Reaction Kinetics

Question 1

Rate of reaction

Answer 1

The change in concentration of reactant or product per unit time.

rate of reaction = change in concentration of a reactant or product/
time taken

units: mol dm–3 s–1 or mol dm–3 min–1 or mol dm–3 h-1

Rate of reaction is always a positive value.
Since the [reactant] is decreasing, d[reactant] is a negative value. The minus sign ensures that rate of
reaction is a positive value.

Question 2

2 types of reaction

Answer 2

(A) Instantaneous Rate
It is the rate at a particular time i.e. the rate at a particular instant during the reaction.

(B) Initial Rate
It is the instantaneous rate at time, t = 0. It is the instantaneous rate at the start of the reaction,
when an infinitely small amount of the reactant has been used up.

Question 3

Rate equation

Answer 3

aA + bB → cC + dD

Experimentally, it can be shown that the rate of the reaction can be related to the concentrations of
individual reactants by a rate equation (or rate law),

rate = k[A]^m[B]^n
where
[A] = concentration of reactant A in mol dm−3
[B] = concentration of reactant B in mol dm−3
k = rate constant
m = order of reaction with respect to [A]
n = order of reaction with respect to [B]
m + n = overall order of reaction

Question 4

Rate equation/law

Answer 4

The rate equation is a mathematical expression that is experimentally
determined to show the exact dependence of the rate of reaction on the
concentrations of all the reactants.

Question 5

rate constant, k

Answer 5

The rate constant, k, of a reaction is the constant of proportionality in the
experimentally determined rate equation.

overall zero order reaction : mol dm–3 s–1
overall first order reaction: s-1
overall second order reaction : mol-1dm3s-1
In general, units of k is (mol dm−3)^1−ntime^−1
, where n is the overall order of the reaction.

Question 6

order of reaction

Answer 6

The order of reaction with respect to a given reactant is the power to
which the concentration of that reactant is raised to in the experimentally
determined rate equation.

Question 7

overall order of reaction

Answer 7

The overall order of a reaction is the sum of the powers of the
concentration terms in the experimentally determined rate equation.

Question 8

Zero order reaction

Answer 8

A zero-order reaction is one in which the reaction rate is independent of the concentration of the
reactant, i.e. the rate is unaffected by changes in the concentration of the reactant, i.e. rate[A]0.

Question 9

First order reaction

Answer 9

A first-order reaction is one in which the reaction rate is directly proportional to the concentration
of a single reactant i.e. rate  [A]1

First-order reaction has a constant half-life, t1⁄2, (time taken for the concentration of a reactant to decrease to half its initial value.

Question 10

Second order reaction

Answer 10

A second-order reaction is one in which the reaction rate is proportional to the product of the
concentrations of two reactants (e.g. rate  [A][B], where A and B are reactants) or to the
concentration of a single reactant raised to the power of two (e.g. rate  [A]^2).

Question 11

Half life

Answer 11

The half-life, t1⁄2, of a reaction is the time taken for the concentration of a reactant to decrease to
half its initial value.

Question 12

For first order reaction, half-life can be calculated by:

Answer 12

t1⁄2 =ln 2/k

The number of half-lives at a particular time, t, can be calculated by

final concentration of reactant/
initial concentration of reactant = (1/2)^n
where n = number of half-lives.

Question 13

Pseudo order reaction

Answer 13

Under certain conditions, a second order kinetics can be approximated to as first order kinetics. The
reactions are termed pseudo first order reaction. There are three situations to consider,
* presence of large excess of a reactant;
* the solvent used is also the reactant; and
* presence of a catalyst.

Question 14

Collision theory

Answer 14

The collision theory is important in explaining the rate of reaction in terms of particle collisions and energy profile of a chemical reaction.

Question 15

Conditions for chemical reaction to occur

Answer 15

• reactant particles undergo collisions;
• collisions take place with the minimum amount of energy known as activation energy, Ea; and
• collisions have the correct geometry (i.e. reactant particles are correctly oriented with respect
  to each other)

Question 16

Activation energy

Answer 16

The activation energy, Ea, of a reaction is the minimum amount of energy that the reactant particles
must possess before they can collide to result in a reaction.

Question 17

Maxwell Boltzmann Distribution curve

Answer 17

The Maxwell-Boltzmann Distribution Curve shows how the kinetic energy of the particles of a sample
of gas varies at a given temperature.

Question 18

Factors affecting reaction rates

Answer 18

1. physical state of the reactants,
2. concentration of the reactants,
3. temperature, and
4. catalyst.

Question 19

Catalyst

Answer 19

A catalyst increases the rate of a reaction by providing an alternative reaction pathway with a lower
activation energy.

Question 20

Features of catalyst

Answer 20

• A catalyst is usually unchanged in quantity but may change physically at the end of reaction.
  For example, pellets may become powder.
• A small amount of catalyst is usually sufficient to bring about a rapid increase in reaction
  rate. Increasing the amount of catalyst indefinitely does not mean that the rate will keep
  increasing indefinitely.
• Each catalyst is specific in character. One may catalyse a class of reactions but not for all other
  reactions.
• A catalyst generally increases the rate constant, k, for a reaction.
• A catalyst alters the reaction mechanism and hence the order and rate equation

Question 21

Homogeneous catalysis and its typical features

Answer 21

In homogeneous catalysis, the catalyst and the reactants are in the same phase, either liquid or gaseous.

typical features of
homogeneous catalysis
* The catalyst takes part in the chemical reaction by being converted into an
intermediate.
* The intermediate subsequently
undergoes reaction to form the
product(s) and the catalyst is
regenerated.

Question 22

Heterogeneous catalysis and its steps

Answer 22

In heterogeneous catalysis, the catalyst and the reactants are in different phases. The catalyst is
usually in the solid phase while the reactants are usually liquids or gases.

Step 1: Adsorption of the reactant particles onto the active site on the surface of the catalyst

Step 2: Reaction at the surface of the catalyst

Step 3: Desorption of the reactants or products from the surface of the catalyst

Question 23

Autocatalysis

Answer 23

Autocatalysis is a type of catalytic action whereby the product of a reaction acts as a catalyst for
the reaction.
A reaction in which a product acts as a catalyst is said to be autocatalytic.

Question 24

enzymes

Answer 24

look at notes

Question 25

Energy profile diagram

Answer 25

When drawing an energy profile diagram,
* The products have lower energy level than the reactants in an exothermic reaction. Likewise,
the products have higher energy level than the reactants in an endothermic reaction.
* The number of steps in the reaction corresponds to the number of humps in the diagram.
* The slow step (rate determining step) has higher activation energy, Ea, than the fast step(s).
* The intermediate, which has an energy minimum, is positioned in between the humps.