Chemical Kinetics

1. Chemical Kinetics

2. Learning Outcomes • recall earlier work on the states of matter and simple collision theory • be able to apply the ideal gas equation and the molar volume to reactions • understand the importance of energy in gas and liquid collisions: the Maxwell Boltzmann distribution and activation energy • use these concepts to give a comprehensive account of the effects of temperature and concentration on the rate of a reaction • understand that the rate of a chemical process can be represented by a rate equation • recall the general rate equation and understand the rate constant and the order of reaction • be able to investigate how the rate of a reaction changes with changing conditions • be able to carry out simple calculations of rates and rate constants • know the meaning and definition of ‘catalyst’ • understand the process of catalysis and how catalysis works • distinguish between homogeneous and heterogeneous catalysts • be able to describe the process of heterogeneous catalysis using a particle explanation

3. Kinetics TedX : Goin’ to the Dance (5 minutes) https://www.youtube.com/watch?v=OttRV5ykP7A Rusting of Iron: 2Fe (s) + O2 (g) + 2H2O (l) → 2Fe(OH)2 (s) (with limited O2, magnetite Fe3O4 is formed: FeO·Fe2O3) Horxn = -884.6 kJ {Thermite.Rxn} Thermite Reaction: Fe2O3 (s) + 2 Al (s) Mg ignition Al2O3 (s) + 2 Fe (l) Horxn = -847.6 kJ

4. Collision Theory “For a reaction between two particles to occur, the particles must collide with: Orientation Minimum energy Fuse School (3min) https://www.youtube.com/watch?v=eSInI1xHvh4

5. How do reactions occur? Reactions take place when particles collide with a certain amount of energy. Incorrect orientation – no reaction

6. How do reactions occur? Reactions take place when particles collide with a certain amount of energy. Enough energy and correct orientation –reaction!

7. Activation Energy • The minimum amount of energy needed for the particles to react is called the activation energy, and is different for each reaction. • If particles collide with less energy than the activation energy, they will not react. Kick-starting Reactions - Activation Energy

8. Factors That Affect Reaction Rates • Concentration of Reactants • concentration increases, increases the likelihood that reactants will collide. • Temperature • At higher temperatures, reactant molecules have more kinetic energy and collide more often and with greater energy. • Catalysts • Speed rxn by changing mechanism. • Nature if Reactants • g>l>s • Strong acids faster • Alkali and alkaline earth metals faster • Crushed solids (powder) faster than whole, which • Why do you chew food? Pressure For gases it actually, when increased concomitantly increases the concentration, but as solids and liquids are both condensed phases of matter, (pressure cannot squeeze them). It only affects gases.

9. Factors That Affect Reaction RatesLet’s go through the film clip Which will react faster? • Physical State of the Reactants In order to react, molecules must come collide So how can we make solids react WITHOUT changing their state? • Gases, liquids or solutions react faster than solids. (Higher pressure and concentration also affects rate.) (1) Gases, Liquids, Solutions (High P & Conc.) • Finely ground substances have more surface areas and react faster than chunk pieces. (see next slide) (2) Solids

10. Surface Area Increasing the surface area of a reactant increases the number of particles available to react. This increases the frequency of collisions which increases the rate of the reaction. Royal Society (surface area 2 min) https://www.youtube.com/watch?v=FBF7auCMy58

11. Factors That Affect Reaction Rates • Concentration of Reactants As the concentration increases, so does the likelihood that reactant molecules will collide. Increased pressure increases gas concentrations {*Rxn.withConcOxy} 0.3 M 6 M .Mg+HCl}

12. Factors That Affect Reaction Rates • Temperature At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. Generally, as temperature increases, so does the reaction rate. Rule of thumb: 10oC rate 2x Increasing the temperature of a reaction increases the kinetic energy of the particles. The particles collide more often andwith more energy. Now to say the same thing in a fancy way. Increasing the temperature increases the frequency of effectivecollisions which increases the rate of reaction

13. Activation Energy • In other words, there is a minimum amount of energy required for reaction: the activation energy, Ea. • Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.

14. INCREASING TEMPERATURE According to KINETIC THEORY, all particles must have energy.Thegreater their KINETIC ENERGY the faster they travel. ZARTMANN heated tin and directed the gaseous atoms at a rotating disc with a slit in it. Any atoms which went through the slit hit the second disc and solidified on it. Zartmannfound that the deposit was spread out and was not the same thickness throughout. This proved that there was a spread of velocities and the distribution was uneven. ZARTMANN’S EXPERIMENT This proved, and is on the graph on the next slide that: 1) no particles have zero energy/velocity 2) some have very low and some have very high energies/velocities 3) most have intermediate velocities

15. INCREASING TEMPERATURE MAXWELL-BOLTZMANN 6 min BotlzmanDistrubution http://www.youtube.com/watch?v=66AxPceP5QY T2> T1 EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER • Temperature is the measure of the average kinetic energy of the molecules in a sample. • Thus at higher temperatures, a larger population of molecules has higher energy. T1 T2 NUMBER OF MOLECUES WITH A PARTICULARENERGY Ea MOLECULAR ENERGY Explanation increasing the temperature gives more particles an energy greater than Ea more reactants are able to overcome the energy barrier and form products a small rise in temperature can lead to a large increase in rate BUT area under curve stays constant - corresponds to the total number of particles

16. INCREASING TEMPERATURE T3 MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY T1 NUMBER OF MOLECUES WITH A PARTICULARENERGY TEMPERATURE T1 > T3 MOLECULAR ENERGY • Decreasing the temperature alters the distribution • get a shift to lower energies/velocities • curve gets narrower and more pointed due to the smaller spread of values • area under curve stays constant

17. INCREASING TEMPERATURE T3 MAXWELL-BOLTZMANN DISTRIBUTION OF MOLECULAR ENERGY T1 NUMBER OF MOLECUES WITH A PARTICULARENERGY T2 TEMPERATURE T2 > T1 > T3 MOLECULAR ENERGY REVIEW no particles have zero energy/velocity some particles have very low and some have very high energies/velocities most have intermediate velocities as the temperature increases the curves flatten, broaden and shift to higher energies

18. Factors That Affect Reaction Rates • Presence of a Catalyst • Catalysts speed up reactions. • Catalysts are not consumed during the course of the reaction. MnO2 2 H2O2 (l) 2 H2O (l) + O2 (g) Catalysis of H2O2 by MnO2

19. Catalysts One way a catalyst can speed up a reaction is by holding the reactants together and helping bonds to break. Increase the reaction rateby changing the mechanism decreases the activation energy

20. Surface Catalysis Some Reactions an in Internal Combustion Engine: 2 C8H18 (l) + 25 O2 (g) 16 CO2 (g) + 18 H2O (g) (+heat) N2 (g) + O2 (g)  2 NO (g)(causes acid rain & ozone depletion)) Pt {Pt Catalytic Converter: 2 NO(g) O2(g) + N2(g)} NO N2 O2 NO Pt Pt Surface Atoms recombine to form product, which are then released from surface Reactant molecules attach to Catalytic Surface Bonds of attached molecules are Broken

21. Catalytic Converters The catalyst(Pt and Pd) is coated onto a ceramic honeycombattached to the exhaust pipe. The catalyst helps to convert CO  CO2 And NO2 N2 + O2

22. Enzymes: biological catalysts substrate • Lock and Key Theory: the substrate (reactant) fits into the active siteof the enzyme much like a key fits into a lock. enzyme

23. Some Cambridge Questions

29. 2006 (A)

31. 2007 (A)