give the relation between time first order reaction t 90% and t50%
Answers
The study of rates of chemical reactions mechanisms and factors affecting rates of reactions is called
chemical kinetics.
• Spontaneous chemical reaction means, the reaction which occurs on it’s own without any external reagent. A
spontaneous chemical reaction may be slow or fast (instantaneous).
For spontaneous reaction ΔG = – ve.
Spontaneity deals with feasibility.
• Based on the speed of reactions, the chemical reactions are of 3 types.
i) Very slow reactions: Ex. Rusting of iron, Formation of H2O from H2 and O2 at room temperature]
ii) Very fast reactions ( instantaneous reactions) :
a) Discharge of pink color of KMnO4 by oxalic acid in acid medium.
b) H2 + F2 ⎯⎯ → sun⎯light ⎯ 2HF
c) H2 + Cl2 ⎯⎯ → sun⎯light ⎯ 2HCl
Al ionic reactions and neutralisation reactions are fast.
iii) Slow reactions: These reactions occur with optimum speeds.
Rates of very slow and very fast reactions can not be determined.
Rate of these slow reactions can be measured
Ex: a) 2H2O2 → 2H2O + O2
b) 2N2O5 → 2N2O4 + O2
c) CH3COOC2H5 + NaOH→ CH3COONa + C2H5OH
Reaction rate or rate of a reaction:
• The change in concentration per unit time is called rate of reaction.
• Unit of rate of reaction : mol. lit–1.sec–1.
• Rate of reaction can be expressed with respect to reactants or products.
Rate =
dt
dc − (-sign indicates decrease in the concentration of reactants)
Rate =
dt
dc
+ (+sign indicates increase in the concentration of products)
• The rate of reaction will not remain constant and it changes with time.
• Average rate of reaction is the rate of reaction measured in long time interval.
• Instantaneous rate of reaction is the rate of reaction measured in infinitesimally
small time interval. For reaction A →B graphical representation of rate is as follows.
• The rate reaction changes with time exponentially.
• No reaction takes place uniformly through out the course of the reaction.
Expressing the rate of reaction:
• General equation : mA + nB → pC + qD
Rate of reaction = [ ] [ ] [ ] [ ]
dt
d D
q
1
dt
d C
p
1
dt
dB
n
1
dt
d A
m
1 − = − = + = +
Time →
Concentration
O
[A]
[B]
↑
Chemical Kinetics
2
Example :
1) 2N2O5 → 4NO2 + O2
rate = [ ] [ ] [ ]
dt
d O
dt
d NO
4
1
dt
d N O
2
1 2 5 2 2 − = + = +
= [ ] [ ] [ ]
dt
2 d O
dt
d NO
2
1
dt
d N2O5 2 × 2 − = − =
2) 2SO2 + O2 → 2SO3
rate = [ ] [ ] [ ]
dt
d SO
2
1
dt
d O
dt
d SO
2
1 2 2 3 − = − = +
= [ ] [ ] [ ]
dt
d SO
dt
2 d O
dt
d SO2 2 3 = + × − = −
• The decrease in the concentration of the reactant per unit time or increase in the concentration of the product
per unit time is called rate of the reaction.
• The rates of chemical reactions differ form one another, since the number and the nature of the bonds are
different in the different substances (reactants products or both)
• A → B
rate = – [ ] [ ]
dt
dB
dt
d A = +
• The unit for the rate of the reaction is
moles – lit–1 – sec–1.
• N2 + O2 → 2NO
rate = [ ] [ ] [ ]
dt
d NO
2
1
dt
d O
dt
d N2 2 − = − = (or)
rate= [ ] [ ] [ ]
dt
d NO
dt
2d O
dt
2d N2 2 = − = +
• pP + qQ → rR + sS
the rate = [ ] [ ] [ ]
dt
d R
r
1
dt
d Q
q
1
dt
dP
p
1 − = − = +
= [ ]
dt
d s
s
1
+
• The rate of the reaction measured with respect to the decrease in the concentration of the reactants
decreases.
• The rate of the reaction measured with respect to the increase in the concentration of the products increases.
• The rate of a reaction at any particular instant of time during the course of a reaction is the rate of change of
concentration of a reactant or a product at that instant of time.
• The rate of the reaction varies with time of the reaction.
• The concentration of the reactants in a reaction varies exponentially with time.
• No reaction takes place with uniform rate throughout the course of the reaction.