2. – Study of reaction rates, or the
changes in the concentrations of
reactants and products with time
– By studying kinetics, insights
are gained into how to control
reaction conditions to achieve a
desired outcome, its mechanism
3. THE RATE OF REACTION
• Consider the hypothetical reaction,
A(g) B(g)
• Equimolar amounts of reactant A will be consumed while
product B will be formed as indicated in this graph:
4. – Reaction whose rate is independent of concentration
– Its differential rate law is rate = k
– One can write their rate in a form such that the exponent of the reactant
in the rate law is 0
rate = – [A] = k[reactant]0 = k(1) = k
t
– Since rate is independent of reactant concentration, a graph of the
concentration of any reactant as a function of time is a straight line with a
slope of –k (concentration decreases with time)
a graph of the concentration of any product as a function of time is a straight
line with a slope of +k
ZEROTH-ORDER REACTIONS
5. – Reaction rate is directly proportional to the concentration of one of the
reactants
– Have the general form A products
– Differential rate for a first-order reaction is
rate = – [A] = k[A]
t
– If the concentration of A is doubled, the rate of the reaction doubles;
– If the concentration of A is increased by a factor of 10, the rate increases
by a factor of 10
– Units of a first-order rate constant are inverse seconds, s–1
– First-order reactions are very common
The order of a reaction can be expressed in terms of either each reactant in
the reaction or the overall reaction.
• For example:
FIRST-ORDER REACTIONS
6. • Two kinds of second-order reactions
1. The simplest kind of second-order reaction is one whose rate is proportional to
the square of the concentration of the reactant and has the form 2A
products.
– Differential rate law is rate = – [A]
2t
– Doubling the concentration of A quadruples the rate of the reaction
– If the [A] is halved the rate of the reaction will decrease by a factor of 4.
(½)2 = ¼
– Units of rate constant is M–1s–1 or L/mols
– Concentration of the reactant at a given time is described by the following
integrated rate law:
SECOND-ORDER REACTIONS
7. Contn..
2. The second kind has a rate that is proportional
to the product of the concentrations of two
reactants and has the form A + B products.
– Reaction is first order in A and first order in B
– Differential rate law for the reaction is
rate = – [A] = – [B] = k[A] [B]
t t
– Reaction is first order both in A and in B and
has an overall reaction order of 2
8. FACTORS THAT AFFECT REACTION RATES
• There are several factors that can influence
the rate of a reaction:
1. The nature of the reactants.
2. The concentration of the reactants.
3. The temperature of the reaction.
4. The presence of a catalyst.
• We will look at each factor individually.
9. Phase and Surface Area Effects
• If reactants are uniformly dispersed in a single homogeneous solution, the
number of collisions per unit time depends on concentration and
temperature.
• If the reaction is heterogeneous, the
reactants are in two different phases,
and collisions between the reactants can
occur only at interfaces between phases;
• therefore, the number of collisions between
the reactants per unit time is reduced, as
is the reaction rate.
• The rate of a heterogeneous reaction depends on the
surface area of the more condensed phase.
10. NATURE OF REACTANTS
• This is a very broad category that encompasses the different reacting
properties of substances.
• For example sodium reacts with water explosively at room temperature to
liberate hydrogen and form sodium hydroxide.
• Calcium reacts with water only slowly at room temperature to liberate hydrogen and
form calcium hydroxide.
11. Contn..
• The reaction of magnesium with water at room temperature is so slow that
that the evolution of hydrogen is not perceptible to the human eye.
• However, Mg reacts with steam rapidly to liberate H2 and form magnesium
oxide.
• The differences in the rate of these three reactions can be attributed to the
changing “nature of the reactants”.
12. Concentrations of Reactants
• Two substances cannot react with each other unless their constituent particles
come into contact; if there is no contact, the rate of reaction will be zero.
• The more reactant particles that collide per unit time, the more often a reaction
between them can occur.
• The rate of reaction usually increases as the concentration of the reactants
increases.
13. The increase in the molecule numbers is
indicative of an increase in concentration.
A(g) + B (g) Products
A B
A B
A B
B
A B
A B
A B
A B
4 different possible A-
B collisions
6 different possible A-
B collisions
9 different possible A-
B collisions
Contn..
14. Temperature Effects
• Increasing the temperature of a system increases the average
kinetic energy of its constituent particles.
• As the average kinetic energy increases, the particles move
faster, so they collide more frequently per unit time and
possess greater energy when they collide, causing increases in
the rate of the reaction.
• Rate of all reactions increases with increasing temperature and
decreases with decreasing temperature.
15. SOLVENT EFFECTS
•The nature of the solvent can affect the reaction
rates of solute particles.
•Solvent viscosity is also important in determining
reaction rates.
1. In highly viscous solvents, dissolved
particles diffuse much more slowly than in
less viscous solvents and collide less
frequently per unit time.
2. Rates of most reactions decrease rapidly
with increasing solvent viscosity.
16. CATALYST EFFECTS
• Catalyst is a substance that
participates in a chemical reaction
and increases the rate of the
reaction without undergoing a net
chemical change itself.
• Catalysts are highly selective and
often determine the product of a
reaction by accelerating only one
of several possible reactions that
could occur.
17. • Catalysts change reaction rates by providing an alternative reaction pathway
with a different activation energy.
Contn..
