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Kinetics V
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Collision theory and Arhenius equation

## Shaillee Kaushal is teaching live on Unacademy Plus

Shaillee Kaushal
Faculty in Chemistry with a teaching experience of 14 years. Specialised in teaching for boards and competitions.

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1. XlI-04 Kinetics-lV

2. Collision Theory of reaction (Arrhenius theory of reaction rate) ) A chemical reaction takes place due to collision among reactant olecules. The number of collisions taking place per second per unit olume of the reaction mixture is called collision frequency (Z). The value f collision frequency is very high, of the order of 1025 to 1028 for binary collisions o Esery lisin not bring aborta chemcalchange. The clilins which actually bring about the products are called effective collisions.

3. For collisions to be effective the following two barriers are to be cleared: Fraction of molecules capable of bringing (i)Energy Barriers: effective collisions The minimum amount of energy which the colliding molecules must possess as to make the chemical reaction to occur, is known as threshold energy. EnergE There is an energy barrier for each reaction the reactive species must be provided with sufficient energy as to cross the energy barrier Activation energy threshold energy-average kinetic energy of reacting molecules. Threshold energy initial potential energy of reactant molecules +activation energy

4. Activated complex Activated complex Every chemical reaction whether exothermic or endothermic has an energy barrier which has to be overcome before reactants can be transformed into products CD If the reacting molecules have enough energy they can reach the peak of the energy barrier after collision and can go to the right side of the slope and change into products AE Reactarts A+B Reactants AE A+B Products If the activation energy of the reaction is low, the rate of the reaction if high and ifthe activation energy is high the rate of reaction is slow Progressofthe reaction Progress of the reaction-,

5. If the reacting molecules have enough energy they can reach the peak of the energy barrier after collision and can go to the right side of the slope and change into products Ifthe activation energy of the reaction is low, the rate of the reaction if high and if the activation energy is high the rate of reaction is slow activation energy Ea E(activated complex) - E ground state. AHActivation energy of forward reaction- activation energy of backward reaction.

6. Orientation Barrier:- The reacting molecules must collide in proper manner it the reaction is to occur. This can be shown by the above figure. Rate of reaction is directly proportional to the number of effective collisions Products dx dt Rate collision frequency X fraction of effective collisions No product

7. Factors affecting rate of reaction: 1. Nature of the reactants: (a) Physical state of reactions: Gaseous state > liquid state > solid state decreasing rate of reaction Collision is not so effective in heterogenous system as in homogenous system

8. b) Physical size of the reactants: Among the solids, rate increases with decrease in particle size. In powdered state rate of reaction is maximum as the surface area is maximum Chemical nature of reactants: Consider the following two reactions: 2NO(g) 02 (g) 2N02(g) CH4202(g)CO2(g) + 2H20 The first reaction is faster than the second because in the first reaction only N-O ond has to be broken whereas in the second reaction four (C-H) bonds have to be roken

9. 2. Concentration of the reactants:- Rate of reaction decreases with decrease in concentration of the reactants. Rate of forward reaction decreases and that of backward direction increases with time.

10. . Effect of temperature: The rate of reaction increases considerably with an increase in temperature For every 10 C rise in temperature the rate of the reaction doubles The temperature co-efficient of the chemical reaction is defined as the ratio of the specific reaction rates of the reaction at two temperatures. u temperature co - effecient kt+10 kt

11. C Intercept log10 A slopes tan = 2.303 R 1/T log(A log B