Determination of reaction order.

Determination of reaction order.
Physical Pharmaceutics - 2
Soniya.M.Sunil 3rd year B.Pharm
Mar Dioscorus College of Pharmacy.

1. Order of reaction
•Order of a reaction refers to the power dependence of the rate
of concentration of each reactant.
•Order is an experimentally determined parameter.
•For first order reaction , rate is dependant on the
concentration of a single species.

2. Determination of reaction order
❖Exact order of a reaction can be determined experimentally.
❖The methods to determine reaction order are:
•Graphical Method
•Substitution Method
•Half life Method
•Van’tHoff differential Method
•Ostwald’s isolation Method

3. Graphical Method
▪In presence of single reactant.
▪Rate versus concentration graph:
•‘Log(a-x)’vs‘t’:straightline=firstorderreaction.
•‘1/(a-x)’vs‘t’:straightline=secondorderreaction.
•‘1/(a-x)^2’vs‘t’:straightline=thirdorderreaction.

4. Substitution Method
•Hit and trial method.
•Integrated rate law method.
•Concentration and time will be given.
•The value of k is determined .
•If the value of k is constant ,the used
equation gives the order of reaction.

5. Half-life method
•Onlyusedwhentheratelawinvolveoneconcentration
term.
• t
(1/2)∞ a
1-n
t
(1/2)= k’a
1-n
logt
(1/2)= log k’+ (1-n)loga
•Graph can be drawn where the slope is (1-n) ,
n = order of reaction.
•If different concentrations are given the ratio of half life
is taken.
,
•By taking logs and rearranging,

6. Van’tHoff differential method
•The rate of a reaction varies as the ‘n’thpower of the
concentration of the reactant , where,
n= reaction order.
•Thus, for two different initial concentrations C
1and C
2,
equations can be written in the form:
,
•Taking logarithm and subtracting,
•Or , n = [log(-(dC
1)/dt)-log((dC
2)/dt)] ÷[logC
1-logC
2]
❑ nA Product
rate = k[A] ⁿ
dxldt= k[A] ⁿ
log(dxldt) = logk+ nlogA
y = c + mx (Slope = n)

7. Ostwald’s isolation method
•Initial rate method.
•Initial rate of reaction is determined by varying the
concentration of the reactants while others are kept constant.
•For a reaction given by:
aA+ bB→cC+ dD
•Rate ∝[A]
x
[B]
y
⇒Rate = k[A]
x
[B]
y
, where ,
•[A] and [B]are concentration of reactants .
•k is the constant.
•x and y are the partial reaction order.
•If [B] is kept constant,
•R
(1)= k[A
1]
x
•R
(2)= k[A
2]
x
•R
(1)/R
(2)= ([A
1]/[A
2])
x

8. Thank You..
Soniya . M . Sunil
B. Pharm
3
rd
year
Mar Dioscorus College of Pharmacy