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Chemical Kinetics

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  1. Chemical Kinetics CHAPTER 6

  2. Copy This Chemical Kinetics (Ch#6.1) • Kinetics is the study of the rate of change of concentration in chemical reactions • To study this we must concern ourselves with exactly how molecules interact during a chemical reaction

  3. Copy This Rate of Reaction(ch#6.1) • Rate: The speed with which the concentration of a reactant or product in a reaction changes rC = Δ[C] = [C]final – [C]initial Δttfinal – tinitial • The rate tells us how much of product C is produced in a certain amount of time. • A variety of units are used to express rate. • No word problems from this chapter. A + B  C + D Rate

  4. Copy This Collision Theory (Ch#6.4) • A chemical system consists of particles that are in constant random motion at various speeds • The average kinetic energy of the particles is proportional to the temperature of the sample

  5. Copy This Maxwell-Boltzmann Distribution • Graph shows how the distribution of kinetic energies changes when a substance is heated or cooled • At any temperature there are some particles with low K.E. and some with high K.E. • The higher the temperature, the more particles there are with higher K.E. T2 > T1

  6. Copy This Collision Theory (pg#383) • A chemical reaction must involve collisions between particles • An effective collision (one that results in a reaction) is one where the particles have sufficiently high energy and the correct orientation to form new bonds • An ineffective collision results in the particles rebounding off of one another unchanged

  7. Copy This Collisions IO- + Cl- ClO- + I- I O- Cl- Ineffective Transition State I O- Cl- I- -O Cl I- O Cl-

  8. Copy This Collision Theory • The rate of reaction is dependent on the frequency of collisions and the fraction of those collisions that are successful • Both the frequency and fraction of successful collisions are dependent on the temperature • Answer Q#1 (a) – (d) on page 387

  9. Copy This Transition States • For a reaction to occur, the two chemicals reacting must come sufficiently close to one another in the proper orientation in order to break old bonds and form new ones • The state in which new bonds are forming and old ones are breaking is called the transition state or activated complex • The transition state is always higher in energy than products and reactants.

  10. Copy This Activation Energy (EA) • The difference in energy between the reactants and the transition state is called the activation energy • It can also be defined as the minimum energy with which particles must collide before they can rearrange in structure, resulting in an effective collision. • This energy is directly related to the rate of reaction

  11. Copy This Activation Energy (EA) A B C B C EA Ep A Reactants A B ΔH C Products A + BC  AB + C

  12. Activation Energy • How is the activation energy related to the rate of reaction? • This goes back to the Maxwell-Boltzmann distributions we saw before and the idea that collision frequency and the number of successful collisions per second determine the rate Rate = collision frequency x fraction of collisions that are effective

  13. Copy This EA and Rate # of Particles Higher activation energies result in there being fewer particles with sufficient energy required to reach the transition state Fewer particles with sufficient energy results in fewer effective collisions, and thus a slower rate EA2 > EA1 EA1 EA2 Kinetic Energy

  14. Copy This Factors Affecting the Rate • There are five things that can affect the rate of a reaction: • The chemical nature of the reactants • The concentration of reactants • The temperature • The presence of a catalyst • Surface area of solid reactants

  15. Copy This Chemical Nature • Ex: Na metal reacts violently with water but Au metal does not react with water at all • This difference in reactivity is directly related to the activation energy required for reaction. • Since Na has a violent reaction it has a relatively low EA • Since Au does not react its EA must be so high that it prohibits reaction at the current temperature

  16. Copy This Concentration and Area • Higher concentrations of reactants lead to higher rates of reaction because there are more particles that can collide • This increases the collision frequency • Surface Area • The surface area of solids affects the rate in the same way. • More surface area equals more particles that are colliding with the reactants in the solution, thus a higher collision frequency

  17. Copy This Temperature • Higher temperatures increase the; • Speed of particles • Amount of particles that have enough energy to overcome the activation energy of the reaction • Since the particles move faster there is a higher collision frequency • Since more particles have the requisite energy the amount of effective collisions increases

  18. Copy This Collision Theory

  19. Copy This Catalysts • Catalysts provide alternate ways for a reaction to proceed that have lower activation energies than the uncatalyzed reaction • These alternate pathways may be single steps or may involve several intermediate steps • Since the EA is lower the rate increases • Enzymes in your body are catalysts

  20. Copy This Catalysts

  21. Copy This Multistep Catalysts

  22. HOMEWORK • Answer Q# 2- 6 on page 396-397 of the textbook

  23. Reaction Mechanisms • To understand the rate of reactions it is important for us to understand how the reaction proceeds • Does it happen in one single collision? • Are there multiple reactions required to get to the final product? • The exact progress a reaction takes is called the mechanism

  24. Copy This Reaction Mechanisms • Elementary Step • A step in a reaction mechanism that involves a single collision of two or three particles (or one particle breaking down) • All reaction mechanisms are composed of elementary steps

  25. Copy This Reaction Mechanisms • Examples: • Unimolecular elementary step Cl2(g) + light  2 Cl(g) • Reaction involving several elementary steps 4 HBr(g) + O2(g)  2H2O(g) + 2 Br2(g) Composed of: HBr(g) + O2(g)  HOOBr(g) HOOBr(g) + HBr(g)  2HOBr(g) 2{ HOBr(g) + HBr(g)  H2O(g) + Br(g)} Bimolecular Elementary steps

  26. Copy This Reaction Mechanisms 4 HBr(g) + O2(g)  2H2O(g) + 2 Br2(g) Composed of: HBr(g) + O2(g)  HOOBr(g) HOOBr(g) + HBr(g)  2HOBr(g) 2{ HOBr(g) + HBr(g)  H2O(g) + Br(g)} • HOBr(g) and HOOBr(g) are both not present in the overall reaction • They are produced and consumed in the mechanism • Compounds like this are called intermediates

  27. Copy This Reaction Mechanisms • Each step has a separate activation energy and therefore a separate speed • The overall rate of a multistep reaction is determined by the rate of the slowest step

  28. Copy This Rate Law • The rate of a reaction is proportional to the concentrations of the reactants aX + bY products r = k[X]m[Y]n • Where k is the rate constant, which is related to the activation energy of the reaction and the temperature

  29. Copy This Rate Law • The exponents are the same as the coefficients for reactions that are elementary steps 2 A + B  C r = k[A]2[B] • If the reaction involves several steps the exponents may not be the same as the coefficients (and in some cases are not integers) 2 NO2(g) + F2(g)  2 NO2F r = k[NO2][F2]