Reaction Rates

1
Reaction Rates EquilibriumChapter 18

• Sections 18.1 18.2

2
Collision Theory

• Rate is a measure of the speed of any change
  that occurs within an interval of time.
• Example growth per week (cm/week)
• Collision Theory atoms, ions, and molecules can
  react to form products when they collide with one
  another, provided that the colliding particles
  have enough kinetic energy.
• Particles lacking the necessary kinetic energy to
  react bounce apart unchanged when they collide.

3
Collision Theory

• Activation Energy the minimum energy that
  colliding particle must have in order to react.

• Activated Complex or Transition State the
  unstable arrangements of atoms that forms
  momentarily.
• Top of the Hill

Activation Energy
4
Factors Affecting Reaction Rates

• Temperature
• Increasing temperature speeds reactions
• Decreasing temperature slows reactions
• Why???? (Think collisions)
• Concentration
• Increasing concentration speeds reactions
• Decreasing concentration slows reactions
• Why ???
• Particle Size
• Small particles-lots of surface area fast
  reaction
• Large particles-small surface area slow reaction
• Use of a Catalyst

5
Catalysts

• Catalyst a substance that increases the rate of
  reaction by lowering the activation energy.
• The catalyst is not used up in the reaction.

6
Chemical Equilibrium

• Chemical Equilibrium when the rates of the
  forward and reverse reactions are equal.
• 2SO2 (g) O2(g) ? 2SO3 (g)
• At chemical equilibrium no net change occurs in
  the actual amounts of the components of the
  system.

7
Le Châteliers Principle

• Le Châteliers principle If a stress is applied
  to a system in dynamic equilibrium, the system
  changes in a way that relieves the stress.
• See Book Examples Pages 352-354.

• French chemist Henri Le Châtelier (1850-1936)

8
Chapter 18Reaction Rates and Equilibrium

• Sections 18.3

9
Equilibrium Constants

• aA bB ? cC dD
• Equilibrium constant (Keq) is the ratio of
  product concentrations to reactant concentrations
  at equilibrium, with each concentration raised to
  a power equal to the number of moles of that
  substance in the balanced equation.
• Keq Cc x Dd
• Aa x Bb
• Write the Keq expression for the following
  reaction.
• H2 (g) I2 (g) ? 2HI(g)

10
Solubility Product Constant

• AgCl(s) ? Ag(aq) Cl- (aq)
• Keq Ag x Cl-
• AgCl
• Solubility product constant (Ksp) equals the
  product of the concentrations of the ions each
  raise to a power equal to the coefficient of the
  ion in the dissociation equation.
• You can leave out the solid because you can
  assume that the concentration of AgCl is
  constant.

• Ksp Ag x Cl-
• Ksp 1.8 x 10-10
• The smaller the numerical value of the solubility
  product constant, the lower the solubility of the
  compound.

11
The Common Ion Effect

• Common Ion An ion that is both in the solution
  and in the salt.
• Common Ion Effect The lowering of the solubility
  of an ionic compound as a result of the addition
  of a common ion.

12
Entropy and Free EnergyChapter 18.4
13
Free Energy and Spontaneous Reactions

• Free Energy-The energy that is available to do
  work
• Internal combustion engine of a car is 30
  efficient
• Only about 30 of the free energy is used to
  propel the car

• Spontaneous reactions occur naturally and favor
  the formation of products at the specified
  conditions
• Produce substantial amounts of products
• Release free energy
• Nonspontaneous reactions a reaction that does
  not favor the formation of products
• Does not give a substantial amount of products

14
Entropy

• Heat (enthalpy ?H) changes accompany most
  chemical and physical processes.
• Entropy (?S) is a measure of the disorder of a
  system
• Law of Disorder the natural tendency is for
  systems to move in the direction of maximum
  disorder or randomness.
• The size and direction of enthalpy changes and
  entropy changes together determine whether a
  reaction is spontaneous that is whether it favors
  products and releases free energy.
• ?G ?H- T?S

15
Gibbs Free Energy

• For a process that occurs at constant temperature
  and pressure
• ?G ?H T ?S
• G Gibbs Free Energy
• H Enthalpy
• T Temperature in Kelvin
• S Entropy

16
Free Energy

• A process carried out at constant temperature and
  pressure will be spontaneous in the direction in
  which free energy decreases.
• Negative ?G spontaneous
• Positive ?G non-spontaneous

17
Effect of ?H and ?S on Spontaneity
-?H exothermic (heat given off )
?S disorder increases