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What is this?. Kinetics. Reaction Rates: How fast reactions occur. How do we measure rxn rates?. Rates must be measured by experiment Indicators that a reaction is happening Color change Gas evolution Precipitate formation Heat and light Many ways to measure the rate  Volume / time

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kinetics

Kinetics

Reaction Rates:

How fast reactions occur

how do we measure rxn rates
How do we measure rxn rates?
  • Rates must be measured by experiment
  • Indicators that a reaction is happening
    • Color change
    • Gas evolution
    • Precipitate formation
    • Heat and light
  • Many ways to measure the rate
    • Volume / time
    • Concentration / time
    • Mass / time
    • Pressure / time
how do we measure rxn rate
How do we measure rxn rate?

A  B

  • How fast product appears
  • How fast reactant disappears
forward vs reverse rxn
Forward vs Reverse Rxn
  • Some rxns are reversible
  • After a sufficient amount of product is made, the products begin to collide and form the reactants
  • We will deal only w/ rxns for which reverse rxn is insignificant

2 N2O5(aq)  4 NO2(aq) + O2 (g)

  • Why is reverse rxn not important here?
rate law
Rate Law
  • Math equation that tells how reaction rate depends on concentration of reactants and products
  • Rates = k[A]n
    • K = rate constant / proportionality constant
    • n = order of reaction
      • Tells how reaction depends on concentration
        • Does rate double when concentration doubles?
        • Does rate quadruple when concentration doubles?
2 kinds of rate laws
2 kinds of rate laws
  • Both determined by experiment
  • Differential Rate Law
    • How rate depends on [ ]
  • Integrated Rate Law
    • How rate depends on time
differential rate law
Differential Rate Law
  • 2 methods
    • Graphical analysis
    • Method of initial rates
graphical analysis
Graphical Analysis
  • Graph [ ] vs. time
  • Take slope at various pts
  • Evaluate rate for various concentrations
graphical analysis10
Graphical Analysis
  • When concentration is halved…
    • Rate is halved
    • Order = 1
    • Rate = k[N2O5]1
graphical analysis11
Graphical Analysis
  • When concentration is doubled…
    • Rate is quadrupled
    • Order = 2
    • Rate = k[N2O5]2
method of initial rates
Method of Initial Rates
  • Initial rate calculated right after rxn begins for various initial concentrations
  • NH4+(aq) + NO2-(aq)  N2(g) + 2H2O(l)
  • Rate = k [NH4+]n[NO2-]m
slide13

When [NO2] doubles, rate doubles,

First order with respect to (wrt) NO2

n = 1

When [NO2] doubles, rate doubles,

First order with respect to (wrt) NO2

m = 1

Rate = k[NH4+] [NO2-]

try this one
Try this one:

Rate = k [NO2-]2

Calculate k, using any of the trials, you should get the same value

integrated rate law
Integrated Rate Law
  • Tells how rate changes with time
  • Laws are different depending on order
  • Overall reaction order is sum of exponents
    • Rate = k  zero order
    • Rate = k[A]  first order
    • Rate = k[A]2  second order
    • Rate= k[A][B]  second order
first order integrated rate law
First order integrated rate law
  • Rearrange and use some calculus to get:
  • This is y = mx + b form
    • A plot of ln[A] vs time will give a straight line
  • If k and [A]0 (initial concentration) known, then you know the concentration at any time
second order integrated rate law
Second order integrated rate law
  • Rearrange and use some calculus to get:
  • This is y = mx + b form
    • A plot of 1/[A] vs time will give a straight line
  • If k and [A]0 (initial concentration) known, then you can now the concentration at any time
zero order integrated rate law
Zero order integrated rate law
  • Rearrange and use some calculus to get:
  • This is y = mx + b form
    • A plot of [A] vs time will give a straight line
  • If k and [A]0 (initial concentration) known, then you can now the concentration at any time
graphs give order of rxn
Graphs give order of rxn
  • Use graphs to determine order
    • If [A] vs time = zero order
    • If ln [A] vs time = first order
    • If 1/ [A] vs time = second order
half life
Half-life
  • Def’n: time it takes for concentration to halve
  • Depends on order of rxn
  • At t1/2 [A]=[A]0/2
half life22
Half-Life
  • First order
  • Second order
  • Zero Order