Kinetics

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Kinetics
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Collision Theory of Kinetics

• Kinetics is the study of the factors that effect
  the speed of a reaction and the mechanism by
  which a reaction proceeds.
• In order for a reaction to take place, the
  reacting molecules must collide into each other.
• Once molecules collide they may react together or
  they may not, depending on two factors -
• Whether the collision has enough energy to "break
  the bonds holding reactant molecules together"
• Whether the reacting molecules collide in the
  proper orientation for new bonds to form.

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Effective Collisions

• Collisions in which these two conditions are met
  (and therefore the reaction occurs) are called
  effective collisions.
• The higher the frequency of effective collisions
  the faster the reaction rate.
• When two molecules have an effective collision, a
  temporary, high energy (unstable) chemical
  species is formed - called an activated complex
• Not a true molecule because its bonds arent
  complete

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Activated Complex

• The difference in potential energy between the
  reactant molecules and the activated complex is
  called the activation energy, Ea
• The larger the activation energy, the slower the
  reaction
• The energy to overcome the activation energy
  comes from the kinetic energy of the collision
  being converted into potential energy, or from
  energy available in the environment, i.e. heat.
• Different reactions have different activated
  complexes and therefore different activation
  energies

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Factors Affecting Reaction Rate

• The kind of molecules and what condition the
  reactants are in.
• Small molecules tend to react faster than large
  molecules
• Gases tend to react faster than liquids which
  react faster than solids
• Powdered solids more reactive than blocks
• More surface area for contact with other
  reactants
• Certain types of chemicals are more reactive than
  others
• Ions react faster than molecules
• Increasing temperature always increases reaction
  rate

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Factors Affecting Reaction Rate

• The larger the concentration of reactant
  molecules, the faster the reaction will go.
• Increases the frequency of reactant molecule
  contact
• Concentration of gases depends on the partial
  pressure of the gas
• Higher pressure Higher concentration
• Concentration of solutions depends on the solute
  to solution ratio (molarity).

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Factors Affecting Reaction Rate

• Catalysts are substances that effect the speed of
  a reaction without being consumed.
• Most catalysts are used to speed up a reaction.
• Homogeneous present in same phase
• Heterogeneous present in different phase
• Molecule gets adsorbed onto catalyst active site
• Enzymes
• Catalysts work by providing a pathway for the
  reaction with a lower activation energy

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Molecular Interpretation of Factors Affecting
Rate

• Catalysts work by providing an alternative
  pathway for the reaction with a lower activation
  energy
• Lowering the activation energy means more
  molecules have enough kinetic energy so that when
  they collide they can form the activated complex
• The result is the reaction goes faster

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Reaction Dynamics

• If the products of a reaction are removed from
  the system as they are made, then a chemical
  reaction will proceed until the limiting
  reactants are used up.
• However, if the products are allowed to
  accumulate they will start reacting together to
  form the original reactants - called the reverse
  reaction.