REACTION KINETICS
Q.01: Multiple choice questions.
(i) In zero order reaction, the rate is independent of:
(a) Temperature of reaction
(b) Concentration of reactants
(c) Concentration of products
(d) None of these
ANSWER: (b) Concentration of reactants
EXPALNATION: The rate of zero order reactions is independent of the concentrations of reactant. Rather, they depend upon other factors like light in case of photochemical reactions.
(ii) If the rate equation of a reaction, 2A+B ¾® Products, is rate = k[A]2[B], and A is present is large excess, then order of reaction is:
(a) 1
(b) 2
(c) 3
(d) None
ANSWER: (a) 1
EXPALNATION: The reaction in which one of the reactants is in large excess, its rate is determined by the concentration of the reactant which is present in less amount.
(iii) The rate of reaction:
(a) Increases as the reaction proceeds.
(b) Decreases as the reaction proceeds.
(c) Remains the same as the reaction proceeds.
(d) May decrease or increase as the reaction proceeds.
ANSWER: (b) Decreases as the reaction proceeds.
EXPALNATION: This is because as the reaction proceeds, the concentration of reactants decreases. With the decrease in concentration of reactants, the rate of forward reaction also decreases according to law of mass action.
(iv) With increase of 10oC temperature, the rate of reaction doubles. This increase in rate of reaction is due to:
(a) Decrease in activation energy of reaction.
(b) Decrease in the number of collisions between reactant molecules.
(c) Increase in activation energy of reactants.
(d) Increase in number of effective collisions.
ANSWER: (d) Increase in number of effective collisions.
EXPALNATION: Increase in temperature increases the number of those reactant molecules which possess energy greater than activation energy barrier. Thus, greater number of molecules can now make effective collisions and the rate of reaction doubles.
(v) The unit of the rate constant is the same as that of the rate of reaction in:
(a) First order reaction
(b) Second order reaction
(c) Zero order reaction
(d) Third order reaction
ANSWER: (c) Zero order reaction
EXPALNATION: Rtae constant or k is the ratio between the rate of reaction and concentration of reactants. i.e., k = Rate/[C]. In case of zero order reaction, the exponent power of C is zero. So, the rate becomes equal to k. Thus, for zero order reactions, the unit of rate constant is the same as that of rate of reaction.
FILL IN THE BLANKS
Q.02: Fill in the blanks with suitable words.
(i)The rate of an endothermic reaction _______ with the increase in temperature. (increases)
(ii)All radioactive disintegration nuclear reactions are of ________ order. (first)
(iii)For a fast reaction, the rate constant is relatively ________ and half-life is ________. (large, small)
(iv)The second order reaction becomes _________ if one of the reactants is in large excess. (pseudo first)
(v)Arrhenius equation can be used to find out __________ of a reaction. (energy of activation)
TRUE/FALSE
Q.03: Indicate true or false as the case may be.
(i)The half-life of a first order reaction increases with temperature. (FALSE)
CORRECTION: The half-life of a first order reaction decreases with temperature.
(ii)The reactions having zero activation energies are instantaneous. (TRUE)
(iii)A catalyst makes a reaction more exothermic. (FALSE)
CORRECTION: A catalyst makes a reaction occur faster.
(iv)There is difference between rate law and the law of mass action. (TRUE)
(v)The order of reaction is strictly determined by the stoichiometry of the advanced equation. (FALSE)
CORRECTION: The order of reaction is determined by the experimentally determined co-efficients of the reactants of rate determining step.