Rates and reaction mechanism

1. Rates and reaction mechanism • The reaction mechanism is the sequence of individual reaction steps that together complete the transformation of reactants to products • The single reaction described by chemical equation may involve several sequential elementary steps

2. Reduction of NO2 by CO • Overall reaction is NO2 + CO = NO + CO2 • Two elementary steps NO2 + NO2 = NO + NO3 NO3 + CO = NO2 + CO2

3. Reaction intermediates • Reaction intermediate: an entity created during one of the elementary steps, but consumed during a subsequent step • Intermediates are not transition states • Intermediate does not appear in overall reaction equation

4. Intermediates are neither product nor reactant • Overall reaction is NO2 + CO = NO + CO2 • NO3 is product of first step: NO2 + NO2 = NO + NO3 • NO3 is consumed in second step NO3 + CO = NO2 + CO2 NO3 is an intermediate NO2 and CO are reactants NO and CO2 are products

5. Molecularity • The number of molecules that participate in the step • Unimolecular involves one molecule • Bimolecular involves two molecules • Termolecular (rare) involves three molecules

6. Laws and mechanism • The rate law for an elementary (single) reaction step follows directly from the molecularity

7. Rate laws for overall reactions • If overall reaction occurs in single step, rate law is obviously the same as for that step • For the single-step reaction CH3Br + OH- = Br- + CH3OH

8. Slow steps, bottlenecks and rate equations • For two or more steps, one step will be slower than the rest. This is rate limiting. The reaction rate will not be faster than the slowest step • In the reaction NO2 + CO = NO + CO2 • NO2 + NO2 = NO + NO3slow • NO3 + CO = NO2 + CO2fast The molecules in the fast step do not participate in the rate equation

9. Rate-limiting step has highest Ea

10. Rate laws inform about reaction mechanism • A logical sequence for determining reaction mechanisms from observed kinetic data

11. Some like it hot: The Arrhenius equation • It is well known that all chemical processes go faster as temperature increases • This suggests • Not all collisions result in product • Fraction of collisions that results in products increases with T

12. Collision theory • Reactions only occur when molecules collide • Not all collisions result in reactions • Not all collisions have the right orientation • Not all collisions have sufficient energy • Why is that?

13. Transition state and activation energy • The approach of A towards BC increases energy due to repulsions between electrons E

14. Maximum energy is transition state • Approach of A causes weakening of B – C and formation of A – B • At transition state, the energy is a maximum E

15. Over the barrier • The B – C bond weakens and C leaves • The A – B bond forms completely • The energy lowers • Final state compared to initial state depends on relative strengths of the A – B and B – C bonds E

16. The activation energy and transition state

17. The problem of energy • Not all molecules have same energy • Distribution in energy is broad • Distribution shifts to higher energy as T increases • Only small fraction have sufficient energy to leap barrier • Fraction increases with T • Therefore rate increases with T

18. The exponent • Fraction of molecules with sufficient energy is given by • f is very, very small: activation energy of 50 kJ/mol yields value of f = 10-9 • Increasing T by 10ºC causes f to increase by a factor of 3 • Collision rate is much less sensitive to T change • T-dependence of rate is determined by fraction of collisions with correct energy, not total number of collisions

19. The problem of orientation • Not all (molecular) orientations are equal • Fraction of collisions that has correct orientation is the steric factor – p Reaction No reaction

20. Putting it all together: The Arrhenius equation • Collision rate • Rate law • But Orientation factor Collision factor Energy factor

21. Experimental determination of Ea • Plot of ln k vs 1/T is linear Ea = -R(slope) • From two temperatures:

22. Graphical determination of Ea • Exploring factors that affect rate

23. Ways over (or around) the barrier • Temperature increases reaction rate by increasing fraction of molecules with sufficient energy to jump barrier • A catalyst lowers the barrier. A catalyst acts to increase the reaction rate, but is not consumed itself during the reaction

24. Catalysis • Catalysis is a fascinating field. It deals with processes which may provide solutions for many of the key problems we face. It provides food through the ammonia synthesis. It creates flexibility in the complex matrix of energy sources, energy carriers and conversion and it contributes in minimizing pollution. J. Rostrup-Nielsen, Catalysis Today, 111 (2006) 4 - 11

25. Catalysts modify the pathway • Addition of chlorine catalyst increases rate of decomposition of ozone into O2 – reason for the destruction of ozone layer by CFCs • Although two barriers are present, both are smaller than the one without the catalyst, and reaction rate is higher

26. Without catalysts, there would be no life at all, from microbes to humans • ENZYMES are biological catalysts • Most enzymes are proteins – large molecules • Have correct shape to bring reactant molecules together in correct orientation Enzyme