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’t Hoff differential Method
•Ostwald’s isolation Method

3. Graphical Method
▪In presence of single reactant.
▪Rate versus concentration graph:
•‘Log(a-x)’ vs ‘t’ : straight line = first order reaction.
•‘1/(a-x)’ vs ‘t’ : straight line = second order reaction.
•‘1/(a-x)^2’ vs ‘t’ : straight line = third order reaction.

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
• Only used when the rate law involve one concentration
term.
• t(1/2) ∞ a1-n
t(1/2) = k’a1-n
log t(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’t Hoff differential method
•The rate of a reaction varies as the ‘n’th power of the
concentration of the reactant , where,
n= reaction order.
• Thus, for two different initial concentrations C1 and C2,
equations can be written in the form:
,
• Taking logarithm and subtracting,
•Or , n = [log(-(dC1)/dt)-log((dC2)/dt)] ÷ [logC1 - log C2]
❑ 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[A1]x
• R(2) = k[A2]x
• R(1) / R(2) = ( [A1]/[A2])x

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