19. One recently discovered allotrope of carbon (e.g., C60) is commonly known as _________ . (1994)

20. Among PCl3, CH3+, NH2-, and NF3, ___________ is least reactive towards water. (1997)

21. A solution of sodium in liquid ammonia at -33˚C conducts electricity. On cooling, the conductivity of this solution ______________ .

22. When an aqueous solution of sodium fluoride is electrolysed, the gas liberated at the anode is ___________ .

23. Silver jewellery items tarnish slowly in the air due to their reaction with ___________. (1997).

24. Compounds that formally contain Pb4+ are easily reduced to Pb2+. The stability of the lower oxidation state is due to _____ . (1997)

answers some time later

Inorganic Chemistry Past JEE Fill Blanks Questions

Fill in the blanks

1. The increase in solubility of iodine in an aqueous solution of potassium iodide is due to the formation of ___________ . (1982)

2. Galvanization of iron denotes coating with ___________ .(1983)

3. If metal ions of group III are precipitated by NH4Cl and NH4OH without prior oxidation by conc. HNO3 ___________ in not completely precipitated. (1984)

4. Sodium dissolved in liquid ammonia conducts electricity because _________. (1985)

5. Hydrogen gas is liberated by the action of aluminum with concentrated solution of ___________. (1987)

6. Silver chloride is sparingly soluble in water because its lattice energy is greater than __________ energy. (1987)

7. _______ phosphorus is reactive because of its highly strained tetrahedral structure.

8. In extractive metallurgy of zinc, partial fusion of ZnO with coke is called _______ and reduction of the ore to the molten metal is called ____________ .
(smelting, calcining, roasting, sintering) (1988)

9. The salts _________ and _______________ are isostructural.
(FeSO4.7H2O; CuSO4.5H2O; MnSO4.4H2O; ZnSO4.7H2O)

10. ______________ acid gives hypo ____________ ion.
(hydrobromic, hypobromous, perbromic, bromide, bromite, perbromate) (1988)

11. Sulphur acts as ___________ agent in vulcanization of rubber. (1989)

12. The basicity of phosphorus acid (H3PO3) is ____________ . (1990)

13. In P4O10, the number of oxygen atoms bonded to each phosphorus atom is _________ . (1992)

14. The lead chamber process involves oxidation of SO2 by atmospheric oxygen under the influence of __________ as catalyst. (1992)

15. Ca2+ has a smaller ionic radius than K+ because it has __________- . (1993)

16. The formula of the deep red liquid formed on warming dichromate with KCl in concentrated sulphuric acid is ______________ . (1993)

17. The two types of bonds present in B2H6 are covalent and __________ . (194).

18. The type of magnetism exhibited by [Mn(H2O)6] 2+ is ________________ . (1994)

19. One recently discovered allotrope of carbon (e.g., C60) is commonly known as _________ . (1994)

20. Among PCl3, CH3+, NH2-, and NF3, ___________ is least reactive towards water. (1997)

21. A solution of sodium in liquid ammonia at -33˚C conducts electricity. On cooling, the conductivity of this solution ______________ .

22. When an aqueous solution of sodium fluoride is electrolysed, the gas liberated at the anode is ___________ .

23. Silver jewellery items tarnish slowly in the air due to their reaction with ___________. (1997).

24. Compounds that formally contain Pb4+ are easily reduced to Pb2+. The stability of the lower oxidation state is due to _____ . (1997)

answers some time later

Inorganic Chemistry - Past JEE - True or False Type questions

State whether the following statements are true or false

1. Red phosphorus is less volatile than white phosphorus because the former has a tetrahedral structure. (1982)

2. MgCl2.6H2O on heating gives anhydrous MgCl2. (1982)

3. When PbO2 reacts with a dilute acid, it gives hydrogen peroxide. (1982)

4. Copper metal reduces Fe2+ in an acid medium. (1982)

5. Silver fluoride is fairly soluble in water. (1982)

6. Dilute HCl oxidizes metallic Fe to Fe2+. (1983)

7. In an aqueous solution chlorine is a stronger oxidizing agent than fluorine. (1984)

8. Silver chloride is more soluble in very concentrated sodium chloride solution than in pure water. (1984)

9. Solubility of sodium hydroxide increases with increase in temperature. (1985)

10. Sodium when burnt in excess of oxygen gives sodium oxide. (1987).

11. Both potassium ferrocyanide and potassium ferricyanide are diamagnetic. (1989)

12. Cu+ disproportionates to Cu2+. (1991)

13. Nitrous oxide, though an odd electron molecule, is diamagnetic in liquid state. (1991)

14. Diamond is harder than graphite. (1993)

15. The basic nature of the hydroxides of Group 13 (Gr. III B) decreases progressively down the group. (1993)

16. The tendency for catenation is much higher for C than for Si. (1993)

17. HBr is stronger acid than HI because of hydrogen bonding. (1993)

Answers sometime later

Thursday, August 14, 2008

Acids and bases - Past JEE questions

(Bronsted and Lewis concepts);

1. State whether the following statement is true or false

Aluminium chloride (AlCl3) is a Lewis acid because it can donate electrons. (1982)

2. Fill in the blanks

The conjugate base of HSO4ˉ in acqueous solution is ___________. (1982)


3. The conjugate acid of NH2ˉ is:
a. NH3
b. NH2OH
c. NH4+
d. N2H4

(1985)


Answers some time later

Wednesday, August 13, 2008

van der Waals Equation - past JEE questions

1. In the van der Waals equation of state for a non-ideal gas the term that accounts for intermolecular forces is

a. (V-b)
b. RT
c. (P + a/V²)
d. (RT) ‾ ¹
(1988)

Answer ( c )

A is a measure of the forces of attraction between molecules

2. Indicate whether the following statement is true or false

In the van der Waals equation

[p + n²a/V²] (V – nb) = nRT

the constant ‘a’ reflects the actual volume of the gas molecules.

(1993)

Answer: False

The constant b reflects the actual volume of one mole of gaseous molecules.