CLASS: XII
MARKS: 70
UNITS: SOLUTIONS
TIME: 3 HRS
ELECTROCHEMISTRY
CHEMICALB KINETICS
GENERAL INSTRUCTIONS:
Read the following instructions carefully.
(a) There are 33 questions in this question paper with internal choice.
(b) SECTION A consists of 16
multiple-choice questions carrying 1 mark each.
(c) SECTION B consists of 5 short
answer questions carrying 2 marks each.
(d) SECTION C consists of 7 short answer
questions carrying 3 marks each.
(e) SECTION D consists of 2
case-based questions carrying 4 marks each.
(f) SECTION E consists of 3 long
answer questions carrying 5 marks each.
(g) All questions are compulsory.
(h) Use of log tables and calculators is not allowed.
SECTION A:
The following questions are multiple-choice questions with one correct
answer. Each question carries 1 mark. There is no internal choice in this
section
1. The unit of ebullioscopy constant is:
a) K kg mol-1
b) K-1 kg mol
c) K kg-1 mol-1
d) K kg-1 mol
2. A 5% solution of cane-sugar (molecular weight = 342) is isotonic with 1% solution of substance A. The molecular weight of X is
a) 342
b) 171.2
c) 68.4
d) 136.8
3. Which of the following solutions will have the highest conductivity at 298 K?
(a) 0.01 M HCl
(b) 0.1 M HCl
(c) 0.01 M CH3COOH
(d) 0.1 M CH3COOH
4. If E°Fe2+/Fe = -0.441 V and E°Fe2+/Fe2+ = 0.771 V, the standard EMF of the reaction,
Fe + 2Fe3+ → 3Fe2+ will be
(a) 1.212 V
(b) 0.111 V
(C) 0.330 V
(d) 1.653 V
5. Which of the following associated with isotonic solutions is not correct?
a) They will have the same osmotic pressure
b) They will have the same vapour pressure
c) They have same weight concentrations
d) Osmosis does not take place when the two solutions are separated by a semipermeable membrane
6. Which of the following statement is true?
(a) molecularity of reaction can be zero or a
fraction.
(b) molecularity has no meaning for complex
reactions.
(c) molecularity of a reaction is an
experimental quantity
(d) reactions with the molecularity three are very rare but are fast.
7. Which of the following solutions has the highest vapour pressure?
a) 0.2 𝑚 urea
b) 0.1 𝑚 glucose
c) 0.1 𝑚 MgSO4
d) 0.1 𝑚 BaCl2
8. If the initial concentration of substance A is 1.5 M and after 120 seconds the concentration of substance A is 0.75 M, the rate constant for the reaction if it follows zero - order kinetics is:
(a) 0.00625 molL-1s -1
(b) 0.00625 s -1
(c) 0.00578 molL-1s -1
(d) 0.00578 s -1
9. Galvanised iron sheets are coated with
(a) Carbon
(b) Copper
(c) Zinc
(d) Nickel
10. The e.m.f. of the cell Zn/Zn2+ (0.01 M) || Fe2+ (0.001 M) Fe at 298 K is 0.2905 volt. Then the value of equilibrium constant for the cell reaction is:
(a) e0.32/0.0295
(b) 100.32/0.0295
(c) 100.26/0.0295
(d) 100.32/0.0591
11. 4.5 g of aluminium (at. mass 27) is deposited at cathode from Al3+ solution by a certain quantity of electric charge. The volume of hydrogen produced at STP from H+ ions in solution by the same quantity of electric charge will be:
(a) 44.8 L
(b) 11.2 L
(c) 22.4 L
(d) 5.6 L
12. For a chemical reaction, X + 2Y → Z, if the rate of appearance of Z is 0.50 moles per litre per hour, then the rate of disappearance of Y is
(a) 0.5 mol L-1 hr-1
(b) 1.0 mol L-1 hr-1
c) 0.25 mol L-1 hr-1
(d) cannot be predicted
In questions 13 to 16 Given are two statements labelled as Assertion (A) and Reason (R) Select the most appropriate answer from the options given below:
(a) Both A and R are true and R is the correct explanation of A
(b) Both A and R are true but R is not the correct explanation of A
(c) A is true but R is false
(d) A is false but R is true
13. Assertion (A): Molarity of a solution in liquid state changes with temperature.
Reason (R): The volume of a solution changes with change in temperature.
14. Assertion: Aquatic species are more comfortable in cold waters rather than in warm waters. Reason: Different gases have different KH values at the same temperature.
15. Assertion (A): During electrolysis of aqueous copper sulphate solution using copper electrodes hydrogen gas is released at the cathode.
Reason (R): The electrode potential of Cu2+ /Cu is greater than that of H +/H2
16. Assertion (A): The half- life for a zero order reaction is independent of the initial concentration of the reactant.
Reason (R): For a zero order reaction, Rate = k
Section B
This section contains 5 questions with internal choice in one question. The following questions are very short answer type and carry 2 marks each.
18. Plot a graph between vapour pressure and mole fraction of a solution obeying Raoult’s Law at constant temperature? 2
19. Calculate the emf of the following cell at 298 K : Cr(s) / Cr3+ (0.1M) // Fe2+ (0.01M) / Fe(s) [Given : EoCell = + 0.30 V]
OR
The conductivity of 10-3 mol/L acetic acid at 25oC is 4.1 x 10 -5
S cm-1. Calculate its
degree of dissociation, if 
for acetic acid at 250 C is 390.5 S cm2 mol-1. 2
20. What are Fuel cells? Write the electrode reactions of a H2- O2 fuel cell.
What advantages do fuel cells have over conventional methods of generating electrical energy? 2
21. The activation energy of a reaction is 75.2 kJmol-1 in the absence of a catalyst and it lowers to 50.14 kJmol-1 with a catalyst. How many times will the rate of reaction grow in the presence of a catalyst if the reaction proceeds at 25oC? 2
Section C:
This section contains 7 questions with internal choice in one question. The following questions are short answer type and carry 3 marks each.
22. The following data were obtained during the first order thermal decomposition of N2O5(g) at a constant volume: 2N2O5 g ® 2N2O4 g +O2 g
Calculate the rate constant. 3
23. A) Name and state the law correlating the pressure of the gas & its solubility in a liquid.
B) What happens to the vapour pressure of a
liquid if a volatile liquid is added to it.
C) How is elevation in b.pt., when a non-volatile solute is added, related to lowering of vapour pressure. 3
24. The element A and B formed purely covalent compounds having molecular formula AB2 and AB4. When dissolved in 20 gram of benzene 1 gram of AB2 lowers the freezing point by 2.3 K and of AB4 by 1.3 K. Calculate atomic mass of A and B. The molar depression constant for freezing is 5.1 KKg mol-1 3
25. Calculate the maximum work done that can be obtained from the Daniell cell Zn/Zn2+//Cu2+/Cu. Given that E°Zn2+/Zn and E°Cu2+/Cu are -0.76 and +0.34 V respectively. 3
26. (a) Can we construct an electrochemical cell with two half-cells composed of ZnSO4 solution and zinc electrodes? Explain your answer.
(b) Calculate the λ0m for Cl- ion from the data given below: Λ0m MgCl2 = 258.6 Scm2mol–1 and λ 0m Mg2+ = 106 Scm2mol–1
c) The cell constant of a conductivity cell is 0.146 cm-1. What is the conductivity of 0.01 M solution of an electrolyte at 298 K, if the resistance of the cell is 1000 ohm? 3
27. A first order gas phase reaction: A2B2(g) → 2A(g) + 2B(g) at the temperature 400°C has the rate constant k = 2.0 × 10-4 sec-1. What percentage of A2B2 is decomposed on heating for 900 seconds? (Antilog 0.0781 = 1.197) 3
28. (i) Solutions of two electrolytes ‘A’ and ‘B’ are diluted. The limiting molar conductivity of ‘B’ increases 1.5 times while that of ‘A’ increases 25 times. Which of the two is a strong electrolyte? Justify your answer.
(ii) The products of electrolysis of aqueous NaCl at the respective electrodes are:
Cathode: H2 Anode: Cl2 and not O2. Explain. 3
Section
D:
The following questions are case -based questions. Each question has an internal choice and carries 4 marks each. Read the passage carefully and answer the questions that follow.
29. Read the given passage and answer the questions that follow:
A Lead storage battery is the most important type of secondary cell having a lead anode and a grid of lead packed with PbO2 as cathode. A 38% solution of sulphuric acid is used as electrolyte. (Density=1.294 g mL-1) The battery holds 3.5 L of the acid. During the discharge of the battery, the density of H2SO4 falls to 1.139 g mL-1. (20% H2SO4 by mass)
a) Write the reaction taking place at the cathode when the battery is in use.
b) How much electricity in terms of Faraday is required to carry out the reduction of one mole of PbO2?
c) What is the molarity of sulphuric acid before discharge?
OR
Write the products of electrolysis when dilute sulphuric acid is electrolysed using Platinum electrodes. 4
30. Read the given passage and answer the questions that follow
In corrosion, a metal is oxidised by loss of electrons to oxygen and formation of oxides. Corrosion of iron (commonly known as rusting) occurs in presence of water and air. The chemistry of corrosion is quite complex but it may be considered essentially as an electrochemical phenomenon. At a particular spot of an object made of iron, oxidation takes place and that spot behaves as anode and we can write the reaction
Electrons released at anodic spot move through the metal and go to another spot on the metal and reduce oxygen in the presence of H+ (which is believed to be available from H2CO3 formed due to dissolution of carbon dioxide from air into water. Hydrogen ion in water may also be available due to dissolution of other acidic oxides from the atmosphere). This spot behaves as cathode with the reaction
The overall reaction: 2Fe(s) + O2 (g) + 4H+
(aq) à 2Fe2 +(aq) + 2 H2O (l), Eo(cell) =1.67 V
The ferrous ions are further oxidised by atmospheric oxygen to ferric ions which come out as rust in the form of hydrated ferric oxide (Fe2O3 . x H2O) and with further production of hydrogen ions.
a) Write the cathode half- reaction during rusting of iron.
b) Iron does not rust when a coating of zinc cracks in a galvanised iron pipe, whereas it rusts when a coating of tin done on its surface cracks. Give reasons.
c) What is meant by galvanisation? 4
Section E
The following questions are long answer types and carry 5 marks each.
31. i) Graphically explain the effect of temperature on the rate constant of reaction? How can this temperature effect on rate constant be represented quantitatively?
ii) The decomposition of a hydrocarbon follows
the equation
Calculate Ea.
OR
i) In
the reaction
Q + R → Products
The time taken for 99% reaction of Q is twice
the time taken for 90% reaction of Q. The concentration of R varies with time
as shown in the figure below:
What is the overall order of the reaction?
Give the units of the rate constant for the same. Write the rate expression for
the above reaction.
ii) Rate constant for a first order reaction has been found to be 2.54 x 10-3s-1. Calculate its three-fourth life. 5
32. (a) Why is glycol and water mixture used in car radiators in cold countries?
(b) Give reason When 30 ml of ethyl alcohol and 30ml of water are mixed, the volume of resulting solution is more than 60ml.
(c) What is cryoscopic
constant? Give the expression to obtain the cryoscopic constant of a solvent.
(d) What do you
mean by a minimum boiling azeotrope? Give an example. 5
33.
A) Explain
construction & working of a standard hydrogen electrode.
B) Give
reasons: (i) Why does the voltage of a mercury cell remain constant throughout
its life? (ii) Dry cells do not have a long life.
C) What is a
salt bridge? Give its functions. 5
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