CHEMISTRY – TEST – 3 @ CHEMICAL KINETICS (THEORY) (50 min.) (Candidates are allowed additional 10 minutes for only reading the paper They must NOT start writing during this time) All questions are compulsory Question 1 is of 10 marks having three sub parts all of which are compulsory. Question numbers 2 - 4 carry 2 marks. Question numbers 5 -7 carry 3 marks. Question numbers 8 carry 5 marks. The intended marks for questions or part of questions are given in brackets [ ]. Balanced equations must be given wherever possible and diagrams where they are helpful. When solving numerical problems, all essential working must be shown. In working out problems, use the following data: Gas constant R = 1.987 cal deg -1mol -1 = 8.314 JK -1 mol-1 = 0.0821 dm 3 atm K -1 mol -1 3 1 l atm = 1 dm atm = 101.3 J 1 Faraday = 96540 coulombs

NAME

SCHOOL

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QUES 2

QUES 3

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MOB. (PARENT –1)

MOB. (PARENT – 2)

QUES 8

M. M. 30

Question 1: [A] Choose the most appropriate word to complete the following statement. -1

-1

-2

[3]

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[independent, dependent, liter mole sec , liter mole sec ,activation energy, decreased, threshold energy] (i) Half life period of a first order reaction is________ of initial concentration. (ii) The units of rate constant for a second order reaction are_____ . (iii) The minimum energy that the reactant molecules must possess before collisions, so that the products are formed is known as____. Ans: (i) independent -1

(ii) litre mol sec

-1

(iii) Threshold energy [B] Choose the correct alternative.

[3]

(i) If the concentration of one of the reactants in a chemical reaction is increases, the rate of the reaction usually (a) decreases

(b) increases

(c) remains the same (d) None of these

(ii) Half-life period of any zero order reaction is: (a) Directly proportional to the initial concentration of the reactant (c) Same for all reactions

(b) half of the rate constant

(d) independent of initial concentration of the reactants.

(iii) In a reaction, A + B → Product, rate is doubled when the concentration of B is doubled, rate law for the reaction can be written as (a) RATE = k [B]

2

(b) RATE = k [A] [B]

(c) RATE = k [B]

2

(d) RATE = k [A] [B]

Ans: (i) (b) increases (ii) (a) Directly proportional to the initial concentration of the reactant (iii) (a) RATE = k [B] [C] Answer the following (i) If the half life of a first order in A is 4 min, how long will it take A to reach 12.5% of its initial concentration? Ans: To reach 12.5% of the initial concentration A will take 3 half lives. Time required for 3 half lives = 3 x 4 = 12 min. (ii) The rate of a reaction increases with increase in surface area of reactant. Give reason.

[4]

Ans: The reaction rate increases with increase in surface area because more number of molecules take part in the reaction at the same time which means that the number of effective collisions increase. (iii) The rate of a reaction is given by, Rate = K [N2O5], what is the order of reaction in this equation Ans: One (1) (iv) What is the effect of change of concentration on the rate of a reaction? Ans: Rae of reaction increases with the increase in concentration and vice – versa. Question 2: Consider two gases A and B in a container at room temperature. What effect will the following changes have on the reaction rate between these gases (increase, decrease, no effect)? (a) The pressure is decreased. (b) The temperature is increased [2] Ans: (a) Reaction rate decreases. (b) Reaction rate increases Question 3: Consider the following reaction: N2 (g) + 3 H2 (g)  2 NH3 (g) -1 -1 If the rate of decomposition of H2 (g) is 0.06 molL s , what is the rate of formation of NH3 (g)? [2] Ans:

= x 0.06 = 0.04 mol/L-sec Question 4: The reaction A+B C has first order. Write rate equation and unit of rate constant for same. Ans: Rate equation: 1 0 1/2 1/2 0 1 R = k [A] [B] or R = k [A] [B] or R = k [A] [B] -1 -1 Unit = sec or time Question 5: 75% of a reaction of the first order was completed in 32 minutes. When was the 40% completed? Ans:

[2]

[3]

After simplification, t= 11.79 min or 707.9 min Question 6: The half life for radioactive decay of 14C is 5730 y. An archaeological artifact contained wood had only 75% of the 14C found in a living tree. Estimate the age of the sample. [3] Ans: t1/2 = 5730 y K = 0.693/t1/2 = 0.693/5730

T = 2379.09 y Question 7: The decomposition of a compound is found to follow a first order rate law. If it takes 20 minutes for 30 per cent of original material to react, calculate: (a) The specific rate constant (b) The time at which 20 per cent of the original material remains unreacted. [3] Ans: (a) -1

-4

= 0.0178 min or 2.9728 x 10 sec

-1

(b) After simplification, t= 90.2467 min or 5414.805 sec

Question 8: From the following data, from the reaction -1 -1 [A] [B] Initial rate (Mol L s ) between A and B. Calculate: -1 -1 Mol L Mol L 300 K 320 K (a) The order of reaction with respect to B. -4 -5 -4 -3 (b) The rate constant at 290 K. 2.5x10 3.0x10 5.0x10 2.0x10 -4 -5 -3 (c) The energy of activation. 5.0x10 6.0x10 4.0x10 --3 -5 -2 or 1.0x10 6.0x10 1.6x10 -(a) The rate of a particular reaction becomes six times when the temperature is increased from 298 K to 308 K. Calculate the energy of activation for the reaction. (b) The first order rate constant for the decomposition of ethyl iodide by the reaction: –5 –1 C2H5I (g) → C2H4 (g) + HI (g) at 500K is 1.60 × 10 s . Its energy of activation is 209 kJ/mol. Calculate the rate constant of the reaction at 600K.

Ans: (a) R = k[A]x[B]y 5 x 10-4 = k [2.5 x 10-4]x[3 x 10-5]y 4 x 10-3 = k [5 x 10-4]x[6 x 10-5]y 1.6 x 10-2 = k [1 x 10-3]x[6 x 10-5]y (a) Divide equation (ii) by (i)

----(i) ----(ii) ----(iii)

Therefore, y = 1 (b) But Ea is unknown Therefore, Ea = 55333.0951 J Substituting the value of Ea in the above equation, k2 = 2.3270 x 10-4 (c) Ea = 55333.0951 J OR (a) Therefore, Ea = 136752.4004J (b) Therefore, k2 = 0.069599968 sec-1