This document discusses key concepts in chemical kinetics including: - Rate of reaction is defined as the change in concentration of reactants or products per unit time. Rate laws describe how the rate of reaction depends on reactant concentrations. - Order of reaction refers to the sum of powers of concentrations in the rate law. Molecularity is the actual number of reacting species. - Reaction orders include zero order (independent of concentration), first order, and second order reactions. Integrated rate equations relate concentration changes to rate constants for each order. - Factors like temperature, solvent, ionic strength, and catalysis influence reaction rates as described by theories like collision theory and Arrhenius equation. Determining

1. Chemical kinetics
(Dr.) Mirza Salman Baig
Assistant Professor (Pharmaceutics)
AIKTC, School of Pharmacy,New Panvel
Affiliated to University of Mumbai (INDIA)

2. Rate of reaction
• Rate of reaction is defined as the
change in concentration of any of
reactant or product per unit time.
• Rate= dA/dt
• Unit is Mole/liter

3. Rate Law
• The rate of reaction is directly
proportional to the reactant
concentration, each concentration
being raised to some power.
• 2A + B --> Product
• Rate =k[A]m[B]n
(k = specific rate constant)
The Equation shows how rate is related to concentration

4. Order of reaction
• Order of reaction is defined as sum of
powers of concentration in rate law.
• Rate =k[A]m[B]n
• Order of reaction in above case is
(m+n)
• It is the number of concentration
terms on which the rate of reaction
depends.

5. Molecularity of reaction
Number of molecules that react to give the
product. It is number (intiger) not a
fraction.
Types
• Unimolecular
cis to trans
• Bimolecular
A+B ---> C
• Termolecular
A+B+C --> D

6. Reaction
Order
• Sum of powers of
concentration
terms in rate law
• Experimentally
determined
• It can have
fractional value
• It can assume zero
value
Molecularity
• Number of
reacting species
in a reaction
• Theriotical
concept
• It is always a
whole number
• It cannot have zero
value

7. Zero Order reaction
• Rate of reaction is independent of
concentration of reactant/product
• Reaction A--> Product
• Initial conc a 0
• Final conc a-x x

8. Zero Order reaction
• Rate of reaction = -d[A]/dt= k0[A]0
• dx/dt= k0 (a-x)0=k0
• x=k0t
• k0 is rate constant (or specific rate
constant) of zero order reaction
• Rate constant is the rate of reaction
at all concentrations x/t

9. First Order Reaction
• A--> Product
• At time t=0 concentration of A is a
mole/lit, If at time t, x mole of A have
changed, the final concentration
after time t will be (a-x)
• For first order reaction dx/dt is
directly proportional to concentration
of reactant...
• dx/dt= k(a-x)
• dx/(a-x)= kdt

10. First Order Reaction
• dx/(a-x)= kdt
• Integrating above equation..
• ∫dx/(a-x)= ∫kdt
• -ln (a-x) = kt + I
• If t=0, x=0 then I= -ln a
• substuting for I
• ln a/(a-x)= kt or k = (1/t) ln a/(a-x)
• changing in common log
• k= 2.303/t . log a/(a-x)

11. First Order Reaction
• k= 2.303/(t2-t1) . log (a-x1)/(a-x2)
• t1 and t2 are time interval at which x1
and x2 amount of reactant changed
respectively
• Example
• N2O5--> 2NO2 + 1/2 O2

12. Second Order Reaction
• Reaction A--> Product
• Initial conc a 0
• Final conc a-x x
• For second order reaction rate of
reaction is proportional to square of
concentration of reactants
• dx/dt= k (a-x)2

13. Second Order Reaction
• dx/dt= k (a-x)2
• dx/(a-x)2=k dt
• On integration ∫dx/(a-x)2= ∫k dt
• 1/(a-x) = kt +I
• At t=0, x=0.... I = 1/a

14. Second Order Reaction
• Substituting value of I in above
equation
• 1/(a-x) = kt + 1/a
• kt = 1/(a-x) - 1/a
• k = 1/t . x/a(a-x)

15. Pseudo order reaction
• Experimental order of reaction which
is not actual is known as pseudo
order
• Reaction A+B --> Product
• If B is in excesses, its concentration
will practically constant and only
concentration of A will affect rate of
reaction hence rate law will be..
• Rate = k' [A]...

16. Pseudo order reaction
(Example)
• Ethyle acetate+ Water (excesses) -->
Acetic acid + EtOH

17. Units of rate constant
• Units of rate constant for different
order reaction are different
• For Zero order
• k= d[A]/dt
• k= mol/lit . 1/time
• k= mol lit-1 time-1

18. Units of rate constant
• For first order
• k= 2.303/t . log a/(a-x)
• k= 2.303/t . log [A]0/[A]t
• k= 1/time
• k= time-1

19. Units of rate constant
• For second order reaction
• k= 1/t . x/a(a-x)
• k= 1/t . x/[A]0([A]0-x)
• k= 1/time . concentration/concentration2
• k= 1/time . 1/concentration
• k= 1/time . 1/(mol/lit)
• k= mol-1 lit time-1

20. How to determine order of
reaction
• Using integrated rate equations
• Graphical Method
• Using half life period
• Oswald isolation method

21. Using integrated rate
equations
• Perform the reaction using different
initial concentration of of reactant (a)
and note the concentration (a-x) at
regular time interwals (t)
• These values are then substituted in
integrated rate equations (first order,
second order, zero order)
• The rate equation which yield
constant value for k corrospond to
the correct order of reaction.

22. Graphical Method
• For straight line y=mx+c
• Fig A: If the plot of log (a-x) vs t is a straight
line, the reaction follows first-order .
• Fig B: If the plot of 1/(a-x) vs t is a straight
line, the reaction follows second order.
log(a-x)
t
t
1/(a-x)
Fig A Fig B

23. Half life reaction
• It is defined as the time required as
for the decrease in concentration of
reactant to half of its initial value.
• When x=a/2, then t=t1/2
• We can substitute this value in
equation of reaction

24. Using Half life method
• Seperate experiments should be
performed using different initial
concentration
• Half life for nth order reaction is
• t1/2= 1/ [A] n-1

25. Order of
reaction
Equation t1/2 (Half life)
0 k0= x/t OR x=k0t t1/2= a/2k0
1 k= 2.303/t . log a/(a-x) t1/2= 0.693/k
2 k = 1/t . x/a(a-x) t1/2= 1/ak

26. Using Half life method
• Zero order: Half life is directly
proportional to initial concentration of
the reactants
• First order: Half life is independent
of the initial concentration of
reactants
• Second order: Half life is inversly
proportional to initial cincentration
of reactants

27. Collision Theory of Reaction Rate
• According to this theory, chemical
reaction take place only when there
is collision between reacting
molecules.
• Colliding molecules must posses
sufficient kinetic energy
• A-A + B-B --> 2 A-B
• Molecules must collide with correct
orientation

28. Energy of activation Ea
• Minimum amount
of energy (Ea)
necessary to
cause reaction
between two.
• Only molecules
that collide with
kinetic energy
higher than Ea
are able to react.

29. Physical and chemical factors
influencing the chemical
degradation of pharmaceutical
product:
• Temperature
• Solvent & Dielectric constant
• Ionic strength
• Specific acid base catalysis
• General acid base catalysis

30. Effect of Temperature
(Arrhenius equation)
• Rate of reaction increace 3-folds by increase in
temperature of about 10oC
• Relation between rate constant, temperature and
Ea .....Arrhenius equation
• k= Ae-Ea/RT
• R= gas constant
• T = Temperature in Kelvin
• A= Factor related to frequency of collision
• Ea= Energy of activation
• log k2/k1 = Ea/2.303R . (T2 - T1)/T2T1
• k2 and k1 are rate of reaction at temperature T2 and
T1 respectively

31. Effect of solvent
(dielectric constant)
• Reactions involving ions of opposit charge
are accelerated by solvents with low
dielectric constant (ability of a substance to
store electrical energy in an electric field)
• Ex. Rate of hydrolysis of sulphate ester is
greater in low dielectric constant solvent like
methylene chloride than in water.
• Reaction between similar charged ions is
favoured by high dielectric constant
solvents
• Reaction between neutral molecules which
produce highly polar transition state is
favoured by high dielectric constant
solvents

32. Effect of Ionic strength
• Ionic strength may affect inter-ionic
attraction
• Increase ionic strength expected to
decrease the rate of reaction
between oppositely charged ions
and increase in rate of reaction
between similar charged ions... as
per Debye-Hukel equation

33. Catalyst
• It is a substance that influence speed
of reaction without itself being
altered chemically.

34. Specific acid base catalysis
• Specific acid base catalysis refer to
catalysis by hydrogen ion (H+) or Hydroxyl
Ion (OH-)
• Ex. Rate of hydrolysis of ester ethylacetate
• CH3-COOC2H5 --> CH3COOH + C2H5-OH
• is studeid at constant pH (buffered soln),
rate of disappearance of ethyl acetate
(ester) will apperantly First order.
• If reaction is studied at different pH (in
acidic range) then different first order rate
conatant k is observed at different pH.

35. Specific acid base catalysis
• Observed rate depend on
concentration of ester and [H+],
therefore it is actually second order
reaction.
• Observed rate constant (kobs) is
proportional to [H+]
• kobs = kacid [H+]
• logkobs = log kacid + log [H+]
• logkobs = log kacid - pH

36. Specific acid base catalysis
• Log kobs = log kacid - pH
• It suggest kobs vs pH is straight line
with slop -1 and y-intercept log kacid
• If we study same reaction in alkaline
pH then we observe different rate
constant at different pH
• log kobs = log kbase + log [OH-]
• kobs = kacid[H+] + kbase [OH-]

37. 1 st order Hydrolysis of atropin

38. General acid base catalysis
• Acid or base catalysis is not
restricted to effect of [H+] or [OH-]
• Undissociated acid and base can
often produce catalytic effect
• Metal can also serve as catalyst
• Ex. Mutarotation of glucose in
acetate buffer is catalysed by [H+],
[OH-], Acetate ion [CH3COO-],
undissociated acid [CH3COOH]
undissociated acetic acid.

39. Problem
Q. Solution of drug contained
500unit/ml of drug when prepared. It
was analyzed after 40 days and
found to contain 300 unit/ml. Assume
decomposition is first order, at what
time will the drug have decomposed
to one half of its original
concentration?

40. • a= 500 unit/ml
• (a-x) = 300 unit/ml
• t= 40 days
• k= 2.303/t . log a/(a-X)
• k= 2.303/40 . log 500/300
• k= 0.0576 × log 1.667
• k= 0.0576 × 0.222
• k= 0.01278 day-1

41. • t1/2 = 0.693/k .... for first order
• t1/2 = 0.693/ 0.01278
• t1/2 = 54.23 Days

42. Problem
• Rate constant for first order
reaction is 1.54 × 10-3

43. Thank You