Introduction to Chemical Kinetics AOSC 433/633 & CHEM 433 Ross Salawitch

Transcription

1 Introduction to Chemical Kinetics AOSC 433/633 & CHEM 433 Ross Salawitch Class Web Site: Goals for today: Loose ends from last lecture Overview of Chemical Kinetics in the context of Atmospheric Chemistry Physical meaning of rate expression numbers Description of different types of reactions Lecture 11 9 March

2 Loose Ends Figure 2.10, Chemistry in Context Equation 2.5, Chemistry in Context O 3 + hν O( 1 D) + O 2 ( 1 g ) Enthalpy = 93.3 kcal/mole λ max = 305 nm O 3 + hν O( 3 P) + O 2 ( 3 Σ g ) Enthalpy = 25.7 kcal/mole λ max = 1120 nm 2

3 Loose Ends Page 83, Chemistry in Context 3

4 Relationship Between UV and Column Ozone Madronich et al., J. of Photochemistry and Photobiology B, Vol. 46, 5 19,

5 Relationship Between Cancer and UV Scotto and Fraumeni, Cancer Epidemiology, W. B. Saunders and Co, Philadelphia,

6 Depletion of the Ozone Layer, Global Figure Q13-1, WMO/UNEP Twenty Question and Answers 6

7 Skin Cancer / Australia 7

8 Reading Chapter 3, Chemical Kinetics, from Photochemistry of Planetary Atmospheres, Yung and DeMore. Additional material for interested students: Chapter 9, Chemical Kinetics, from Introduction to Atmospheric Chemistry, Jacob. Short, easy to read overview Chapter 2, Chemical Concepts in the Atmosphere, Aeronomy of the Middle Atmosphere, Brasseur and Solomon. Treatment of partition functions and quantum effects relevant to atmospheric chemistry Chapter 28, Chemical Kinetics I: Rate Laws, Physical Chemistry: A Molecular Approach, McQuarrie and Simon. Rigorous treatment of kinetics from a pchem point of view 8

9 Types of Reactions Reading: 1. Unimolecular A B + C 2. Bimolecular A + B C + D 3. Termolecular A + B + M C + M Of course, reactions must balance in a stoichiometric manner photochemical reactions break and reform chemical bonds; they do not rearrange protons 9

10 Types of Reactions Atmospheric Chemistry: HONO 2 same as HNO 3 (nitric acid) We ll use both notations interchangeably 1. Unimolecular 1a. Photolysis : O 3 + photon O + O 2 1b. Heterogeneous: N 2 O 5 + H 2 O (aqueous) 2 HONO 2 1c. Thermal Decomposition: ClOOCl + heat ClO + ClO 2. Bimolecular 2a. Gas Phase: OH + CH 4 CH 3 + H 2 O 2b. Heterogenous: ClONO 2 + HCl (adsorbed) Cl 2 + HONO 2 3. Termolecular 3. OH + NO 2 + M HONO 2 + M 10

11 Importance of Radicals With a few exceptions, the only reactions between molecules that proceed at appreciable rates are those involving at least one radical Radicals require significant energy to form: a bond must be broken Radical formation is tied to absorption of photons that photodissociate a compound, leading to radical formation Initiation O 2 + photon O + O Propagation O + O 2 + M O 3 + M O 3 + photon O( 1 D) +O 2 O( 1 D) + H 2 O OH +OH OH + O 3 HO 2 + O 2 HO 2 + O OH + O 2 Termination OH + HO 2 H 2 O + O 2 11

12 Radicals Radicals: unpaired electron in outer valence shell Is a species a radical? Count the electrons: HNO 3 : = 32 electrons no NO : = 15 electrons yes NO 2 : 23 electrons yes Other radicals: OH, HO 2, Cl, Br, ClO, BrO Important exception: Atomic oxygen : two unpaired electrons in its triplet ground state O( 3 P) (1s 2 2s 2 2p x2 2p y 1 2p z1 ) therefore a biradical : we ll call O( 3 P) a radical What is O( 1 D)? higher energy singlet state with all electrons paired but last orbital empty: O( 1 D) (1s 2 2s 2 2p x2 2p 2 y ) O( 1 D) is even more reactive than O( 3 P) : it is hungry for more electrons! 12

13 Admission Ticket Lecture 11 Gibbs Free energy involves both enthalpy and entropy. Briefly describe the relative roles of the change in enthalpy and entropy in affecting the rate of a chemical reaction. Under what conditions will enthalpy dominate the rate? Under what conditions will entropy dominate the rate? Briefly: why is kinetic information needed, in addition to thermodynamic information, to quantity our understanding of atmospheric chemistry? Kinetic information is needed because thermodynamic information doesn t give a clue to the time constant needed for equilibrium to be reached 13

14 Bimolecular Gas Phase Reactions 8.9 kcal/mole 35.1 kcal/mole 17.8 kcal/mole 57.8 kcal/mole OH + CH 4 CH 3 + H 2 O dch dt 4 Rate of Reaction = = k [OH][CH 4] Enthalpy = 13.8 kcal/mole Exothermic! Arrhenius Expression for rate constant: k = e cm sec / T 3 1 R = erg / (K mole) = erg / ( K gm) for air Yung and DeMore, Photochemistry of Planetary Atmospheres, Oxford,

15 Bimolecular Gas Phase Reactions 8.9 kcal/mole 35.1 kcal/mole 17.8 kcal/mole 57.8 kcal/mole OH + CH 4 CH 3 + H 2 O dch dt 4 Rate of Reaction = = k [OH][CH 4] Enthalpy = 13.8 kcal/mole Exothermic! Arrhenius Expression for rate constant: k = e cm sec / T (2010 Evaluation) 15

16 Bimolecular Gas Phase Reactions 8.9 kcal/mole 35.1 kcal/mole 17.8 kcal/mole 57.8 kcal/mole OH + CH 4 CH 3 + H 2 O dch dt 4 Rate of Reaction = = k [OH][CH 4] Enthalpy = 13.8 kcal/mole Exothermic! Arrhenius Expression for rate constant: k = e cm sec / T (2010 Evaluation) 16

17 OH + CH 4 CH 3 + H 2 O Bimolecular Gas Phase Reactions k = e cm sec / T (2010 Evaluation) 17

18 Bimolecular Gas Phase Reactions IUPAC recommendation: k = T e cm sec / T

19 Photolytic Production of OH H 2 O + hν H + OH λ MAX = 242 nm Figure 4.11, Seinfeld and Pandis, 2006 (from DeMore et al., 1994) 19

20 Bimolecular Production of OH a. H 2 O + O( 1 D) OH + OH Enthalpy = 28.1 kcal/mole b. H 2 O + O( 3 P) OH + OH Enthalpy = 17 kcal/mole c. H 2 + O( 1 D) OH + H Enthalpy = 43.7 kcal/mole d. H 2 + O( 3 P) OH + H Enthalpy = 1.4 kcal/mole k a = e (60/T) cm 3 s -1 k b = 0.0 k c = cm 3 s 1 k d = e ( 4570/T) cm 3 s -1 20

21 Bimolecular Production of OH O( 3 P) O( 1 D) At surface, [O 1 D] 10 5 [O( 3 P)] 21

22 Heterogeneous Reactions Pseudo Uni-Molecular 13.3 kcal/mole 57.8 kcal/mole 2 32 kcal/mole N 2 O 5 + H 2 O (aqueous) 2 HONO 2 Enthalpy = 19.5 kcal/mole Reaction is exothermic HONO 2 same as HNO 3 (nitric acid) We ll use both notations interchangeably Gas phase rate is exceedingly slow Proceeds on surfaces (e.g., sulfate aerosols) because the ionic state of H 2 O provides access to a reaction mechanism that is not accessible in the gas phase 22

23 Heterogeneous Reactions Pseudo Uni-Molecular N 2 O 5 + H 2 O (aqueous) 2 HONO 2 Rate of Reaction = k[n O ] ; Units of k are s k = γ ( Velocity N2O5 ) (Α erosol Surface Area per Unit Volume) 4 γ = sticking coefficient or reaction probability (dimensionless) Velocity N 2 O 5 = (8 k T / π m) 1/2 = ( T / 108 ) 1/2 cm/sec 1 Aerosol Surface Area per Unit Volume = 4 π r a 2 N a where 108 = Molecular Weight of N 2 O 5 r a = radius of aerosol N a = number density of aerosol For this type of reaction: γ will depend on temperature and aerosol type γ does not depend on gas phase abundance of H 2 O because, reacting surface is primarily composed of H 2 O 23

24 Heterogeneous Reactions Pseudo Bi-molecular ClONO 2 + HCl (adsorbed) Cl 2 + HONO 2 1 k = γ ( VelocityClONO2 ) (Α erosol Surface Area per Unit Volume) 4 γ = sticking coefficient or reaction probability (dimensionless) Velocity ClONO 2 = ( T / 97.5 ) 1/2 cm/sec For this type of reaction: γ will depend on temperature and aerosol type γ depends on partial pressure (e.g., gas phase abundance) of HCl because, reacting surface is not primarily composed of HCl Gas phase H 2 O >> gas phase N 2 O 5 N 2 O 5 +H 2 O(aqueous) can never deplete gas phase H 2 O 24

25 Heterogeneous Reactions In all cases, γ must be measured in the laboratory Reaction probabilities given for various surface types, with formulations of various degrees of complexity, in Section 5 of the JPL Data Evaluation. Atmospheric Chemistry and Physics by Seinfeld and Pandis provides extensive treatment of aqueous phase chemistry, properties of atmospheric aerosol, organic aerosols, etc. 25

26 Thermal Decomposition 30.5 kcal/mole kcal/mole ClOOCl + M ClO + ClO + M H = 18.1 kcal/mole k k THERMAL FORMATION GREACTANTS GPRODUCTS = = Κ e ( ) / RT EQUILIBRIUM Rate of Reaction = k [ ClOOCl]; Units of k are s THERMAL THERMAL 1 G Gibbs Free Energy = H T S where H = enthalpy T = temperature S = entropy See section 3.2, Chapter 3, Yung and DeMore, for an excellent intuitive discussion of enthalpy, entropy, and Gibbs free energy 26

27 Thermal Decomposition 30.5 kcal/mole kcal/mole ClOOCl + M ClO + ClO + M H = 18.1 kcal/mole k k THERMAL FORMATION GREACTANTS GPRODUCTS = = Κ e ( ) / RT EQUILIBRIUM JPL Data Evaluation gives values of K EQUILBRIUM and k FORMATION In equilibrium: K EQ = e (8744/T) cm -3 k THERMAL [ClOOCl] = k FORMATION [ClO] [ClO] where k THERMAL = k FORMATION K EQ Energetically, system favors ClOOCl Entropically, system favors ClO & ClO at low T, ClOOCl stable: energy wins! at high T, ClOOCl unstable: entropy rules! Equilibrium constants given in Section 3 of the JPL Data Evaluation. 27