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Chemical Kinetics

Chemical Kinetics is the branch of physical chemical which

deals with the rate and mechanisms of chemical reactions.

The reaction rate (R) for a reactant or product in a particular

reaction is intuitively defined as how fast or slow a reaction

takes place: R= t

C

С

t

R – the increase in C(x) of

products of a reaction per unit

of time or the decrease in C(x)

of reactants per unit of time.

1

2

1. Forward2. Backward

Factors That Affect Reaction Rates

1) the physical state (nature) of the reactants,

2) the concentrations of the reactants,

3)the temperature at which the reaction occurs,

4) and whether or not any catalysts are present in the

reaction.

2. Concentration plays a very important role in reactions, because according to the collision theory of chemical reactions, molecules must collide in order to react together.

As the concentration of the reactants increases, the

frequency of the molecules colliding increases, striking each

other more frequently by being in closer contact at any given

point in time.

The rate of a chemical reaction at any instant at a given t° is

proportional to the active mass at that instant of each of the reactants

present in the systems; the active mass in a homogeneous system, is

defined as the number of moles of the substance present per unit

volume (mol/l – molar concentration).

The Law of Mass Action.The effect of mass on the rate of reaction was studied by Guldberg and

Waage who in 1864 stated Law of mass action as follows.

Homogeneous reactions are chemical reactions in

which the reactants are in the same phase.

2CO(g) + O2(g) 2CO2 (g) R1 =k1 [CO]2 [O2] R2 =k2 [CO2]2

Heterogeneous reactions have reactants in two or more

phases. Reactions that take place on the surface of a catalyst

of a different phase are also heterogeneous.

! Solid state is not on account

S(s) + O2 (g) SO2 (g)

R 1 =k1[O2]

R 2 =k2[SO2]

3. Effect of Temperature. Vant – Hoff’s Rule. A rise in t° leads to a tremendous increases in reaction rate. The rate of many reactions increases 2-4 times for a 10°C rise in t°.

Ea

Most reactions involving neutral molecules

cannot take place at all until they have

acquired the energy needed to stretch, bend,

or otherwise distort one or more bonds. This

critical energy is known as the activation

energy (Ea) of the reaction.

Finally, in 1899, the Swedish chemist Svante Arrhenius (1859-

1927) combined the concepts of activation energy and the

Boltzmann disribution law into one of the most important

relationships in physical chemistry:

It is often convenient to estimate the activation energy from

experiments at only two temperatures.

Activation energy plots.

Activation energy diagrams can describe both

exothermic and endothermic reactions:

A catalyst is usually defined as a substance that

speeds up a reaction without being consumed by it.

More specifically, a catalyst provides an alternative,

lower activation energy pathway between reactants

and products. Most biochemical processes that

occur in living organisms are mediated by enzymes,

which are catalysts made of proteins.

4. Effect of catalysis.

catalysts affect the forward and reverse rates

equally; this means that catalysts have no effect on the

equilibrium constant.

Catalysts are conventionally divided into two categories:

homogeneous and heterogeneous. Enzymes, natural

biological catalysts, are often included in the former group,

but because they share some properties of both but exhibit

some very special properties of their own, we will treat them

here as a third category.

Mechanism of catalysis.There are many types of mechanisms of catalysis .

The catalyst combines with one of the reactants to form a more

reactive but unstable intermediate compound which then reacts with

other reactants to yield the products.

1) 2NO + O2 2NO2

2) NO2 +SO2 SO3 + NO cat.

2SO2 + O2 2SO3 NO

Molecularity - is the member of molecular taking part in a simple reaction or as the number of molecules taking part in the rate determining step (slowest step) of a complex chemical reaction.By molecularity reactions may be classify to: Unimolecular : Ca CO3 t° CaO + CO2

Dimolecular : H2 + Cl2 2HCl

Trimolecular : 2CO + O2 2CO2

The Order of an elementary reaction. The term order of a

reaction is used to denote the dependence of

experimentally determined reaction rate on concentration.

It is the sum of the exponents of concentration term

in the experimentally observed rate reaction.

The First order reactions.The rate of the reaction will be directly proportional to the concentration of the reactant.First order main formulas:

C0 -initial concentration C τ – final concentration τ – time

K=time-1

R = kC

Half – life of a reaction (τ0.5)is the time it takes for the reactant concentration to decrease to one half of its initial value.

The First order reactions.The rate of the reaction will be directly proportional to the concentration of the reactant.First order main formulas:R = kC K= 1/time K = τ – time

C0 – initial concentration Cτ

– final concentration

Half – life of a reaction (τ0.5) is the time it takes for the reactant concentration to decrease to one half of its initial value.

ChemicalEquilibrium

Chemical Equilibrium is the state reached by a reaction

mixture when the rates of forward and backward reactions

have become equal.

The equilibrium constant Kc

is the value obtained for the equilibrium

constant expression when equilibrium

concentrations are substituted.

A reversible reaction is a chemical

reaction that results in an equilibrium

mixture of reactants and products. For a

reaction involving two reactants and two

products this can be expressed

symbolically as: 3H2 + N2 ↔ 2 NH3

At Equilibrium the rate of the forward reaction is equal to the rate of the backward:

R1 = R2

Le – Chatelier,s Principle: when a system in

chemical equilibrium is disturbed by a change of t°,

p or C, the system shifts in equilibrium composition

in a way that tends to counteract this change of

variable ( or: if system at equilibrium is subjected to

a change (such as: t°, p, C), the system will tend to

adjust itself so as to neutralize the effect of change).

T = absolute temperature, ln = natural logarithm,

ΔG0 = change of reaction in Gibbs Free energy,

Kc = equilibrium constant.

Thermodynamic equilibrium.

ΔG0= - lnKc R T

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