A Model for Reaction Rates

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A Model for Reaction Rates
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Expressing Reaction Rates

• Average Rate ?quantity
• ?time
• The amount of increase or decrease depends on
  mole ratios
• Units Molarity/s or mol/Ls

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Expressing Reaction Rates Cont.

• As a reaction proceeds, there is a decrease in
  concentration of reactants and an increase in the
  concentration of the products
• N2 3H2 ? 2NH3
• N2 and H2 decrease in concentration over time,
  while NH3 increases in concentration over time.

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Expressing Reaction Rates

• Average reaction rate
• Concentration M2 - Concentration M1
• t2 - t1
• ?Concentration
• ?time
• Reaction rates must always be positive
• If you get a negative reaction rate, change the
  sign to positive

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The Collision Theory Summary

• Reacting substances (atoms, ions, or molecules)
  must collide.
• Reacting substances must collide with the correct
  orientation
• Reacting substances must collide with sufficient
  energy to form the
• activated complex.

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Activation Energy

• The minimum amount of energy that reacting
  particles must have to form the activated complex
  and lead to a reaction
• Symbol Ea
• Direct influence on the rate of a reaction

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Exothermic
inc energy
inc time
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Endothermic
inc energy
F
inc time
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Calculating Average Reaction Rates

• ?Concentration
• ?time

• Given H2 Cl2 2HCl

• Calculate the average reaction rate expressed in
  moles H2 consumed per liter per second
• Calculate the average reaction rate expressed in
  moles Cl2 consumed per liter per second

0.025 0.060 5.00 0.00
0.007 M/s
0.035 0.070 5.00 0.00
0.007 M/s
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Rate Law

• An expression that relates rate of a reaction and
  reactant concentration
• Rate of reaction depends on reactant
  concentrations
• never includes products
• Given 2A 3B 2C
• In general, rate k Ax By

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Rate Constant

• Symbol k (lowercase)
• A constant specific and unique for every reaction
• If k is large, products form quickly
• If k is small, products form slowly

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Order of reaction

• Exponents in rate law (x and y)
• Determine how much the rate depends on the
  respective concentration(s)
• Can ONLY be determined by experiment
• Overall order of a reaction is the sum of all
  orders (x y)

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Example 1

• 2A 3B 2C
• Skeleton Rate k Ax By
• Overall order of reaction (x y)
• If x1, 1st order in A
• If y2, 2nd order in B
• Overall order 1 2 3rd order

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Example 2

• Write the skeleton rate law
• 2H2O(l) 2H2(g) O2(g)

rate k H2Ox
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Relationship of order and rate

• Example 1 rate k A 1st order
• If concentration of A doubles, rate doubles
• If concentration of A triples, rate triples
• If concentration of A is halved, rate halves
• Example 2 rate k A2 2nd order
• If concentration of A doubles, rate quadruples
• If concentration of A triples, rate is 9 times as
  fast

directly proportional
exponentially proportional
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• Example 3 rate k A0 0th order
• Recall anything raised to the 0th power 1
• If the concentration of A changes in any way
  NOTHING happens to the rate
• Rate is not dependent on concentration
• Rate k