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Atmospheric Chemistry

ATOC-5151 / CHEM-5151

Spring 2013Prof. Jose-Luis Jimenez

Lecture 2: Review of Fundamentals

1

For next time• Took almost 3 days in 2013

• Add brief description of oxidation and reduction

• Maybe more problems on relative lifetimes?

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Clicker Question• Should we do an Igor intro & Q&A session?

A. Yes, this is very important

B. Yes, this would be useful

C. Maybe

D. May not be that useful

E. It’d be a waste of time

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Business Items• Today is last clicker test day

– They may be used for in-class pop quizzes @ start of class starting next class

• You are responsible for having a working clicker

• Will choose office hours for the next two weeks today

• Reading associated with a lecture– Specified on course web page

– Due on the lecture AFTER the material has been covered

– Good to do before esp. if limited background

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Business Items• EVERYTHING should be linked from

http://tinyurl.com/AtmChem (e.g. office hours)

• Will assign HW #2 later today:– http://tinyurl.com/hw-5151

• I keep adding clarifications to HW as people ask questions. Double-check the page before turning in the HW

• Clicker questions: I’ll wait 45 s, or # seconds in question, or until all have answered, whichever is shorter

• Questions or items that need clarification?5

Clicker Q

• What is true for the distribution of molecular speeds of N2 at ambient T & P (45s)

A. The average speed is ~472 m/s

B. The minimum speed is 0 m/s

C. The maximum speed is not bound

D. All of the above

E. I really need a coffee*

*: this clicker question is sponsored by

For review: http://en.wikipedia.org/wiki/Maxwell%E2%80%93Boltzmann_distribution 6

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Reminder of Kinetic Theory of Gases

• Will discuss molecular speed, collisions, mean free path (65 nm), mass vs speed, meaning of P & T; effect of changing P & T (ideal gas law)

http://phet.colorado.edu/en/simulation/gas-propertieshttp://celiah.usc.edu/collide/1/http://en.wikipedia.org/wiki/Kinetic_theoryhttp://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kinthe.html#c1

Clicker Q: is the size of the molecules as drawn realistic compared to the empty space? (30s)

A. Molecules are x10 too small

B. Molecules are about ok

C. Molecules are x10 too large

D. Molecules are x1000 too large

E. Where is my coffee?

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Clicker Qs• The average thermal speed of SF6 at ambient T & P is (30s)

A. Smaller than that of N2

B. Larger than that of N2

C. Same as that of N2

D. None of the above

E. A, B, and C

• Each air molecule in this classroom is colliding with others approximately (60s):

A. 100 times per second

B. 10,000 times per second

C. 106 times per second

D. 1010 times per second

E. I don’t know8

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More Clicker Q

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• Each air molecule in the polar stratosphere (~50 mbar, 200 K) is colliding with others approximately (60s):

A. 107 times per second

B. 108 times per second

C. 109 times per second

D. 1010 times per second

E. I don’t know

The Periodic Table

10Adapted from Heald

Clicker Q: What is the reason for periodic elemental properties? (30s)

A. Similar number of neutrons in nucleus

B. Similarity of entire electronic structure

C. Similarity of outer electron shell

D. Similar orbital energies

E. I don’t know

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Orbitals and Filling Rules

• Clicker Q: what is the electronic structure of calcium (60s)

A. 1s2 2s2 2p10 3s6 B. 1s2 2s2 2p6 3s2 3p6 4s2

C. 1s2 2s2 2p6 3s2 D. 1s2 2s2 2p10 3s4 E. I don’t know11

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Electronic Structure in Periodic Table

12From Jacobson: Fund. Atmos. Mod.

“Lewis Diagrams”

Represent only the electrons in the outer highest occupied orbital(“valence shell”)

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Covalent bonds: sharing of paired electrons

Polar covalent bonds: When 2 atoms from different elements share e- unequally

Chemical BondsBond formation involves the electrons (e-) in the outermost (valence) shell. A complete outer shell consists of 8 valence electrons (except H and He which have 2)Breaking of a bond leads to a release of energy. Generally double or triple bond energies are higher.

Ionic bonds: electrostatic attraction between positive and negative ions e- transfer

13Adapted from HealdAdditional reading at: http://www.smallscalechemistry.colostate.edu/

/PowerfulPictures/ChemicalBonding.pdf

Lewis Structures of Bonded Molecules I

• Hydrogen shares e- until it is surrounded by 2 e-

• Atoms other than H share e- until surrounded by 8 valence e- (“Octet Rule”)

14http://www.smallscalechemistry.colostate.edu/PowerfulPictures/LewisDiagrams.pdf

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More Lewis Structures

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MethaneCH4

AmmoniaNH3

Molecular OxygenO2

Radicals• Radical = species

with an unpaired electron– High energy, high

tendency to pair the electron to reduce the free energy

– Often rapid reactions

– Radicals drive most atmospheric gas-phase chemistry

From Paul Ziemann

A

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Organic Compound Classes• Hydrocarbons: only C and H

Adapted from Paul Ziemann

Mostly Biogenic

Mostly Anthropogenic

• Clicker Q: are hydrocarbons and carbohydrates the same thing? (30s)

A. Always B. Never C. Often but not always

D. No, except in a few cases E. I don’t know

Organic Functional Groups• FG: atom or group of atoms with specific

structure and reactivity

http://www.uwec.edu/carneymj/Adobe%20handouts/Nomenclature.pdf (nice page for catcthing up on organic nomenclature)

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Ionic SpeciesAmmonium NH4

+

Acetate CH3COO-

Nitrate NO3-

Nitrite NO2-

Hydroxide OH-

Hypochlorite ClO-

Chlorite ClO2-

Chlorate ClO3-

Perchlorate ClO4-

Permanganate MnO4-

Carbonate CO32-

Sulfate SO42-

Sulfite SO32-

Peroxide O22-

Silicate SiO32-

Phosphate PO43-

19Adapted from Heald

• Ions are very important in water solution (aerosols, clouds, oceans…)

• Not important in the gas-phase except very high in the atmosphere (“ionosphere”)• Very reactive but…• Too low concentrations• Useful for measurements

A + R-A- + B“Chemical ionization”

Business Items• Schedule posted at http://tinyurl.com/Sched-5151

– Linked on course web page

– Will be updated during the semester

– Midterm exam on 21-March-13 (just before Spring Break)• Will include all the material covered until then

– Final exam either on 2-May-13 or during final exam week (TBD)

• Cumulative for the whole course

• OK to install Igor on ATOC computers– Should update if using for research

• HW comments: Log axis, use of Functions

• iClicker Roster collection

• Questions from last time?20

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Energy dominated by ions and bonds

21http://www.smallscalechemistry.colostate.edu/PowerfulPictures/ChemicalEnergetics.pdf

Standard Heat of Formation

H f0 "standard" heat of formation

Standard: all reactants and products are in their standard state

“standard state”: solids and liquids = most common formgases: 298 K, 1 atm pressure

Hrxn0 H f

0 (products) H f0 (reactants)

H f0 of any element in standard state=0,

all others tabulated

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Kinetics vs Thermodynamics• Thermo: Can a reaction occur?

2H2 + O2 2 H2O

Hf=

• Kinetics: How fast is a reaction?

2H2 + O2 2 H2O

due to kinetics!

0 0 -57.8

Hrxn=-115 kcal/mol exothermic

Enthalpy• Enthalpy = internal energy

• Hrxn > 0 endothermic;

• Hrxn < 0 exothermic

• Always trying to minimize energy – go “downhill”

From Seinfeld & Pandis

Example of exothermic reactionEprod < Ereact

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Enthalpy: Exothermic or Endothermic?

O + O3 O2 + O2

Hf0(kcal/mol) 60 34 0 0

Clicker Q: is this reaction…? (30s)A. ExothermicB. EndothermicC. Neither (“Thermoneutral”)D. IonicE. I don’t know

Thermodynamics: enthalpy & entropy• Exothermic rxn favored:

• Entropy important too

• Entropy is a measure of randomness

• Rxn are favored by increasing entropy:

Hrxn0 0

Srxn0 S0(products) S0(reactants)

S0tabulated

Srxn0 0

G rxn0 0 for spontaneous rxn

Grxn0 Hrxn

0 TSrxn0

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Entropy• Entropy = amount of disorder

• 2nd Law of Thermodynamics = nature tends to increase in entropy

• For gas phase reactions, big entropy changes driven by number of compounds involved in reaction– Two reactants two products = S ~ 0

– Association and dissociation reactions have S ~ 30 entropy units (cal K-1mol-1) per compound lost/gained

• ~ 10 entropy units per degree of freedom lost/gained

CQ: Entropy increase or decrease?

O + O3 O2 + O2

(30s)a. increaseb. decreasec. stay about the samed. I don’t knowe. I am not paying attention

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For many reactions, Srxn~0

If number of moles is equal in products and reactantsSrxn~ 0

Grxn0 Hrxn

0

O + O3 O2 + O2

Hf0(kcal/mol)

S0(cal/mol-K)60 34 0 038.5 57 49 49

Grxn0 94 T(S0) 94 (273)2.5x103 94.6 kcal /mol

Elementary vs Overall Reactions• Overall = sum of more than one elementary reaction

– CH4 + 2O2 2H2O + CO2

• Too many bonds broken and formed for a single collision

• Elementary = simplest reaction, cannot be broken down, cannot have more than 3 reactants– OH + CO H + CO2

– H + O2 + M HO2 + M

– They occur in a single step!

• To predict concentrations, we need to work with elementary reactions

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Rate Laws• Rate of reaction: change of concentration with

respect to time

• Example: OH + CH4 H2O + CH3

• How do you express the rate of this reaction?

• What does this rate depend on?

Kinetic Rate Equations• Rate of reaction: change of concentration with

respect to time

• Example: OH + CH4 H2O + CH3

• How do you express the rate of this reaction?– -d[OH]/dt = -d[CH4]/dt = d[H2O]/dt = d[CH3]/dt

– [X]: concentration of molecules per cm3 of air

• What does this rate depend on?– -d[OH]/dt = k * [OH] * [CH4]

– k = rate coefficient• k is essentially a probability that a collision between

reactants will lead to products

• Can vary with temperature and pressure

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Elementary reactionsUnimolecular: 1 molecule falls apart

A products

Bimolecular: 2 molecules collide and react

A+B C+D

Termolecular: 3 molecules collide and react

A + B + M AB + M

Unimolecular Rxn are first order

A products

Rate d[A]

dt kI [A]

molecules/(cm3-s) molecules/cm3sec-1

d[A]

[A] kdt

A0

A

d[A]

[A]

0

t

kdt

ln[A]

[A0] kt

time

ln(A/A0)slope=-k

Units???

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Lifetime for Unimolecular Rxn

Lifetime: time for A to decay to 1/e of its original value

[A]=[A0]e-kt ktA

A

][

][ln

0

=1/k

CQ: What is the half-life of A?

ln(12)

k

ln(12)

k

ln(e)

klog(e)

kDon’t know

Examples of Unimolecular Reactions• PAN decomposition:

• All photolysis reactions: CH3COONO2

kI CH3COO NO2

O O

O3 h O O2

Clicker Q: which atmreaction is not

unimolecular? (30s)

A) N2O5 + h NO2 + NO3

B) N2O5 NO2 + NO3

C) NO2 + NO3 N2O5

D) all are unimolecular

E) I don’t know

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Bimolecular Reactions• Bimolecular – 2 molecules collide and react

– Very common in the atmosphere

– A + B C + D

– Many two body reactions:

– OH + CH4 H2O + CH3

Second Order

Rate d[A]

dt kII [A][B]

molecules/(cm3-s) molecules/cm3cm3

molec sec

CQ: What is the rate expression for NO2?

O3 + NO NO2 + O2

d(NO2)/dt = ...

A) k(NO2)(O2)B) k(O3)(NO)C) -k(NO2)(O2)D) -k(O3)(NO)E) I don’t know

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Pseudo-first order…

Rate d[A]

dt kII [A][B]

In atmosphere, [B] may be constant even if it is much smaller than [A]!

Rate d[A]

dt k '[A] where k ' kII [B]

CQ: What is lifetime of A wrt reaction with B? (45s)A 1/kII

B [B]/kII

C 1/kII [B]D 0E I don’t know

Termolecular Reactions I• Trimolecular (3 molecules collide “simultaneously”)

– Three body reactions, association reactions

– Classic example: O + O2 + M O3 + M

What is M in the atmosphere? (30s) A) O2 B) N2 C) N2 or O2 D) H2O E) dunno

M is a third body to take away excess energy

Rate d[A]

dt kter[A][B][M]

molecules/(cm3-s) molecules/cm3

CQ: units of termolecular reaction k? A. cm6 molecule-2 s-1

B. cm3 molecule-2 s-1 C. cm6 molecule-1 s-1

D. cm6 molecule-2 s-2 E. I don’t know

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Rate of formation from 3rd rxn:

But assume AB* has short lifetime, the can use steady state approximationformation rate = loss rate

Re-arrange:

Termolecular Reactions IIA + B AB* 3AB* A + B 4AB* + M AB + M* 5M* M + heat 6A + B + M AB + M 7

M = third body (usually inert: O2, N2) stabilizes the excited products AB*

5[ ] [ *][ ]d

AB k AB Mdt

3 4 5[ ][ ] [ *] [ *][ ]k A B k AB k AB M

3 5

4 5

[ ][ ][ ][ ] [ ] [ ]

[ ]

k k A B Md d dA B AB

dt dt dt k k M

3 5

4

[ ] [ ] [ ] [ ][ ][ ]k kd d d

A B AB A B Mdt dt dt k

3[ ] [ ] [ ] [ ][ ]d d d

A B AB k A Bdt dt dt

Low-pressure limit [M] << k4/k5:

rate depends linearly on [M]High-pressure limit [M] >> k4/k5

rate independent of [M] (all AB* will stabilize)R3 is the rate-limiting step

41From Heald

Clicker Q• Which graph approx represents the dependence

of a termolecular reaction rate on pressure?

A B

C D

E: I don’t know42

P

Rate

P

Rate

P

Rate

P

Rate

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Chemical Equilibrium

A + B C + D, kf A + B ↔ C + DC + D A + B, kr

At equilibria (or ss) : [ ][ ] [ ][ ]

[ ][ ]

[ ][ ]

f r

feq

r

k A B k C D

k C DK

k A B

[ ][ ]

[ ][ ]

C DQ

A B

Reaction Quotient (not in equilibrium):

if Q < Keq then rxn will shift to R (more products)if Q > Keq then rxn will shift to L (more reactants)

Le Châtelier’s Principle: Perturbance of a system at equilibrium system will shift to minimize perturbance

43From Heald

Acids and Bases (in solution)

pH = -log[H+] the activity of H+

< 7 = acidic> 7 = basic7 = neutral

Note: in atmosphere neutral pH=5-5.7 because pure water takes up CO2

Acid-Base Equilibrium: example ionization of acetic acid

3 3

3

3

[ ][ ]

[ ]a

CH COOH H CH COO

H CH COOK

CH COOH

H2O(l) ↔ H+(aq) + OH-(aq)14 2[ ][ ] 10 (mol/L)wK H OH

NOTE: OH- (hydroxide ion) OH (hydroxyl radical)!

44From Healdhttp://www.smallscalechemistry.colostate.edu/PowerfulPictures/AcidBaseEquilibrium.pdf

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Solubility & Henry’s LawHenry’s Law: Distribution of species between aqueous and gas phases

2 2

2

( )

[ ]A A

A

A g H O A H O

A H OK H

p

HA = Henry’s Law ConstantUnits here are mol/L/atm(sometimes reciprocal – be careful!)

Some Henry’s Law Constants of Atmospheric Relevance:

Chemical Species Henry’s Law Constant @ 25C (mol/L/atm)

HNO3 2.1x105

NH3 57.5

SO2 1.2

CO 9.6x10-4

45Adapted from Heald

Clicker Q: if the concentration of CO is 1000 times larger than that of HNO3, which of the 2 will be present at higher concentrations in cloud water? (60s)

A. HNO3 B. CO C. Neither will be present

D. Approx. equal concentrations E. Don’t know