CHEMICAL KINETICS QUIZ-1

Q1. Which statement is correct?

•  Reactions with low activation energy are usually exothermic
•  The rate law sometimes enable to deduce the mechanism of a reaction
•  The rate law for reaction is an algebraic expression relating the forward reaction rate to product concentration
•  Increase in the total pressure of a gas phase reaction increase the fraction of collisions effective in producing reactions

Solution
Increase in pressure or concentration brings in an increase in collision frequency as well as increase in effective collision. Recall that energy of activation is not at all related with exothermic or endothermic nature

Q2.The half life for a reaction … of temperature

•  Independent
•  Decreased with increase
•  Increased with increase
•  Dependent

Solution
Half-life depends upon rate constant and rate constant (K) varies with temperature as K=A∙e^(-E_n/RT);K increase with temperature. Also t_(1/2)∝1/K

Q3.  For a given reaction, pressure of catalyst reduces the energy of activation by 2 kcal at 27℃. The rate of reaction will be increased by:

•   20 times
•  14 times
•  2 times
•  28 times

Solution
K1=Ae^(-Ea/RT) K2=Ae^(-[E_a-2]/RT) K1/K2 =e^(2/RT)=e^(2/2×〖10〗^(-3)×300)=28

Q4. What is the formula to find value of t_(1/2) for a zero order reaction?

•  k/[R]_0
•  2k/[R]_0
•  0.693/k
•  [R]_0/2k

Solution
T_(1/2)=T_50,x=R/2 〖∴ T〗_50=R/k_0 So 〖 T〗_50∝R T_50∝R/k_0 Therefore, the formula of t_(1/2)for a zero order reaction is [R]_0/2k

Q5.In a reaction, the threshold energy is equal to:

•  Activation energy + normal energy of reactants
•  Activation energy – normal energy of reactants
•  Activation energy
•  Normal energy of reactants

Solution
 Activation energy is the needed by reactant molecules to gain threshold energy level.

Q6. Consider the reaction 2A+B→product When concentration of B alone was doubled, the half-life did not change. When the concentration of A alone was doubled, the rate increased by two times. The unit of rate constant for this reaction is

•  No unit
•  L mol^(-1) s^(-1)
• mol L^(-1) s^(-1)
•  s^(-1)

Solution
2A+B→product [B] is doubled, half-life didn’t change Half-life is independent of change in concentration of reactant i.e., first order First order w.r.t. to B +-+ When [A] is doubled, rate increased by two times ⇒ First order w.r.t.A Hence, net order of reaction =1+1=2 Unit for the rate constant=conc.^((1-n)) t^(-1) =(mol^(-1) )^(-1).s^(-1) L.mol^(-1) s^(-1)

Q7.Which of these does not influence the rate of reaction?

•  Nature of the reactants
•  Concentration of the reactants
•  Molecularity of the reaction
•  Temperature of the reaction

Solution
Nature and concentration of the reactants and temperature of the reaction influence the rate of reaction. But molecularity does not affect the rate of reaction as it includes the number of atoms, ions or molecules that must collide with one another to result into a chemical reaction.

Q8.If a plot of log_10⁡C versus t give a straight line for a given reaction, then the reaction is

•  Zero order
•  First order
•  Second order
•  Third order

Solution
A graph between the log concentration (log c) of reactant and time t for the first order reaction gives a straight line, whose slope is equal to -k/2.303 log_10⁡〖C_A 〗=-kt/2.303+log_10⁡〖(C_A )_0 〗 Hence, the order of the above reaction is one.

Q9.If the reaction rate at a given temperature becomes slower then

•  The initial concentration of the reactants remains constant
•  The entropy changes
•  The free energy of activation is lower
•  The free energy of activation is higher

Solution
Slow reaction rate indicates higher free energy of activation

Q10. Which statement is not correct?

•  For endothermic reactions, heat of reaction is lesser than energy of activation
•  For exothermic reactions, heat of reaction is more than energy of activation
•  For exothermic reactions energy of activation is less in forward reaction than in backward reaction
• For endothermic reactions energy of activation is more in forward reaction than in backward reaction

Solution