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Kinetics

Kinetics. Rates, rate laws, orders, and differential rate laws. 12.1 CHEMICAL REACTION RATES. The speed of a reaction is expressed in terms of its “rate”, some measurable quantity is changing with time.

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Kinetics

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  1. Kinetics Rates, rate laws, orders, and differential rate laws.

  2. 12.1 CHEMICAL REACTION RATES The speed of a reaction is expressed in terms of its “rate”, some measurable quantity is changing with time.

  3. The rate of a chemical reaction is measured by the decrease in concentration of a reactant or an increase in concentration of a product in a unit of time. Rate = change in concentration of a species time interval

  4. Collision theory of reaction rates • Particles must collide • Only two particles may collide at one time • Colliding molecules must have proper orientation to become products • Collision must occur with enough energy (activation energy) to overcome the electron/electron repulsion of the valence shell electrons so that the electrons can rearrange and form new bonds

  5. Factors that affect reaction rates • Nature of reactants Strength of bonds; # of bonds; aqueous vs. solid • Concentration of reactants More molecules  more collisions More collisions  faster reaction • Temperature Higher temperature  higher kinetic energy  faster molecules  more collisions  faster reaction • Catalysts Accelerate chemical reactions but are not transformed Lower activation energy Change orientation  more favorable collisions • Surface area more exposed area  more collisions

  6. How can you tell by looking, which substance is which? • What would the graph look like if you continued to collect data? • How could we express the rate of this reaction?

  7. Ways to Express Rates • Relative • Average • Instantaneous • More commonly, rates are expressed by a rate law or rate expression– two methods: • Differential – expresses how rate depends on initial concentration • Integrated – expresses how concentration depends on time

  8. 12.2 RATE LAWS: AN INTRODUCTION Reactions are reversible. So far, we’ve only considered the forward reaction. The reverse is equally important.

  9. When the rate of the forward = the rate of the reverse, we have EQUILIBRIUM! To avoid this complication we will discuss reactions soon after mixing--initial reaction rates, and not worry about the buildup of products and how that starts up the reverse reaction.

  10. The rate expression or rate law is the relationship between reaction rate and the concentrations of reactants given by a mathematical equation.

  11. CONCENTRATION AND REACTION RATE: THE RATE LAW OR RATE EXPRESSION: the relationship between reaction rate and the concentrations of reactants given by a mathematical equation. To find the exact relation between rate and concentration, we must do some experiments and collect information.

  12. Initial rxn rate = k[A]om[B]on k = rate constant [A] = initial concentration of reactant A [B] = initial concentration of reactant B m = order of reaction for reactant A n = order of reaction for reactant B the little subscript “o” means original.

  13. ORDER OF A REACTION Order with respect to a certain reactant is the exponent on its concentration term in the rate expression. Order of the reaction is the sum of all the exponents on all the concentration terms in the expression.

  14. Exponents (orders) can be zero, whole numbers, or fractions -- AND MUST BE DETERMINED BY EXPERIMENTATION!!

  15. Zero order • The change in concentration of reactant has no effect on the rate. • These are not very common. • General form of rate equation: Rate = k[A]0 = k

  16. First order • Rate is directly proportional to the reactants concentration; doubling [rxt], doubles rate. These are very common! Nuclear decay reactions usually fit into this category. • General form of rate equation: Rate = k [A]1 = k[A]

  17. Second order • Rate is quadrupled when [rxt] is doubled and increases by a factor of 9 when [rxt] is tripled, etc. These are common, particularly in gas-phase reactions. • General form of rate equation: Rate = k [A]2

  18. THE RATE CONSTANT, k Is temperature dependent & must be evaluated by experiment. The units of k depend on the rate law and the units in which the data was collected

  19. A + B  C

  20. A + B  C

  21. A + B  C

  22. Since the rate stays the same regardless of the concentration of [A], it is zero order with respect to A. However, the rate doubles with a doubling of [B] and triples with a tripling of [B]. This indicates the rate is first order with respect to [B].

  23. Adding the orders of each reactant gives the overall order of the reaction. rate = k[A]o1 [B]o1 overall order is 2 rate = k [A]o [B]o2 overall order is 3 rate = k[A]oo [B]o1 overall order is 1

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