Collision Theory

Sem 1 Jyotsana Panwar

1. SAMRAT PRITHVI RAJ CHOUHAN GOVERNMENT COLLEGE, AJMER. 2021-22 DEPARTMENT OF CHEMISTRY SUBMITTED BY:- JYOTSANA PANWAR M.Sc. SEMESTER FIRST

2. COLLISION THEORY Image by-tmcleod.org

3. CONTENTs Introduction Main postulates of collision theory Mathematical treatment of collision theory Test of simple collision theory Failure of simple collision theory Modification of collision theory General weakness of collision theory Points to remember

4. introduction Proposed by Max Trautz & William Lewis . Based on the kinetic theory of gases. According to this theory, the reactant molecules have to collide in a certain way with a certain amount of energy in order to form a new product. Since, for the collision at least two molecule are must involve, therefore this reaction is known as Bimolecular reaction .

5. Main postulates of collision theory Collision is required between the reactant molecules to convert them into product. Reactant molecules are assumed to be a solid sphere . Due to collision, kinetic energy of the reactant molecules increases, & the molecules which acquire the energy either equal to or more than activation energy , converts into the product. The number of collision between the reactant molecule can be increased by increasing the temperature of the reactant. Thus, Rate of reaction Temperature .  

6. Not all the molecules of reactant acquire kinetic energy equal to activation energy , so not all reactant molecules are converted into the product. The activation energy is that minimum additional energy which is required for effective collision between molecule so that they may got threshold energy. Image by-nagwa.com

7. Mathematical treatment of collision theory A+B Product According to collision theory, Rate of reaction Number of collision between A&B According to Rate law, Rate of reaction So, Number of collision concentration of reactants According to Arrhenius law, Rate of reaction= Z …(1) Where, Z= no. of binary collision per unit time, = activation energy According to kinetic theory, the rate of reaction is related with the no. of molecule per unit volume i.e. rate nxn  

8. rate , rate = k …(2) Comparing eq. (1) & (2), we get k k= Z …(3) From kinetic theory of gases the value of Z is given by- Z= 4 …(4) where, n= no. of molecules per unit volume (A,B) Putting eq. (4) into (3) we get,  

9. k= 4 x After simplifying we’ll get, k= 4 x …(5) k= z , where z= 4 is the collision number. Collision number / collision frequency is the number of collisions per unit time in per unit volume. When two molecules of different types are involved in collision, z becomes- z= [ ] where, ( )= reduced mass  

10. Test of simple collision theory Theory can be tested by comparing values of “z” calculated from the mathematical eq. derived above & the experimental value of “z”. Alternatively theory can be tested by comparing the values of rate constant (K) obtained from theory & from observed value. For e.g. 2HI Experimental value of k= 3.5x at 550K Calculated value of k= 5.4x at 550K  

11. FAILURE OF SIMPLE COLLISION THEORY In polymerization of ethene :- there is vast difference between the observed & theoretical value of K. Polymerization of 1,3-butadiene :- it occurs at rate which is slower of times than theoretical value. Many chain reactions proceed with much higher rate than theoretical value. For e.g. specifically reaction between hydrogen and bromine occurs at a very high speed than the calculated value.  

12. Modification of collision theory Collision is the only factor for a chemical reaction in gaseous state then all reactions must have same rate, which is not same for all reactions. Thus, some modifications have been suggested in simple theory- Collision between all molecules doesn’t bring a chemical reaction. It means out of total collisions only a fraction of collision is effective to bring a chemical reaction. Image by- savemyexams.co.uk

13. the molecules which comes close to each other for collision must have a definite or certain orientation so that their collision may be effective. Image by- opentextbc.ca Image by- slidetodoc.com

14. Thus the rate constant of reaction depends upon the no. of effective collision & ‘k’ is given by- k= P x Z or A= pre-exponential factor to make allowance for such effect with increase or decrease the rate of reaction- The value of ‘P’ reaches- theoretically= 0-1 practically= 1-  

15. General weakness of collision theory Co-relation between ‘P’ & characteristic of reaction molecules have not been very successful. Theory doesn’t give any explanation for abnormally high rate of some reactions for e.g. Chain reactions Simple collision theory doesn’t give any physical explanation of relationship between the “P” & “ Δ S” (change in entropy) The theory is unsuccessful for unimolecular reaction until Lindmann’s theory is considered.

16. Theory is based upon classical mechanics, which needs a lot of modifications. There is no consideration have been taken for the internal motion (rotational motion) of the reactants. Theory considers molecule as a hard sphere which is not true for the gaseous molecules specifically.

17. POINTS TO REMEMBER Image 1 by- learnpick.in Image 2 by-brainkart.com

18. references Dalal , Mandeep (2018). A textbook of physical chemistry-Volume-1. Dalal Institute, Main market, Sector 14, Rohtak, Haryana 124001, India. Denise granger ( 2017). What is collision theory of chemical reactions? Socratic Q & A, Google search.

19. Image by- memesgenerator.net