Kinetics Revision

1
Kinetics Revision

• for aA bB ? cC ,
• Rate k Ax By
• x order of reaction with respect to A
• k rate coefficient ( or rate constant)

2
Elementary Reactions

• Occur in a single collision
• have simple rate laws which follow stoichiometry
• A ? products unimolecular Rate k A
• Cl2 ? Cl Cl
• A A ? products bimolecular Rate kAA
• Cl Cl ? Cl2
• A B ? products bimolecular Rate kAB
• Cl H2 ? HCl Cl

3
Example

• For reaction A 3B ? product,
• initial A / M initial B / M
  initial rate R / M s1 1 0.10
  1.0 5.0 2 0.14 1.0 10.0 3
  0.14 0.5 5.0
• Answer
• sets 2 3 A constant, halving B halves rate
• hence R µ B1 first order in B

• sets 1 and 2 B constant, look at order with
  respect to A

• For B constant, R µ Ax

4
Hence second order with respect to A 2nd order in
A, 1st order in B, 3rd order overall
overall order sum of individual orders

• Find rate coefficient k where R k A2 B
• from set 1 5.0 M s1 k ? (0.10 M)2 ? 1.0 M
• hence k 5.0 ? 102 M2 s1
• Units found from rate law.

5
Rate LawsDependence On Time

• Reaction obeying 1st-order rate law A ? products

(negative sign because A disappears its
concentration is decreasing)

• Solve this differential equation by integrating
  both sides

6
First Order lnA vs time

• Plot of lnA as function of time
• lnA -kt lnA0

7
Half Life
Integrated rate law

• Half Life is independent of initial concentration
• Solutions also exist for second-order reactions...

8
Half life for a First Order Reaction
A0
9
Reaction Mechanism andRate-determining Step

• Mechanisms can be complex, but sometimes all
  steps are fast except onethen slow step
  rate-determining step

10
Example

• H2(g) 2ICl(g) ? I2(g) 2HCl(g)
• Suggested mechanism
• (1) H2 ICl ? HI HCl slow Rate(1)
  k1H2ICl
• (2) HI ICl ? I2 HCl fast Rate(2)
  k2HIICl

H2 2ICl ? I2 2HCl net reaction
(1) is slow rate determining step ?
Overall rate k1H2ICl Experimental rate
kH2ICl, supports mechanism HI is an
intermediate, it doesnt appear in the overall
reaction
11

• Often no simple solution to rate equations,
  solved coupled differential equations
  numerically.Example
• oscillating reaction iodate, malonic acid
  (CH2(CO2H)2), H2O2
• ozone layer destruction by CFCs
  (chlorofluorocarbons) simplified mechanism is
• CCl2F2 ? CClF2 Cl
• Cl O3 ? ClO O2
• ClO NO2 ? ClONO2
• ClONO2 HCl ? HNO3 Cl2
• Cl2 ? 2Cl

hn
hn
12
Example

• Fast reversible reaction followed by a slow step
• (1) 2NO(g) ? N2O2 (g) fast equilibrium, K
• (2) N2O2 (g) N2O2(g) ? 2NO slow (rate
  coefficient k)

2NO(g) O2 (g) ? 2NO2 (g) net reaction
Rate rate of slow step k N2O2O2
So rate k N2O2 O2 k K NO2 O2 hence
rate constant ? NO2 O2 consistent with
observed rate law