Synopsis

1. Meaning of reaction rate

2. Reaction rate and concentration and time
3. Models for reaction rate
4. Reaction rate and temperature
5. Catalysis
6. Reaction mechanisms
 Rates of Reactions

Rates of reactions are usually expressed in units of moles per liter per unit time. If we know the chemical
equation for a reaction, its rate can be determined by following the change in concentration of any
product or reactant that can be detected quantitatively.

The rate of reaction is a positive quantity that expresses how the concentration of a reactant or product
changes with time.

Chemical kinetics is the area of chemistry concerned with the speeds, or rates, at which a chemical
reaction occurs. The word “kinetic” suggests movement or change; and it refers to the rate of a reaction,
or the reaction rate, which is the change in the concentration of a reactant or a product with time (M/s).

On a practical level, a knowledge of reaction rates is useful in drug design, in pollution control, and in
food processing. Industrial chemists often place more emphasis on speeding up the rate of a reaction
rather than on maximizing its yield.

The reaction can be represented by the general equation

This equation tells us that during the course of a reaction, reactants are consumed while products are
formed. Thus, the progress of a reaction can be monitored either by the decrease in concentration of
the reactants or the increase in concentration of the products.

To illustrate what this means, consider the reaction

1
𝑁2 𝑂5(𝑔) → 2𝑁𝑂2(𝑔) + 𝑂2(𝑔)
2

The concentrations of 𝑁2 𝑂5 decreases with time; the concentration of 𝑁𝑂2 and 𝑂2 increases. When one
mole of 𝑁2 𝑂5 decomposes, two moles of 𝑁𝑂2 and one-half mole of 𝑂2 are formed. This means that
 ∆[𝑁𝑂2 ] ∆[𝑂2 ]
−∆[𝑁2 𝑂5 ] = =
2 1
2
Where ∆[ ] refers to change in concentration in mole per liter. The minus sign in front of the 𝑁2 𝑂5
term is necessary because [𝑁2 𝑂5 ] decreases as the reaction takes place, the numbers in the
denominator of the terms are the coefficients of these species in the balanced equation.

The rate of reaction can now be defined by dividing by the change in time, ∆𝑡:
−∆[𝑁2 𝑂5 ] ∆[𝑁𝑂2 ] ∆[𝑂2 ]
𝑟𝑎𝑡𝑒 = = =
∆𝑡 2∆𝑡 1
∆𝑡
2
The rate of a reaction is a positive quantity, so a minus sign is needed in the rate expression to make the
rate positive. On the other hand, the rate of product formation does not require a minus sign because
∆[𝑁𝑂2 ] ∆[𝑂2 ]
and 1 are positive quantity (the concentration of 𝑁𝑂2 & 𝑂2 increases with time).
2∆𝑡 ∆𝑡
2

More generally, for the reaction

𝑎𝐴 + 𝑏𝐵 → 𝑐𝐶 + 𝑑𝐷
Where A, B, C, and D represent substances in the gas phase (g) or in aqueous solution (aq), and a, b, c, d
are their coefficients in balanced equation,
−∆[𝐴] −∆[𝐵] ∆[𝐶] ∆[𝐷]
𝑟𝑎𝑡𝑒 = = = =
𝑎∆𝑡 𝑏∆𝑡 𝑐∆𝑡 𝑐∆𝑡
Suppose that for the formation of ammonia

𝑁2(𝑔) + 3𝐻2(𝑔) → 2𝑁𝐻3(𝑔)

The molecular nitrogen is disappearing at the rate of 0.10 mol/L per minute, that is ∆[𝑁2 ]⁄∆𝑡 =
𝑚𝑜𝑙
−0.10 𝐿𝑚𝑖𝑛.

From balanced equation above, we see that the concentration of 𝐻2 must be decreasing three times as
𝑚𝑜𝑙
fast: is ∆[𝐻2 ]⁄∆𝑡 = −0.30 𝐿𝑚𝑖𝑛 and the concentration of 𝑁𝐻3 must be increasing at the rate of
𝑚𝑜𝑙
2 × 0.10 𝐿𝑚𝑖𝑛

−∆[𝑁2 ] −∆[𝐻2 ] ∆[𝑁𝐻3 ] 𝑚𝑜𝑙

𝑟𝑎𝑡𝑒 = = = = 0.10
∆𝑡 3∆𝑡 2∆𝑡 𝐿𝑚𝑖𝑛
Example
 Factors that influence reaction rate

Four factors have marked effects on the rates of chemical reactions. They are (1) nature of the
reactants, (2) concentrations of the reactants, (3) temperature, and (4) the presence of a catalyst.

I. nature of the reactants: The physical states of reacting substances are important in determining
their reactivities. A puddle of liquid gasoline can burn smoothly, but gasoline vapors can burn
explosively. Two immiscible liquids may react slowly at their interface, but if they are intimately
mixed to provide better contact, the reaction speeds up. White phosphorus and red phosphorus
are different solid forms (allotropes) of elemental phosphorus. White phosphorus ignites when
exposed to oxygen in the air. By contrast, red phosphorus can be kept in open containers for
long periods of time without noticeable reaction.
II. concentrations of the reactants:

There are four factors namely

1. Concentration: Molecules must collide to react, a major factor influencing the rate of a given
reaction is reactant concentration, a reaction can occur only when the reactant molecules
collide. Thus the reaction rate is proportional to the concentration of reactants:
𝑟𝑎𝑡𝑒 ∝ 𝑐𝑜𝑙𝑙𝑖𝑠𝑖𝑜𝑛 𝑓𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦 ∝ 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑖𝑜𝑛
2. Physical State: molecules must mix to collide. The frequency of collisions between molecules
also depend on the physical states of the reactants. When the reactants are in the same phase,
as in an aqueous solution, random thermal motion brings them into contact. When they are in
different phases., contacts occur only at the interface, so vigorous stirring and grinding may be
needed. In these cases, the more finely divided is solid and liquid reactants, the greater its
surface area per unit volume, the more contact it makes with the other reactants and the faster.
the reaction occurs.
3. Temperature.; Molecules must collide with enough energy to react. Temperature usually has a
major effect on the speed of your reaction. Recall that molecules in his sample of gas have a
range of speeds, With the most probable speed depends on the temperature. Thus, at higher
temperature, Molecule collision occur in a given time. At the higher temperature, more of the,
sufficiently energetic collisions occur. Thus, Raising the temperature increases the reaction rate
by increasing the number and especially the energy of collisions.
4. Catalyst.; speeding up a chemical reaction. Hey, catalysis is a substance. That increases the rate
without being consumed in the reaction, because Catalysts are not consumed., only very small.,
non-stoichiometric quantities are generally required.

Rate Expression and Rate constant

The rate law expresses the relationship of the rate of a reaction to the rate constant and the
concentrations of the reactants raised to some powers.

For the general reaction:

𝑎𝐴 + 𝑏𝐵 → 𝑐𝐶 + 𝑑𝐷
the rate law takes the form
 𝑅𝑎𝑡𝑒 = 𝑘[𝐴]𝑥 [𝐵]𝑦
where 𝑥 and 𝑦 are numbers that must be determined experimentally. Note that, in general, x and y are
not equal to the stoichiometric coeffi cients 𝑎 and 𝑏.

Reaction order, defined as the sum of the powers to which all reactant concentrations appearing in the
rate law are raised. The overall reaction order is 𝑥 + 𝑦. Alternatively, we can say that the reaction is 𝑥𝑡ℎ
order in 𝐴, 𝑦𝑡ℎ order in 𝐵, and (𝑥 + 𝑦)th order overall. Reaction order enables us to understand how
the reaction depends on reactant concentrations.

Suppose, for example, that for the general reaction 𝑎𝐴 + 𝑏𝐵 → 𝑐𝐶 + 𝑑𝐷 we have 𝑥 = 1 and 𝑦 = 2.
The rate law for the reaction is

𝑅𝑎𝑡𝑒 = 𝑘[𝐴][𝐵]2

The dependency of reaction rates on concentration is readily determined for the decomposition. Of
N205. the plots of rate versus Concentration is a straight line through the origin which means that the
rates must be directly proportional to the concentration.

𝑟𝑎𝑡𝑒 = 𝑘[𝑁2 𝑂5 ]
This equation is referred to as the rate expression for the decomposition of 𝑁2 𝑂5 and the
proportionality constant 𝑘 is called rate constant. The rate constant is independent of other quantities
in the equation. The rate expression can take various forms, depending on the nature of the reaction.
An expression which shows how the reaction rate is related to concentrations is called the rate law or
rate equation.