CHEM 2 Lab Midterm Study Guide p. 3

Question.18. What is a complex reaction ?

A reaction involving more than one step is called a complex reaction.

Question.25. What is the effect of increase in cone, of iodide ions on the following reaction : 2H30+ + 2I- +H2O2 —-> 4H20 + I2

Rate of the reaction increases.

Question.13. "For an exothermic reaction activation energy for the forward reaction is less than that for the backward reaction." Is this statement true or false ?

True.

Question.23. What is the oxidation number of oxygen in H2O2 ?

-1.

Question.21. What is the equivalent mass of H2O2 ?

17.

Question.22. What is the normality of 1 M H2O2 solution ?

2 N.

Question.15. What is the effect of catalyst on the activation energy and heat of the reaction ?

A catalyst decreases the activation energy of the reaction. It has no effect on the heat of the reaction.

Question.12. Why reactions with molecularity more than three are rare ?

Because simultaneous collision between more than three particles is rare on the basis of probability considerations.

Question.10. Why certain reactions are very fast ?

Because they have very low activation energy.

Question.24. What is the colour of starch-iodine complex ?

Blue.

Question.27. Suppose the above rate law expression for the precipitation of sulphur holds good, then on doubling the concentration of S2O3 2- ion and H+ ion, by how many times will the rate of the reaction increase ?

By eight times.

Question.1. What is chemical kinetics ?

Chemical kinetics is that branch of chemistry which deals with the study of the rates of reactions and their mechanisms.

Question.14. What is rate determining step ?

In complex reactions, the slowest step determines the over all rate of the reaction. This step is known as rate determining step.

Question.9. What is the effect of temperature on rate constant of a reaction ?

It increases with increase in temperature.

Question.19. What do you understand by '4 volume' H2O2 solution ?

It is a way of expressing the cone, of H202 solution. 1 litre of '4 volume' H2O2 solution gives 4 litres of oxygen at N.T.P. on decomposition.

Question.16. On increasing the concentration of reactants the rate of the reaction does not change. What can you say about the order of the reaction ?

It is a zero order reaction.

Question.11. What is threshold energy ?

It is the minimum energy which the colliding molecules must possess so as to have effective collision.

Question.5. What is the law of mass action ?

Law of mass action states that the rate of a reaction is directly proportioned to the product of molar concentrations of the reactants.

Question.3. What are the units of the rate of reaction ?

Mol L^-1 s^-1 (Moles per litre per second)

Question.26. The reaction under examination is as follows : S2O3 2- (aq) + 2H+ (aq) —-> H2O(l) + SO2(g) + S(g) Write the conditions under which the rate law expression for this reaction can be written in the following manner. Rate of precipitation of sulphur = k [S2O3 2- ] [H+ ] 2 .

None of the reactants should be used in excess and the reaction should be elementary.

Question.7. What are the units of rate constant for zero order reactions ?

Same as rate of the reaction, i.e. moles/litre/sec.

Question.8. What are the units of rate constant for first order reactions ?

Sec^-1 .

Question.6. What is temperature coefficient of a reaction ?

Temperature coefficient of a reaction is the ratio of rate constants at two temperatures differing by 10°. Its value is generally equal to 2.

Question.28. How does the rate constant of a reaction vary with temperature ?

The rate constant of a reaction increases with increase in temperature.

Question.2. What do you understand by the rate of reaction ?

The rate of a reaction is defined as the change in the molar concentration of any one of the reactant or the product per unit time.

Question.4. What are the factors on which the rate of reaction depends ?

The rate of reaction depends upon : (i) Nature of the reactants ; (ii) Concentration of the reactants ; (iii) Temperature ; (iv) Presence of catalyst; and (v) Presence of radiations.

Question.17. Can order of a reaction be fractional ?

Yes. For example, for the reaction CH3CHO ——> CH4 + CO the order is equal to 3/2.

21. Calculator problem - Cyclopropane rearranges to form propene: CH2CH2CH2 --> CH2=CHCH3 by first-order kinetics. The rate constant is k = 2.74 x 10^-3 s^-1 . The initial concentration of cyclopropane is 0.290 M. What will be the concentration of cyclopropane after 100 seconds?

a) 0.220 M

15. What is the rate constant of the reaction in Problem #14?

a) 2.31 x 10^-2 s^-1

19. The relationship between the rate constant and temperature is expressed by the:

a) Arrhenius equation

3. Consider the following kinetic data collected at the very beginning of a reaction: Run #: 1, 2, 3 Initial [A] (mol/L): 0.10, 0.20, 0.10 Initial [B] (mol/L): 0.10, 0.10, 0.20 Initial Rate of Reaction (mol/L-s): 0.0090, 0.036, 0.018 Which of the following is an appropriate expression for the rate law and rate constant? a) Rate = k[A]2 [B]; k = 9.0 L2 /mol2 -s b) Rate = k[A]2 [B]; k = 0.9 L2 /mol2 -s c) Rate = k[A]4 [B]2 ; k = 9.0 x 10^3 L / mol-s d) Rate = k[A][B]; k = 0.9 L / mol-s

a) Rate = k[A]2 [B]; k = 9.0 L2 /mol2 -s

5. Of the following factors, which cannot affect the rate of a chemical reaction? a) all can affect the rate b) temperature c) presence of a catalyst d) concentration of reactants of the forward reaction e) physical state or state of subdivision of solid reactants

a) all can affect the rate

11. What is the rate constant of the reaction in Problem #10?

b) 0.231 seconds-1

20. Identify the INCORRECT statement below: a) The rate of a typical reaction doubles with a 10° C rise in temperature. b) The overall rate of reaction is determined by the rate of the fastest elementary step. c) The reaction mechanism is a step-by-step pathway by which reaction occurs. d) The reaction mechanism is typically a series of elementary reaction steps. e) Reaction orders for a single elementary step are equal to the balancing coefficients for that step.

b) The overall rate of reaction is determined by the rate of the fastest elementary step.

1. Identify the INCORRECT statement below concerning chemical kinetics: a) The rate of a chemical reaction changes with time. b) The rate constant of a reaction generally depends on the concentrations of species. c) The rate of a chemical reaction is affected by the temperature of the reaction. d) The rate law of a chemical reaction bears no relationship with the balancing coefficients of the overall reaction. e) The rate law expresses how the rate varies with concentration of species.

b) The rate constant of a reaction generally depends on the concentrations of species.

13. The catalytic converter in an automobile uses NiO and Pt metal to speed the combustion of CO to CO2. This is an example of:

b) heterogeneous catalysis

14. The decomposition of N2O5(g) --> NO2(g) + NO3(g) proceeds as a first order reaction with a half-life of 30.0 seconds at a certain temperature. If the initial concentration [N2O5]0 = 0.400 M, what is the concentration after 120 seconds?

c) 0.025 M

10. In a given 1st order reaction A ---> products, the initial concentration of A is 0.40 M. What will be the concentration of A after 15 seconds if the half-life of the reaction is 3 seconds?

c) 1.25 x 10-2 M

7. The gas phase reaction A + B ---> C has a reaction rate which is experimentally observed to follow the relationship Rate = k[A]2 [B]. If the concentration of A is tripled and the concentration of B is doubled, the reaction rate would be increased by a factor of _____.

c) 18

6. Consider the following rate law expression: Rate = k[A]2 [B]. Which of the following is NOT TRUE about the reaction having this expression? a) The reaction is overall third order. b) The reaction is first order in B. c) Doubling the concentration of A doubles the rate. d) The reaction is second order in A.

c) Doubling the concentration of A doubles the rate.

2. The rate law for the iodine clock reaction is given by: Rate = k[IO3 - ][I- ] 2 [H+ ] 2 a) This reaction is first order with respect to IO3 - and third order overall. b) This reaction is second order with respect to I- and 3rd order overall. c) This reaction is first order with respect to IO3 - and fifth order overall. d) This reaction is third order with respect to H+ .

c) This reaction is first order with respect to IO3 - and fifth order overall.

16. Which equation below best gives the concentration of N2O5 versus time in Problem #14?

c) [N2O5] = [N2O5]0e^-kt

18. The collision theory of reaction rates: I. helps to expose how temperature affects the rate. II. assumes that the rate depends on the frequency at which reactants collide. III. assumes that reactants must be in correct orientation to react. IV. assumes that only collisions with energy above the activation energy are successful.

d) All are correct statements.

12. A reaction has the following stoichiometry: H2 + 2 ICl ---> I2 + 2 HCl Which of the following would be a correct definition of the rate?

d) Rate = +(1/2)Δ[HCl]/Δt

17. Radioactive isotope concentrations typically decay by:

d) first order kinetics

4. In the overall reaction given by: H2(g) + 2 ICl(g) --> I2(g) + 2 HCl(g)

d) the concentration of HCl increases twice as fast as I2.

9. The initial rate of a reaction is studied with several initial concentration conditions and the following results are found: (Table on page 3 Kinetics Practice Qz and answers)

e) Rate = k[A]2 [B]

8. Determine the rate-law expression for the reaction below: 2A + B2 + C --> A2B + BC Run # Initial Rate (M-1s -1) [A]o [B2]o [C]o 1-> 1.6 x 10^6-> 1.2 M-> 1. x 10^-2 M-> 1.0 M 2-> 1.6 x 10^6-> 2.4 M-> 1. x 10^-2 M-> 1.0 M 3-> 3.2 x 10^6-> 1.2 M-> 2. x 10^-2 M-> 1.0 M 4-> 6.4 x 10^6-> 1.2 M-> 1. x 10^-2 M-> 2.0 M

e) Rate = k[B2][C]2