Reaction Rates

1
Reaction Rates

• AP chapter 14.3

2
Reaction Rates

• Describe how quickly concentration of reactants
  or products are changing
• Units typically DM/Dt for aqueous reactants and
  products
• Could be units of DP/Dt for gaseous products
• Effective concentration solids and liquids does
  not change over the course of a reaction, so it
  will be more difficult to model these changes

3
Reaction equations

• RatekAmBn
• A is a symbol roughly meaning concentration
  of (in units of molarity or partial pressure)
• More precisely, it means Activity of or
  effective concentration. Remember, that even
  in an aqueous solution, not all of the ions are
  available to react.
• This is given only as one example
• The equation for each type of reaction must be
  tested experimentally. The values of m and n are
  determined experimentally

4
Reaction orders

• RatekAmBn
• The reaction above is m order for reactant A,
  n order for reactant B, and mn order for the
  reaction overall. Reaction order describes the
  influence which increasing concentration has on
  the reaction rate

5
You Try

• Describe the reaction orders for the following
  rate equation
• RatekA2B1
• Sketch the shape of the graph showing the
  relationship between
• A and rate
• B and rate
• If A and B are measured in molar units, what
  is the unit for the rate constant?

6
Whats the relationship between concentration and
time?

• Note that as time passes, the concentrations
  change, so the rate changes.
• Based on this analysis will rate increase or
  decrease as time passes?
• The general solutions for these relationships
  require some calculus, but even without calculus,
  you can learn the equations for some simple cases.

7
Integrated rate laws

• These show the relationship between A and time
  over the course of a single experiment
• First order RatekA
• First order ln A -kt lnA0
• Other integrated rate laws
• Sketch the graph of lnA against t

8
Collision model

• Reactions occur when molecules collide
• Even for unimolecular reactions, collisions are
  necessary to change the kinetic energy of
  colliding particles
• Example 3 O2 hn ? 2 O3
• Example Cl2 F2 ? 2 ClF

9
Distribution of kinetic energies

• Temperature is proportional to average kinetic
  energy for a collection of molecules
• However, the kinetic energy of any individual
  molecule could be a little less or a little more.
• Distribution of molecular energies is a
  predictable function
• http//intro.chem.okstate.edu/1314F00/Laboratory/G
  LP.htm

10
Reactions at a molecular level

• Molecules only react if they possess enough
  combined energy to overcome the activation
  energy.
• The orientations of the molecules also matters.
• http//www.mp-docker.demon.co.uk/chains_and_rings/
  mechanisms/index.html

11
Arrhenius equation

• Relates Temperature, reaction rate and activation
  energy.
• Various forms of the Arrhenius equation

12
Reaction pathway diagrams

• Show the relative potential energy of reactants,
  products, intermediates and transition states.
• intermediates are the various molecular forms
  which appear as reactant becomes product.
• Depending on context, intermediate could mean
  only the stable intermediates, or could also
  include short lived (transient) transition
  states
• Energy diagram

13
Catalysts

• Catalysts are substances which speed up a
  reaction, without being altered or consumed in
  the process
• Catalysts may be temporarily altered as part of
  one of the reaction intermediates.
• How Catalysts work

14
How catalysts work at a chemical level

• Catalysts lower activation energy, either by
  bringing reactants closer together, or otherwise
  stabilizing the reaction transition state.
• A catalyst speeds up both forward and reverse
  reactions, so the mixture comes to equilibrium
  more rapidly. A catalyst can not change the
  equilibrium concentrations
• What do you think would be the result of adding a
  catalyst to a mixture already at equilibrium?

15
Enzymes

• Enzyme is a term for a catalyst found in, or
  obtained from a biological system.
• Enzymes are primarily made of protein, but may
  also include metal ions, nucleic acids, or other
  structural materials.
• How enzymes work

16
Non-enzyme catalysts

• Both enzyme and non-enzyme catalysts are
  important in manufacturing processes, such as the
  synthesis of ammonia
• Catalysts can be homogeneous (in the same phase
  as the reaction) or heterogeneous (a finely
  divided metal in solution, for example.

17
Heterogeneous catalysts

• Why do you think heterogeneous catalysts must be
  finally divided?
• What other methods do chemical engineers to
  increase the surface area of a catalyst?