9 Reaction Kinetics

Question 1

Definition

Rate of reaction

Answer 1

Change in concentration of reactant or product per unit time

Question 2

Definition

Rate equation

Answer 2

Experimental relationship between the reaction rate and the concentration of the reactants.

Rate=k[A]^m [B]^n

Question 3

Definition

Order of reaction (with respect to the reactant)

Answer 3

The sum of the powers of the reactant’s concentration term in the experimentally-determined rate equation

Question 4

Definition

Rate constant, k

Answer 4

The proportionality constant in the experimentally determined rate equation.

Question 5

Formula

Half-life of a first order reaction

Answer 5

ln(2)/k

Question 6

First order reaction

Answer 6

Rate is directly proportional to [A]

Question 7

Second order reaction

Answer 7

Rate is directly proportional to [A]^2

Question 8

Zero order reaction

Answer 8

Rate is independent of [A]

Question 9

Pseudo-order reaction

Answer 9

All reagents except one to be in great excess that their concentrations can be regarded as constant

Question 10

Rate-determining step

Answer 10

The slowest step in a reaction mechanism

Question 11

Collision Theory

Answer 11

A reaction will occur only if particles collide with:
1. minimum amount of kinetic energy (activation energy)
2. appropriate collision geometry/orientation

Question 12

Transition State Theory

Answer 12

Reacting particles with the minimum activation energy will come together in the appropriate orientation to form a transition state/activated complex, which decomposes back into the reactants or rearranges to form products.

Question 13

Thermodynamic stability

Answer 13

Related to ΔG (For enthalpy driven reactions, ΔG≈ΔH)
ΔH<0 (exothermic), reaction is thermodynamically stable
ΔH>0 (endothermic), reaction is thermodynamically unstable

Question 14

Kinetic stability

Answer 14

Related to activation energy
Large activation energy, reaction is slow
Low activation energy, reaction is fast

Question 15

Factors affecting rate of reaction

Answer 15

1. Concentration of reactant particles
2. Surface area (solid reactants)
3. Temperature
4. Catalyst

Question 16

Characteristics of a catalyst

Answer 16

1. Provides alternative reaction pathway of lower activation energy
2. Usually specific
3. Catalyst is regenerated at the end of the reaction
4. Does not affect the overall stoichiometry of the reaction
5. Does not affect the position of equilibrium

Question 17

Homogenous catalyst

Answer 17

Catalyst and reactants are in the same phase

Question 18

Mechanism of a homogenous catalyst

Answer 18

Catalyst forms an intermediate with one of the reactants.
Intermediate undergoes further reaction to form the product and regenerate the catalyst
(Multi-step reaction expected)

Question 19

Heterogenous catalyst

Answer 19

Catalyst and reactants are in different phases

Question 20

Adsorption Theory

Answer 20

1. Adsorption: Reactant particles form weak bonds to active sites of the catalyst surface. Weakens/breaks the bonds within the reactant molecules, making them more reactive
2. Reaction: The reactant particles are held on the catalyst surface in close proximity and in the correct orientation so they can readily react
3. Desorption: Weak bonds between products particles and the catalyst surface are broken and products formed diffuse away

Question 21

Autocatalyst

Answer 21

When one of the products catalyzes the reaction

Question 22

Enzymes

Answer 22

Proteins that catalyze the chemical reactions in living systems

Question 23

Factors affecting enzymes

Answer 23

1. Substrate concentration (at low [S], reaction is first order, at high [S], reaction is zero order)
2. pH of medium
3. Temperature

Question 24

Sampling method

Answer 24

1. Reactions carried out at constant temperature
2. Known concentration and volumes of reactants are mixed and stopwatch is started
3. Samples of reaction mixture are withdrawn at regular time intervals
4. Sample is quenched
5. Concentration of product/reactant is analyzed via titration