Wednesday, February 3, 2016

6. Chemical Kinetics - JEE Main - CBSE XII - Core Revision Points

Importance of  Core Revision Points: Core Revision Points are important because if you remember them strongly, many more points related to them will come out of your memory and help you to answer question and problems. Read them many times and make sure you remember them very strongly.


Sections in the Chapter Jauhar

6.1 Rate of a chemical reaction
6.2 Experimental measurement of reaction rate
6.3 Factors which influence rates of chemical reactions
6.4 Dependence of reaction rates on concentration
6.5 Order of a reaction
6.6 Integrated rate expansion
6.7 Experimental determination of order of a reaction
6.8 Half life period of a reaction
6.9 Collision theory: Energy and orientation barriers to reactions
6.10 Dependence of reaction rates on temperature
6.11 Concept of activation energy and activated complex theory
6.12 Arrhenius equation and calculation of activation energy
6.13 Effect of radiations on reaction rates: photochemical reactions
6.14 Mechanism of a reaction
6.15 Fast reactions

Revision of the Chapter Video - Hindi
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Revision Points


The topic "Chemical kinetics" consists of reaction rate and reaction mechanism.

The branch of chemistry which deals with the rates of chemical reactions and the mechanism by which they occur, is called chemical kinetics.

Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.

Reaction mechanism is the path by which a reaction takes place.


6.1 Rate of a chemical reaction


Reaction rate is the speed with which a reaction takes place. This shows the rate or speed at which the reactants are consumed and products are formed.

Reaction mechanism is the path by which a reaction takes place.


Rate of reaction

The rate of reaction is a quantity that tells how the concentration of reactants or product changes with time.

So this can be expressed as Δ concentration/Δ time. That is change in concenation divided by time taken for the change.

Molar concentration i.e., moles per liter (M), is used in these equations.

The brackets, [ ] are always used to to indicate molar concentrations.


6.2 Experimental measurement of reaction rate


6.3 Factors which influence rates of chemical reactions


Temperature

As temperature increases, the average kinetic energy increases. So there are more molecules with activation energy and hence reaction rate increases.

As a general approximation, the rate roughly doubles for each 10°C rise in temperature.


6.4 Dependence of reaction rates on concentration


Law of Mass Action

In 1867, Cato Guldberg, and Peter Waage, proposed this law. According to this law, for the rate determining step in a reaction, the rate of reaction is proportional to the product of the concentrations of the reactants, each raised to the power of its coefficient in the balanced equation.

For the reaction aA + bB → cC (when it is a rate determining step)

Rate of reaction is proportional to [A]^a[B]^b

The above proportionality can be written as an equation, by putting in a proportionality constant k.

Rate = k *[A]^a[B]^b

K is called the specific rate constant

Rate law

The rate for a reaction is a mathematical expression that relates the rate of reaction to the concentrations of the reactants.

For the reaction aA + bB → products

The rate law is expressed as, rate of reaction is proportional to [A]^x[B]^y.
x and y are determined experimentally. These values can be whole or fractional numbers or zero.

Rate = k[A]^x[B]^y

k = the rate constant.
[A] and [B] are molar concentrations of reactants mol/litre

Units of rate: Rate is the change in concentration with time.

If the concentrations are expressed in moles/litre and time in seconds, then the units for rate of reaction are mol litre-1 s-1 or mol L-1s-1

Units of rate constant

Units of rate constant are different for different orders of reaction.

For zero order reactions units of rate constant are mol L-1s-1

For first order reactions units of rate constant are s-1

For second order reactions units of rate constant are L mol-1s-1

Basically units of rate constant are changing to give the rate of reaction in required units mol L-1s-1 (change in concentration with time).

In case of gases, the concentrations are expressed in terms of pressure in the units of atmosphere. Therefore the rate of reaction has the units of atm per second.


Integrated Rate Expressions

For zero order reactions

k0 = {[A]0 - [A]}/t

Where k0 = rate constant in the case of zero order reactions
[A]0 = Initial concentration of reactant A

[A] = concentration of reactant A at time t.
t = time

This can be alternatively expressed.

a = Initial concentration of reactant A (in moles per litre)
x = moles reactants that changed into products in time t
a-x = concentration of reactant A after time t

k0 = x/t

Where k0 = rate constant in the case of zero order reactions
x = moles reactants that changed into products in time t

For first order reactions

k1 = (2.303/t)log{[A]0/[A]}

Where k1 = rate constant for first order equations

Alternative expression



a = Initial concentration of reactant A (in moles per litre)
x = moles reactants that changed into products in time t
a-x = concentration of reactant A after time t

k1 = (2.303/t)log{a/(a-x)}

6.5 Order of a reaction

Order of a reaction

The sum of the powers to which the concentration terms are raised in the rate law expression.

For the expression Rate = k[A]^x[B]^y, the order of the reaction is x+y. The order of the reaction is represented by n.

When n = 1, the reaction said to be first order reaction.
n = 2 second order reaction etc.

There are number of reactions where rate of reaction is independent of concentration of reactants. The order of reaction is zero.





6.6 Integrated rate expansion
6.7 Experimental determination of order of a reaction
6.8 Half life period of a reaction
6.9 Collision theory: Energy and orientation barriers to reactions
6.10 Dependence of reaction rates on temperature
6.11 Concept of activation energy and activated complex theory

6.12 Arrhenius equation and calculation of activation energy


Reaction Rate Depends on Temperature

Temperature has influence on reaction rates. In general, an increase in temperature increases the rate of almost all reactions.

A general approximate rule is that the rate of a reaction becomes almost double for every 10° rise in temperature.

Activation Energy

For many reactions some extra energy is to be supplied to the reactants to initiate the reaction. This excess energy is required to bring the energy of reactants to the energy that is required to start the reaction. The energy of the reactants at which the reaction starts is called threshold energy.

Activation energy is the extra energy supplied to initiate the reaction. Thus activation energy is equal to the difference between the threshold energy and the average kinetic energy of the reacting molecules at the the given temperature (Note as activation energy is being given the temperature of the reactants increases)

Arrhenius Equation

Arrhenius proposed a quantitative relationship between rate constant and temperature

k = Ae(–Ea/RT)

where k = rate constant
A is a constant known as frequency factor. In a JEE problem it was termed as preexponential factor
–Ea is the activation energy
Both A and –Ea0 are characteristic of the equation
T is the absolute temperature and R is the gas constant

In log form the equation becomes

log k = log A - (Ea)/2.303 RT

As the activation energy –Ea0 increases, the value of k decreases and therefore, the reaction rate decreases.

Find the value of –Ea0

If log k is plotted against 1/T (both found through experiments), the intercept of the line will be equal to - (Ea)/2.303 R. Hence from the slope found from the graph - (Ea) can be found out as -2.303 R multiplied by slope.

Second Method

Measure rate constant at two temperatures k1 and k2 at T1 and T2

log (k2/k1) = (Ea/2.303 R)[ (1/T1) - (1/T2)]

JEE 2009 problem


For a first order reaction A→P, the temperature (T) dependent rate constant(k) was found to follow the equation logk = – (2000)(1/T) + 6.0
The pre-exponential factor A and the activation energy Ea, respectively, are -

Answer:
1.0 × 1066 s-1 and 38.3 kJ mol-1



6.13 Effect of radiations on reaction rates: photochemical reactions
6.14 Mechanism of a reaction
6.15 Fast reactions

Chemical Kinetics - 28 Videos Playlist - Examfearvideos
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Updated 3 Feb 2016, 22 May 2015

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