Chemical Kinetics

1
Chemical Kinetics Equilibrium
2
Factors th Affect Rxn Rate

• Physical state of nature of reactants
• Concentration of reactants
• Temp _at_ wh rxn occurs
• Presence of a catalyst

3
Rxn Rates

• Change in the conc of Reactants/ OR Products per
  unit time
• Rate at wh Reactants disappear OR
• Rate at wh Products are formed
• M/s (molarity/second)

4
Rxn Rates-cont

• Average rxn rate Change in Conc
• Change in time
• ?X
• ?t
• Instantaneous rate rise
• run

5
Concentration Rate

• conc of a reactant will rate
• 2x X 2x rate
• 4x x 4x rate, etc.

6
Temperature Rate

• the temp the rate
• The rate approx doubles for every 10?C in temp
• Collision Theory
• Molecules must collide w/ enough E a suitable
  orientation to react
• conc temp both collisions

7
Temperature Rate- cont

• Activation E (Ea) - the min amt of E needed to
  start a rxn
• activated complex - the arrangement of the
  elements at the peak of the E of activation
• Energy Profiles
• lower Ea, faster the rxn rate
• ?E does not effect rate
• determine if rxn is endo or exo

8
Catalysis

• A sub th the speed of the rxn w/out changing
  itself
• Lowers the activation Ea by providing a
  completely diff rxn mechanism
• Homogeneous Catalysis
• Present in the same phase as the reactants

9
Catalysis-cont

• Heterogeneous Catalysis
• valence requirements Catalyst is in a diff phase
  fr reactants
• often solids metals or metal oxides
• Rxn occurs on their surface lg SA beneficial
• Adsorption- initial phase where reactants are
  bound to the surface (high reactivity) of the
  catalyst
• Active site- site where binding occurs

10
Catalysis-cont

• Enzymes - biological catalysts
• Usu lg soluble protein molecules
• Very specific to rxns v. efficient v. low Ea
• Optimal conditions e.g. pH, temp
• The substrate (reactants) fit into a active site
  on an enzyme lock-n-key model
• Enzyme-substrate complex- combo of these 2
• Enzyme inhibitors - block active sites (poisons,
  toxins)

11
Recognizing Rxn Equilibrium

• 1. The system is closed.
• Opposite rxns occur at the same rate.
• Equilibrium is reached by either starting w/
  reactants or products.
• Temp is constant.
• Radioactive tracers can prove th rxn proceeds
  even w/ no net observable changes dynamic
  equilibrium
• Equilibrium does NOT mean conc of PR are equal,
  but th RATES ARE EQUAL!

12
Equilibrium Constant, Keq

• A ltgt B
• Once equilibrium is reached, the ratio of A and B
  is constant.
• Law of Mass Action
• Shows the relationship between the concs
  (expressed as partial pressures for gases as
  molarities for solutions) of the R P present at
  equilibrium in any reaction.

13
Equilibrium Constant, Keq-cont

• aA bB ltgt cC dD
• Equilibrium-constant Expression
• Keq CcDd
• AaBb
• Temp dependent
• Dimensionless no units!!!

14
Magnitude of Keq

• Keq can be very large or very small
• CO(g) Cl2 (g) ltgt COCl2 (g)
• Keq PCOCl2 1.49 x 108
• PCOPCl2
• For Keq to be so large gtgt 1, the numerator,
  PCOCl2, must be large
• The equilibrium lies to the right (toward the P)
• Keq ltlt1 equilibrium lies to the left (reactants
  dominate)

15
Solubility Equilibria

• AB(s) H2O(l) -gt A (aq) B-(aq)
• Equilibrium expression
• Ksp AB-
• Ksp the solubility product constant
• Pure liquids solids are NEVER included in
  this expression

16
Heterogeneous Equilibria

• Homogeneous equilibria -involve subs all in the
  same phase
• Heterogeneous equilibria - subs are in diff
  phases
• Partial pressures of gases are used in Keq
  expression.
• Molar conc of dissolved species are used.
• Pure solids, pure liquids, and solvents are NOT
  included in Keq expression.
• these subs must be present for equilibrium

17
Le Chatliers Principle

• If a sy at equilibrium is disturbed by a change
  in temp, press, or the conc of comps, the sy will
  shift the eq position to counteract the effect of
  the disturbance.

18
Le Chatliers Principle-cont

• Change in R or P Conc
• Adding a R or P disturbs equil
• Equil will re-establish by consuming the added R
  or P
• Removing R or P
• Equi will re-establish by forming more sub
• N2(g) 2H2(g) ltgt 2NH3(g)
• Add N2 or H2 shifts right
• Add NH3 shifts left
• Remove N2 or H2 shifts left
• Remove NH3 shifts right

19
Le Chatliers Principle-cont

• Effects of Volume Press Change
• ONLY effect gas phase!
• A decrease in vol will press
• Eq will shift to decrease the press
• Eq will shift to the side with the least number
  of gas particles
• N2O4(g) ltgt 2NO2(g)
• press will shift to the left

20
Le Chatliers Principle-cont

• Effect of Temp Change
• Temp will change the Keq
• Endothermic R heat ltgt P
• the temp will favor the endothermic rxn
• T Keq
• Exothermic R ltgt P heat
• Decreasing the temp will favor the exo rxn
• Decreasing T decrease in Keq

21
Le Chatliers Principle-cont

• The Effect of Catalysts
• Lowers the Ea for both the forward reverse rxns
• Increases the rate at wh equil is achieved, it
  will not shift the equilibrium or change the
  equilibrium mixture.

22
Haber Process

• N2(g) 3H2(g) ltgt 2NH3(g)
• High conc of H2(g) N2(g) maintained.
• NH3(g) (g) th forms is removed.
• High temp to the forward rxn.
• 300-600C
• Catalyst used to rxn rate (est equilibrium
  faster)
• High press used 200-600 atm