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Types of Rate Laws

Types of Rate Laws. Differential Rate Laws i.e. The rate law or rate equations Rate is given as a function of concentration of reactants Takes the form for A + B --> C rate = k[A] a [B] b use inital rate data to determine order with respect to each reactant rate constant.

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Types of Rate Laws

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  1. Types of Rate Laws • Differential Rate Laws • i.e. The rate law or rate equations • Rate is given as a function of concentration of reactants • Takes the form for A + B --> C • rate = k[A]a[B]b • use inital rate data to determine • order with respect to each reactant • rate constant

  2. Types of Rate Laws • Integrated Rate Law • expresses how concentration depends on time • is related to a differential rate law

  3. Types of Rate Laws • Which Rate Law then? • Depends on which experiment is easiest to run. • Measure how the rate changes as concentration changes...then write a differential rate equation • Measure how the concentration changes with time, then write an integrated rate law • If you can write one rate law, then you can easily write the alternate rate law

  4. Types of Rate Laws • Why know the rate law? • Using the rate law, we can determine the steps by which a reaction occurs. • We can determine which is the slowest step in a series of steps. • Knowing the slowest step allows a chemist to work on altering the overall rate of a reaction

  5. Integrated Rate Law • Look at Concentration vs. Time data (not Concentration vs Rate!!) • Based on graphing the data...a backwards approach you might say. • The linear graph will give the integrated rate law...but which graph. Trial and error...guess, if the first one doesn’t work, try another!

  6. Integrated Rate Law • First Order (Integrated) Rate Law • Try graphing this first • Plot ln[A] vs. Time • If this graph is a straight line, then you know the reaction is first order with respect to A • The equation then would be: • ln[A] = -kt + ln[Ao] • y = ln[A] • m = -k • x = time • b = ln[Ao] (initial concentration of A)

  7. Integrated Rate Law • Second Order (Integrated) Rate Law • Try this one second. • Plot 1/[A] vs. Time • If straight, rate is second order with respect to [A] and the rate law is: 1/[A] = kt + 1/[Ao] • y = 1/[A] • m = k • x = time • b = 1/[Ao]

  8. Integrated Rate Law • Zero Order (Integrated) Rate Law • If the other two graphs don’t result in a straight line, try plotting [A] vs. Time. • Since the reaction rate is zero order, the rate does not change as the concentration of A changes. • The form of the zero order (integrated) rate law is: [A] = -kt + [Ao] • y = [A] m = -k x = time b = [Ao]

  9. Integrated Rate Laws • Memorize the three forms of the integrated rate laws • Know the graph and its associated integrated rate law.

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