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Types of Rate Laws

Types of Rate Laws. First Order Second Order Zero Order. Why study rate laws?. Can work backwards from the rate law to infer steps by which the reaction occurs Most reactions do not occur in a single step Most result from a series of steps.

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Types of Rate Laws

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  1. Types of Rate Laws First Order Second Order Zero Order

  2. Why study rate laws? • Can work backwards from the rate law to infer steps by which the reaction occurs • Most reactions do not occur in a single step • Most result from a series of steps • Chemist designing insecticide may study growth to see what type of molecule might inhibit cycle

  3. Types of Rate Laws • Differential • simply called rate law • Shows how rate of rxn depends on [concentration] • DERIVATIVES • Integrated • Shows how [concentrations] depend on time • INTEGRALS

  4. Related Rates • Yesterday: Rate = function of [concentration] • Today: Concentration = function of time IF IF IF … we were using only calculus, we would start with a differential eqn, separate variables, then integrate.

  5. GOOD NEWS! • You can calculate reaction orders from rate laws without using calculus! ALL DERIVED from 3 Rules of Logs: 1. ln A x B = ln A + ln B lnA = ln A – ln B B 3. ln A r = r ln A

  6. HOCUS POCUS! Purple Sheet to the Rescue!

  7. Integrated Rate Law • Shows how concentration of [A] depends on time • Plot of ln[A] vs. t always yields a straight line • VERY IMPORTANT TEST for 1st Order Reactions EXPRESSIONS: As a line: ln [A] = -kt + ln [A]0 y = mx + b As a ratio: ln[A]0= kt [A]

  8. To test if a Rxn is First Order • Plot data on calculator • IF linear THEN 1st Order

  9. Let’s do an example! • Hand Out!

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