Objectives:

1. Define reaction rate, average rate, instantaneous rate and initial rate.2. Determine the reaction rate based on a differential equation.

11.0 REACTION KINETICS

Chemical kinetics is the study of the rates of chemical reactions, the factors that affect these rates, and the reaction mechanisms by which reactions occur.

-Time-Optimum yield-Optimum conditions

control over reaction, obtain products economically,using optimum conditions

Important

industrial process

REACTION KINETICS

rate = d[B]dt d[B] = change in concentration of B

Because [A] decreases with time, d[A] is negative.

rate = d[A] = change in concentration of A

Example; A B

Rate of reaction

• Reaction rate is the change in the concentration of a reactant or a product with time.• Unit of rate (mol L-1 s-1)

• rate

dt = period of time

Time

1

d[A]dt

-

A B

time

rate = -d[A]dt

rate = d[B]dt [A] ↓

[B] ↑

• The average rate is the rate over a period of time.• The rate of reaction at a given time is called an

instantaneous rate of reaction.• The instantaneous rate at the beginning of a

reaction is called the initial rate of reaction.• Instantaneous rate is determined from a graph of

concentration vs time by drawing a line tangent to the curve at that particular time.

Rate of reaction

Reaction:

H2O2(aq) H2O(l) + ½ O2(g)

Reaction rates are obtained from the slopes of the straight lines;

An average rate from the purple line.

The instantaneous rate at t =300 s from the red line.

The initial rate from the blue line.

Rate of reaction

blue red

purple

instantaneous rate = rate at a specific time

Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)

average rate = -d[Br2]dt

= -[Br2]final – [Br2]initial

tfinal - tinitial

The differential rate equation

Rate =dt

d[D]

d

1

dt

d[C]

c

1

dt

]d[B

b

1

dt

d[A]

a

1

Consider the reaction,

aA + bB cC + dD

A differential rate equation enables the relationship between the rate of disappearance of reactants andthe formation of products.

a,b,c and d are the stoichiometric coefficients

Example:

The formation of NH3,

The differential rate equation

N2(g) + 3H2(g) 2NH3(g)

The differential rate equation is;

dt

]d[NH

2

1

dt

]d[H

3

1

dt

]d[N 322 Rate =

The equation means that the rate of disappearanceof N2 is 1/3 the rate of disappearance of H2 and 1/2the rate of formation of NH3.

Consider the reaction, 2HI H2 + I2,

determine the rate of disappearance of HI when

the rate of I2 formation is 1.8 x 10-6 M s-1.

dt

]d[I

dt

]d[H

dt

d[HI]

2

1 22

Example 1:

Rate =

Rate =dt

]d[I

dt

d[HI]

2

1 2dt

]d[I2 = 1.8 10-6

Solution:

dt

d[HI]= 2 1.8 10-6 = 3.6 10-6 M s-1

EXERCISE 1:

Hydrogen gas produced nonpolluting product is water vapour when react in O2 due to this reaction has been used for fuel aboard the space shuttle, and may be used by Earth-bound engines in the near future.

2H2(g) + O2(g) 2H2O(g)

• Express the rate in terms of changes in [H2], [O2] and [H2O] with time.

• When [O2] is decreasing at 0.23 mol L-1 s-1, at what rate is [H2O] increasing?

(0.46 mol L-1 s-1)

Consider the reaction,

NO(g) + O2(g) 2NO2(g).

Suppose that at a particular time during the

reaction nitric oxide (NO) is reacting at the rate of

0.066 M s-1

a) At what rate is NO2 being formed?

b) At what rate is molecular oxygen reacting?

Exercise 2:

Consider the reaction,

N2(g) + 3H2(g) 2NH3(g)

Suppose that at a particular moment during the

reaction molecular hydrogen is reacting at the rate

of 0.074 M s-1

a) At what rate is ammonia being formed?

b) At what rate is molecular nitrogen reacting?

Exercise 3: