kinetics

Question 1

define the collision theory

Answer 1

it states that chemical reactions occur only if reactant particles undergo effective collision.

Question 2

what are the conditions for effective collision to occur?

Answer 2

1. Reactants particles/molecules collide with a minimum amount of energy known as the activation energy.
2. They have to collide in the correct orientation that can lead to the rearrangement of the atoms.

Question 3

define what activation energy is.

Answer 3

It is the minimum amount of energy, which the reactant particles must possess to overcome the energy barrier in order for effective collisions to result in a chemical reaction.

Question 4

what happens in the transition state theory?

Answer 4

1. when reactant molecules collide, an activitated complex(transition complex) is formed in which the old bonds in the reactant molecules are partially broken and new bonds between the reactant molecules are partially formed.
2. the energy required to form the activated complex is activation energy, Ea.
3. the source of activation energy comes from the kinetic energy of the reactant particles
4. the transition state has the maximum energy in the reaction pathway and is very unstable. it is a transient species that cannot be isolated.

Question 5

state the characteristics of Maxwell-Boltzmann Distribution curve.

Answer 5

1. the total area under the graph represents the total number of particles in the system.
2. the highest peak in the graph gives hte most probable kinetic energy that the system would have at a particular temperature.

Question 6

explain how temperature affects the rate of reaction.

Answer 6

1. At higher temperature, total number of reactant particles remain unchanged but average kinetic energy of the reactant particles increases.
2. The number of reactant particles with energy greater or equal to activation energy increases.
3. The frequency of effective collisions in the reaction increases (no. of effective collisions taking place per unit time increases).
4. Since the rate of reaction is proportional to the frequency of effective collisions, the rate of reaction increases.

must know how to draw the Maxwell Boltzmann energy distribution curve at higher temperature.

it displaces towards the right and its peak is lowered.

Question 7

define catalyst.

Answer 7

it is a substance that increases the rate of reaction by providing an alternative pathway with lower activation energy. it remains chemically unchanged at the end of a reaction.

Question 8

explain how catalysts increase the rate of reaction.

Answer 8

using the Maxwell-Boltzman distribution curve:

1. In the presence of a catalyst, activation energy is lowered.
2. The number of reactant particles with energy greater or equal to activation energy increases.
3. the frequency of effective collisions in the reaction increases.
4. Since rate of reaction is proportional to the frequency of effective collisions, the rate of reaction increases.

must know how to draw the energy profile diagram for catalysed reaction as well.

Question 9

explain what happens on the Maxwell-Boltzmann distribution curve when the no. of molecules is increased at a constant temperature.

Answer 9

1. the value of the most probably energy remains unchanged.
2. the no. of molecules with the most probably energy increases
3. area under graph increases
4. No. of molecules with energy greater or equal to activation energy increases.

Question 10

explain what happens on the Maxwell-Boltzmann distribution curve when temperature decreases w/o changing the no. of molecules.

Answer 10

1. the value of the most probably energy remains unchanged.
2. the no. of molecules with the most probably energy increases
3. area under graph remains unchanged.
4. No. of molecules with energy greater or equal to activation energy decreases.

Question 11

explain what happens on the Maxwell-Boltzmann distribution curve when a catalyst is introduced.

Answer 11

1. the value of the most probably energy remains unchanged.
2. the no. of molecules with the most probably energy remains unchanged.
3. area under graph remains unchanged.
4. No. of molecules with energy greater or equal to activation energy increases.

Question 12

state how the concentration of reactants increases the rate of reaction.

Answer 12

1. the no. of reactant particles per unit volume increases.
2. the frequency of effective collisions in the reaction increases.
3. since the rate of reaction is proportional to the frequency of effective collisions, the rate of reaction increases.

Question 13

state how the physical state of reactant affects the rate of reaction.

Answer 13

when a solid reactant is in a more finely divided state, the SA over which the solid can come into contact with liquid/gaseous reactants is larger, reuslint in an increase in the frequency of collisions and an increase in the rate of reaction.

Question 14

state how light affects the rate of reaction.

Answer 14

1. Upon absorbing light energy, the average kinetic energy of the reactant particles increases, leading to a larger proportion of particles with sufficient energy to overcome the activation energy barrier.
2. Hence, the frequency of effective collisions increases, resulting in an increase in the rate of reaction.

Question 15

state the units for the rate of reaction.

Answer 15

mol dm^-3s^-1/ mol dm^-3 min^-1.

Question 16

define instantaneous rate of reaction

Answer 16

it is the rate at a specified time.

use tangent to find

Question 17

define the initial rate of reaction

Answer 17

it refers to the rate at the start of the reaction.

tangent at t=0.

Question 18

define the average rate of reaction

Answer 18

it refers to the change in concentration of a reactant/product over a specified time interval.

Question 19

define the order of reaction

Answer 19

it is the power to which the concentration of a reactant is raised to in the rate equation.

Question 20

define overall order of reaction.

Answer 20

it is the sum of the individual order with respect to each of the reactant.

Question 21

how do temperature and catalyst affect rate constant, K?

Answer 21

when temperature increases, K increases.

when a catalyst is added, the activation energy is lowered and K becomes larger.

Question 22

define zero-order reaction

Answer 22

with respect to a reactant A, it is defined as when the rate of reaction is independent of the concentration of A.

Question 23

state what the graphical representations of a zero-order reaction are.

Answer 23

The reactant concentration-time graph is a downward sloping striaght-line graph showing that the reactant concentration decreases at a constant rate until reaction is complete.

Question 24

define first-order reaction

Answer 24

with respect to reactant A, it is a reaction where the rate of reaction is directly proportional to [A].

Question 25

state the equation and units for a first-order reaction.

Answer 25

rate = k[A] unit for K: S^-1

Question 26

define half-life

Answer 26

it is defined as the time taken for the concentration of the reactant, A to decrease to half its original value

Question 27

state the characteristics of half-life for first-order reactions and the equation involved.

Answer 27

for first-order reactions, half-life is **constant**. T1/2 = (ln2)/K **Half-life of an overall-first order reaction does not depend on the reactant concentration**. ## Footnote if half-life is not constant, we can only deduce that reaction is not first-order, but we cannot deduce whether it is 2nd or 3rd order.

Question 28

define second-order reaction.

Answer 28

with respect to a reactant A, it is a reaction where the rate of reaction is directly proportional to [A]^2.

Question 29

state rate equation and units involved for second-order reaction.

Answer 29

rate = k[A]^2 units for K : mol^-1 dm^3 S^-1

Question 30

define pseudo-order reaction and further explain.

Answer 30

it occurs when one of the reactants is present in a **large excess(about 10 times)** concentration relative to other reactants.

Question 31

state the rate equation for a pseudo-order reaction

Answer 31

rate = K[A(excess)][B] rate = K'[B] where K' =K[B]. this is because the change in concentration of the excess reactant is negliglbe compared to change in concentration of the other reactant.

Question 32

state equation of half-life for pseudo-order reaction.

Answer 32

t1/2 = ln(2)/K' where K' = K[B]. half-life of A in the pseudo-first oder reaction does not depend on its concentration, but on the initial concentration of B(large excess).