Tutorial Quiz this week

1
Tutorial Quiz this week

• E hn hc/l
• De Broglie equation
• VSEPR
• intermolecular forces...
• BRING A CALCULATOR

2
Kinetics

• Silberberg, Chapter 16, p 664...
• Reactions not instantaneous they take finite
  time.
• Demonstration 2H 3I H2O2 ? 2H2O I3

3

• 2H 3I H2O2 ? 2H2O I3

• Rates go as 1/(stoichiometric coefficient)
• reactants products have opposite sign

4
Learn by understanding reason

• total mass of iodine constant in time, whether
  present as I or I3 , i.e., total mass iodine µ
  I 3I3

5
Rate Laws Dependence On Concentration

• differential rate law
• How does rate vary with concentration of each
  species?
• For iodine reaction

?reaction rate law ? does not follow
stoichiometry
6
Complex Rate Laws

• for aA bB ? cC ,
• Rate k Ax By
• where x, y can have any value (0, 1, 1/2 ).
• Exponent of A (ie value of x)
• ? order of reaction with respect to A
• 1, first order in A 2, second
  order in A,
• k rate coefficient (or rate constant)
• Units of k depend on the overall order of the
  reaction

7
Rate Laws
2H 3I H2O2 ? 2H2O I3

• First order in I-, first order in H2O2
• Second order overall
• k has units M-1 s-1
• Reason for non-stoichiometric dependence on
  concentration overall reaction actually many
  elementary reactions occurring simultaneously

8
Example H2 Cl2 ? 2HCl

• 4 elementary reactions occurring simultaneously
• Cl2 ? 2Cl
• Cl H2 ? HCl H
• H Cl2 ? HCl Cl
• 2Cl ? Cl2
• An H2 molecule Cl2 molecule never react by
  colliding together to form two HCl molecules!
• Instead
• isolated Cl2 molecule falls apart to form two Cl
  atoms
• a Cl atom can collide with H2 molecule to form an
  HCl molecule and an H atom...

9
Elementary Reactions

• Only two types of elementary reactions
• unimolecular, e.g. Cl2 ? 2Cl
• isolated molecule reacts
• bimolecular, e.g. Cl H2 ? HCl H
• two molecules (atoms) collide and react
• Elementary reactions have rate lawsthat do
  follow stoichiometry
• Proof
• since 1970, possible to study isolated
  elementary reaction in molecular beam

10
Bimolecular Reactions

• Example Cl H2 ? HCl H
• Suppose we detect HCl in Cl/H2 beam experiment
• if Cl in beam doubled, then we find the no. of
  HCl molecules seen per second is also doubled.
• Similarly doubling H2 doubles no. HCl molecules

• All bimolecular reactions are 2nd-order

11
Unimolecular Reactions

• Example Cl2 ? 2 Cl
• apparatus with single beam of Cl2,
• look at Cl with detector find number Cl atoms
  per second is be proportional to Cl2

• All unimolecular reactions are first-order

12
Finding Rate Law Experimentally

• Simplest initial rate method.
• Concentrations vary with time, but at early time,
  concentrations of reactants have hardly changed.

• Demonstration iodine clock reaction

2IO3 5SO3 2H ? I2 5SO42 H2O
13
Example

• For reaction A ? product,

initial rates
initial A / M R / M s1 0.10 5.0
0.20 10.0
Answer doubling A doubles rate, so R k A1
( k A ) ? first order with respect to
A To find rate coefficient k R k A from
first data line 5.0 M s1 k ? 0.10 M