Chemical Kinetics

The role of a catalyst is to change :

• enthalpy of reaction

• equilibrium constant

• activation energy of reaction

• Gibbs energy of reaction

Which one of the following options is not correct for the decomposition reaction of NH₃ ?

2NH₃ (g) $\underset{\mathrm{Pt} \mathrm{catalyst}}{\overset{1130 \mathrm{K} , \mathrm{at} \mathrm{high} \mathrm{p}}{\to }}$ N₂ (g) + 3H₂ (g)

• Rate of reaction = rate constant

• Further increase in pressure will change the rate of reaction

• Rate of decomposition of NH₃ will remain constant until NH₃ disappears completely

• Rate of reaction depends on concentration of NH₃

For the reaction , aA + bB $\to$ cC , if -3$\frac{\mathrm{d}\left[\mathrm{A}\right]}{\mathrm{dT}}$ = -3$\frac{\mathrm{d}\left[\mathrm{B}\right]}{\mathrm{dt}}$ = +1.5$\frac{\mathrm{d}\left[\mathrm{C}\right]}{\mathrm{dt}}$ then a , b , c respectively are :

• 1 , 3 , 2

• 1 , 2 , 3

• 3 , 2 , 1

• 3 , 1 , 2

For reaction aA $\to$ xP, when [A] = 2.2 mM, the rate was found to be 2.4 mM s^-1. On reducing concentration of A to half, the rate changes to 0.6 mM s^-1.The order of reaction with respect to A is

• 1.5

• 2.0

• 2.5

• 3.0

75% of a zero order reaction complete in 4 h 87.5% of the same reaction completes in

• 6h

• 12h

• 8h

• 2h

For the reaction A_(g)$\to$B_{( g)}+ C_(g), the rate constant is given as (P_i is initial pressure and P_t is pressure at time t)

• $k=\frac{2.303}{\mathrm{t}}\log \frac{{\mathrm{P}}_{\mathrm{i}}}{{\mathrm{P}}_{\mathrm{t}}}$

• $k=\frac{2.303}{\mathrm{t}}\log \frac{{\mathrm{P}}_{\mathrm{i}}}{(2{\mathrm{P}}_{\mathrm{i}}-{\mathrm{P}}_{\mathrm{t}})}$

• $k=\frac{2.303}{\mathrm{t}}\log \frac{2{\mathrm{P}}_{\mathrm{i}}-{\mathrm{P}}_{\mathrm{t}}}{{\mathrm{P}}_{\mathrm{i}}}$

• $k=\frac{2.303}{\mathrm{t}}\log \frac{{\mathrm{P}}_{\mathrm{i} }-{\mathrm{P}}_{\mathrm{t}}}{2{\mathrm{P}}_{\mathrm{i}}}$

The activation energy for most of the reactions is approximately 50 kJ mol^-1 .The value of temperature coefficient for such reactions is :

• > 2

• > 3

• < 1

• > 4

For a first order reaction at 27°C , the ratio of time required for 75% completion to 25% completion of reaction is :

• log $\frac{4}{3}$

• log $\frac{3}{4}$

• $\frac{\log 4}{\log 4/3}$

• $\frac{\log 4/3}{\log 4}$

If the rate of reaction , A $\to$ B doubles on increasing the concentration of A by 4 times , the order of the reaction is :

• 2

• 1

• 0.5

• 4

The plot of a concentration of the reactant versus time for a reaction is a straight line with a negative slope. The reaction follows a

• first order rate equation

• zero order rate equation

• second order reaction

• third order rate equation