An Overview of Reaction Rate

1. An Overview of Reaction Rate

2. Factors Affecting Reaction Rate: 1. the nature of the reactant Hey, some reactions are just slow. 2. concentration of the reactant rate = k [A]x[B]y 3. temperature (Arrhenius equation) the rate constant, k, has different values depending on the temperature even for the same reaction 4. catalysts 5. Surface area (of a solid)

3. Energy Profiles of Reactions

4. fhave this much energy. enthalpy Reactants start with this much enthalpy…. and can’t get here unless they…... reaction progress

5. enthalpy Ea = activation energy -the minimum amount of energy required to initiate a reaction. DE or DH = enthalpy change reaction progress

6. Question14.32 For the uncatalyzed decomposition of H2O2(aq) to form H2O and O2, the activation energy and overall DE are 75.3 KJ/mole and -98.1 KJ/mole respectively. (a) Sketch the energy profile for this reaction. (b) What is the activation energy for the reverse reaction?

7. enthalpy Ea = 75.3 kJ Ea for reverse reaction 173.4 kJ DE or DH = -98.1 kJ exothermic reaction progress

8. enthalpy 200KJ Here’s another….. 110KJ Ea = 180 KJ Ea = ? DE = +90 KJ endothermic DE = ? 20.0KJ reaction progress

9. Temperature

10. T2 >T1 At a higher temperature, more molecules have activation energy. # molecules # of molecules with activation energy Ea Energy of molecules

11. Arrhenius equation Ea = activation energy (given) k1 k2 T2 T1 R is a constant, 8.31 J/mol.K Given a rate constant, k1 at T1, a new rate constant, k2 can be calculated at a given T2. The problem is more of an exercise in Algebra than Chemistry

12. Catalysis

13. A catalyzed reaction: A + B a AB Mechanism: intermediates A + C a AC A B C AC + B a ACB catalyst ACB a AB + C A + B a AB An intermediate is produced and then used up. A catalyst is used and then reproduced.

14. enthalpy A catalyst provides a different “path” or mechanism for the rxn one with lower activation energy reaction progress

15. # molecules Adding a catalyst….. Energy of molecules Ea

16. # molecules Adding a catalyst….. lowers the activation energy and more molecules have activation energy. Ea Energy of molecules

17. Surface Area

18. A 2 cm cube 2 cm 2 cm

19. 2 cm x 2 cm = 4 cm2 2 cm 6 faces with areas of 4 cm2 2 cm Total surface area: 6 x 4 cm2 = 24 cm2

20. Breaking up the solid into smaller pieces…..

21. Each cube: 6 x 1cm2 = 6 cm 2 1 cm Total surface area: 8 cubes x 6 cm 2 = 48 cm 2 Surface area increases with decreasing particle size. In this case, from 24 cm2 to 48 cm2

22. That's all, folks