Review

1
Review Solution Concentration

• 1. Aqueous sodium benzoate, 50 mL at a
  concentration of 1.1 M, was dilututed to produce
  a 0.02 M solution. What is the volume of this
  sample?
• 2. An solution (density 1.22 g/mL) contains
  5.0 mass NaBr and 10 mass Ca(NO3)2 in water.
  What is the molarity of NaBr?

2
Chapter 12 General Rate Expressions

• Rate change in a quantity divided by
  the change in time

Examplesspeed change is distance/change in
time mph miles/hours rpm
revolutions/minute flow rate gallons/minute



3
Reaction Rates 01

• Reaction Rate The change in the concentration of
  a reactant or a product with time (change in
  molarity per second M/s) Reactant ?
  Products aA ? bB

4
Reaction Rates - Table

• Consider decomposition of N2O5 to give NO2 O2
• 2 N2O5(g) 4 NO2(g)
  O2(g)

5
Reaction Rates - Graph
6
Calculating Rate Over a Time Interval
Calc average rate of N2O5 reaction from 100 to
300 seconds
? N2O5 .0120 .0169
0.0049 M ? time 300 100
200 sec
rate


ave. rate 2.45 x 10-5 M/sec
7
Rate Law Reaction Order 01

• Rate Law Shows the relationship of the rate of a
  reaction to the rate constant and the
  concentration of the reactants raised to some
  powers.
• For general reaction a A b B ? c C
  d D rate k Ax By
• x and y must be determined experimentally
• k the rate constant

8
Rate Law Reaction Order 01

• Rate Law Example
• For the reaction 2 N2O5 ? 4 NO2 O2
• The rate law determined by experiment is
• rate -k N2O51

9
Rate Law Reaction Order 01

• Rate Law Example
• For the reaction 4HBr O2 ? 2Br2
  2H2O
• The experimentally-determined rate law is
• rate - k HBr1 O21
• Note that the rate law exponents are NOT
  taken from the coefficients of the balanced
  equation.

10
Rate Law Reaction Order 02

• Reaction Order The sum of the powers to which
  all reactant concentrations appearing in the rate
  law are raised.

Example For the rate law rate k NH31
CH2O0 k NH31 Reaction order
sum of exponents
11
Rate Law Reaction Order 02

• Reaction Order The sum of the powers to which
  all reactant concentrations appearing in the rate
  law are raised.

• Example
• For the rate law rate k HBr1 O21
• Reaction order sum of exponents

12
Rate Law Reaction Order 02
For the rate law rate k NH31
CH2O0 k NH31 Reaction order
1 0 1 or first order For NH3
rate varies as the conc. of NH3 changes
high NH3 conc. gives high reaction rates low
NH3 conc. means the rate is slower
For CH2O CH2O0 is always equal to 1
13
Rate Law Reaction Order 02

• Review The overall order of the reaction is
  the sum of the powers to which all reactant
  concentrations appearing in the rate law are
  raised.
• Each component also has its own order.Example
  if rate k HBr1 O21
• for O2, reaction is
• Reaction order must be determined
  experimentally.Exponents are not taken from
  balanced equation.

14
Rate Law Reaction Order 07

• Rate Constant A constant of proportionality
  between the reaction rate and the concentration
  of reactants.

plot rate vs. Br2 to get a straight line
(assume first order in Br2
rate ? Br2 rate kBr2 0
In this linear relationship, what does k
represent?
15
Rate Law Reaction Order 02

• Reaction Order The sum of the powers to which
  all reactant concentrations appearing in the rate
  law are raised.
• Reaction order is determined experimentally
• By inspection Ex see how rate changes when
  conc. is doubled
• From slope of the line, using the appropriate
  plot

16
Rate Law Reaction Order 05

• Reaction of nitric oxide with hydrogen at
  1280C 2 NO (g) 2 H2 (g) ? N2 (g)
  2 H2O (g)
• From the following data, determine the rate law
  (by inspection). Find the value of the rate
  constant

17
Rate Law Reaction Order 05

• Reaction of nitric oxide with hydrogen at
  1280C 2 NO (g) 2 H2 (g) ? N2 (g)
  2 H2O (g)
• The generic rate law for this reaction is

18
Rate Law Reaction Order 05

• 1. Find rate law exponents by inspection
• Compare Exp. 1 and Exp 2 NO doubles rate
  goes up 3.9 times
• Compare Exp. 2 and 3 H2 doubles rate
  goes up 2.0 times

19
Rate Law Reaction Order 05

• 2. Calculate Rate Constant
• Use first reaction to find k
• rate k NO2 H2
• k rate/( NO2 H2) (1.3 x
  10-5)/(5.0 x 10-3)2 x (2.0 x 10-3) 260

Or second reaction
20
Rate Law Reaction Order 04
Find rate law