Environmental Chemistry

Chapter 3:The Detailed Chemistry of the Atmosphere

Copyright © 2007 DBS

Review: How to Draw Lewis Structures

1. Determine the sum of valence electrons

2. Use a pair of electrons to form a bond between each pair of bonded atoms

3. Arrange the remaining electrons to satisfy octet rule (duet rule for H)

4. Assign formal charges (valence – directly surrounding e-)

CH4 and H2O

but unlike methane, two e- pairs are bonding and two are non-bonding

The non-bonding e- pairs take up more space than bonding pairs, so the H-to-O-to-H bond angle is compressed

Methane, CH4

Water, H2O

VSEPR

Valence Shell Electron Pair Repulsion Theory assumes that the most stable molecular shape has the electron pairs surrounding a central atom as far away from one another as possible

No. e- pairs around central atom

Shape of molecule Bond angle

4 pairs, all bonding:CH4, CF4, CF3Cl, CF2Cl2

Tetrahedral 109.5°

4 pairs, three bonding, one non-bonding:

NH3, PCl3

Triangular pyramid ~107°

4 e- pairs, two bonding, two non-bonding:

H2O, H2S

Bent ~105°

Lewis Structures of Free Radicals

• Free radicals possess an unpaired e-

• The unpaired e- is not in actual use as a bonding e-

• Carbon centered radical in which the carbon atom has one unpaired e- forms 3 bonds rather than four

• Oxygen forms one rather than 2 bonds:

•O – H

• A halogen forms no bonds:

Cl•

•H―C―H | H

Lewis Structures of Free Radicals

• Choice of assigning the unpaired e-

• Hydroperoxy radical, HO2:

• Complicated in molecules containing multiple bonds• For hydroxy formyl, HOCO a reasonable structure is:

Does not go to O since C must have 4 valence

H-O-O•

•

H-O-C=O

Lewis Structures of Free Radicals

A simple formula, ClO• does not indicate which atom carries the e-.

Draw Lewis structures for:

OH•

CF2Cl•

ClO•

NO•

•O – H

•F – C –F | Cl

Cl – O•

•N = O

Hydroxyl Radical: The Atmosphere’s Detergent

•OH is the prominent oxidizing species in the atmosphere

• Despite very low atmospheric concentrations, currently estimated at 106 molecules cm-3, corresponding to a mean tropospheric volume mixing ratio of 4 x 10-8 ppmv

• The lifetimes of most atmospheric gases are, therefore, largely determined by [OH] and the corresponding reaction coefficients

• Radical reactions that are spontaneous produce stable products with strong bonds

Hydroxyl Radical: The Atmosphere’s Detergent

The major route for the formation of the hydroxyl radical in the troposphere is:

NO2• + h ( < 400 nm) → NO• + O•

O• + O2 + M → O3

O3 + h ( < 320 nm) → O2 + O*

O* + H2O → 2 •OH

NO2• + H2O → NO• + 2 •OHOthers:O* + CH4 → OH + CH3OHHNO2 → OH + NOH2O2 + h → 2OH

Interactions with Hydroxyl Radical• Usually it reacts by adding itself to a molecule at the multiple bond• It can also abstract hydrogen atom to produce carbon centered radicals• •OH addition does not occur to O=O bonds since the bonding that

would result will be weak• For example, in the case of SO2, the OH radical adds to the sulfur atom

forming a strong bond but not to an oxygen atom

• Hydroxyl radicals do not add to CO2 since C=O bonds are very strong• However, it adds to CO, the addition favors conversion of triple bond to

stable double bond

Radicals React with O2 to produce Peroxy and Hydroperoxy Radicals

• Predominant fate is ‘add-on’ reaction with O2,

e.g. •CH3 + O2 → CH3OO•

HOO• / HO2• (hydroperoxy) and CH3OO• are called peroxy radicals- Less reactive than other radicals - Do not readily abstract H- Do not react with O due to low conc.

• Main reactions:

HOO• + NO• → •OH + NO2•

R-OO• + NO• → RO• + NO2• (where R = carbon chain)

H3C – O – O•

Successive reactions will completely oxidize the organic compound

H Atom Abstraction by O2 from Nonperoxy Radicals

Gases that undergo decomposition by absorbing UV-A or visible light can generate free radicals. e.g., formaldehyde

H2CO + UV-A (<338 nm) → H• + HCO•

If there is no suitable hydrogen atom for O2 to abstract then it adds-on

H-abstraction occurs provided a new bond is formed

peroxy radical

CH3-O• + O2 → H2C=O + HOO•

H-C=O + O2 → C=O + HOO••

Fate

Decision tree illustratingthe fate of gases emittedinto the air

HNO3, HCl, NH3, etc

H2CO

CH4 + OH• H2O + CH3•

Fate of Free Radicals

Decision tree illustrating the fate of airborne free radicals

ROO· + NO NO2 + RO·

CH3· + O2 CH3OO·

Oxidation of CH4

• Produced in inefficient (anaerobic) burning of fuels• Predominant HC in atmosphere• No multiple bonds• Not soluble in water, does not absorb sunlight• Slow oxidation initiated by hydroxyl radical

(hydrogen abstraction reaction)

CH4 + •OH → •CH3 + H2O abstraction

•CH3 + O2 → •CH3OO• O2 adds forming peroxyCH3OO• + NO → CH3O• + NO2 transfer of OCH3O• + O2 → H2CO + HOO• O2 absracts H

…conversion of methane to formaldehyde

H2CO + UV-A (338 nm) → H• + HCO• unstable

H• + O2 → HOO• O2 abstracts

HCO• + O2 → CO +HOO• O2 abstracts

Note: CO is a stable intermediate and can further undergo transformations

C O + OH• → HO-C=O

H-O-C=O + O2 → O=C=O + HOO•

….. Production of CO2 as the final product

CH4 + 5O2+ NO + 2OH• + UV-A →

CO2 + H2O + NO2 + 4HOO•

Notice the radicals consumed

and produced.

What happens to the HO2 produced?

What happens to the NO2 produced?

(see fate of free radicals)

Reaction intermediates during hydride oxidation

Problems

• 3-4• 3-5• 3-6

Part 2

Photochemical Smog

Saturated hydrocarbons such as CH4 react with hydroxyl radical by hydrogen abstraction

Hydrocarbons with double bond (e.g., ethene) react with •OH by addition because of lower activation energy

…formation of carbon centered radical

Oxidation of Reactive Hydrocarbons

Energetics favor addition over abstraction

Photochemical SmogCarbon centered radical reacts with O2 to produce a peroxy radical which in turn oxidizes NO to NO2

Photochemical decomposition of NO2 to NO and O and formation of ozone results in photochemical smog

NO2 → NO + O (1)

O + O2 → O3 (2)

NO + O3 → 2NO2 + O2 (3)

NO2 is the only significant source of O

Formation of Aldehydes

Decomposition of carbon centered radical

Aldehydes further decompose in sunlight

RHCO + sunlight → R• + HCO•

….further increase in the number of radicals

Original C=C is split into 2 aldehydes

Overall

RHC=CHR + OH• + 2O2 + NO• → 2RHC=O + HOO• + NO2•

Mechanism of the RHC=CHR oxidation process in the smog

NO is oxidized by the C-O-O·

Addition of O2 to radical center

Energetics favor addition over abstraction

Cleavage allows formation of aldehyde double bond

Reaction with O2 allows 2nd aldehyde to form

Photolysis follows.

Peroxy radicals are formed.

NO is oxidized to NO2

Radicals formed: HO2 (2); OH (1), RO (1), NO2 (3)

Also CO

Problems

• 3-8

The Fate of Free Radicals

Rate of reaction between two radicals increase as the radical concentration increases

R• + R’• → R-R’ stable molecule

e.g.,

OH• +NO2• → HNO3

OH• + NO• → HNO2 OH• + NO•

(HONO accumulates only in the night)

When the concentration of NOx is low,

2OH• → H2O2

2HOO• → H2O2 + O2

sunlightStarts AM cycle

Fate of Other Radicals

•O2 + R-C=O

(peroxyacetylnitrate)Peroxyacetylnitrate is eye irritant and toxic to plants

Thus in the afternoon hours a build up of oxidizing agents such as nitric acid, hydrogen peroxide and PAN is encountered

Hourly Variation of Concentration of Gases

• Early morning traffic increases the emissions of both nitrogen oxides and VOCs as people drive to work

• Later in the morning, traffic dies down and the nitrogen oxides and volatile organic compounds begin to react forming nitrogen dioxide, increasing its concentration

• As the sunlight becomes more intense later in the day, nitrogen dioxide is broken down and its byproducts form increasing concentrations of ozone

• As the sun goes down, the production of ozone is halted. The ozone that remains in the atmosphere is then consumed by several different reactions

Source: http://jan.ucc.nau.edu/~doetqp-p

NO → NO2

HC → Aldehydes

Role of NO3•

• Nitrate radical produced from NO2 and O3

NO2• + O3 → NO3• + O2

• Photolysis yields NO2 and O

• Abstracts H from RH during evening

NO3• + RH → HNO3 + R•

Similar to OH

Part 3

Oxidation of SO2 (g)

Addition of OH• followed by the formation of SO3

SO3 + H2O(g) → H2SO4(g)

H2SO4(g) + nH2O → H2SO4 (aq)

Oxidation of SO2 (aq)

Determination of total sulfur content in water

SO2 is soluble in water. It exists in the dissolved form if there is significant cloud or mist in the atmosphere. The oxidation to sulfuric acid occurs in the aqueous phase after rain drops accumulate on earth.

SO2 (g) + H2O (aq) ⇌ H2SO3 (aq)

Typically SO2 conc. is 0.1 ppm or (0.1/106) =1 x 10-7 atm

From Henry’s law, KH = 1 M atm-1 = [H2SO3]/P

[H2SO3] = 1 M atm-1 x 1x10-7 atm = 1 x 10-7 M (or moles/L)

But H2SO3 dissociates readily with a dissociation constant of K = 1.7 x 10-2 M-1

H2SO3 ⇌ H+ + HSO3-

As HSO3 dissociates, more of SO2 dissolves until it reaches an equilibrium with H+ and HSO3

1.7 x 10-2 M-1 (or K) = [H+][HSO3-]/[H2SO3]

1.7 x 10-2 M-1 (or K) = [HSO3-]2/[H2SO3] = [HSO3

-]2 / 1 x 10-7 M …[H+] = [HSO3-]

[HSO3-]2 = 17 x 10-10 M2 = 4 x 10-5 M Total dissolved S is 4 x 10-5 M

Oxidation of SO2 (aq)

• Dissolved SO2 is oxidized by trace amounts of H2O2 and O3

• Sunlight is a dominant factor in forming O3 and H2O2

• If strong acids are present in the droplet they control the pH.

• Any freshly dissolved SO2 has no effect

[HSO3-] = K x [H2SO3]/[H+]

=1.7 x 10-2 x 10-7/[H+]

=1.7x10-9/[H+]

…inversely proportional to H+

Since strong acids dissociate readily,

[H+] concentration controls the overall concentration of HSO3

-

• Acidity of the droplet has effect on the rate of SO2 oxidation

• At pH below 5 H2O2 dominates oxidation and above pH 5 ozone or other catalytic reactions dominate the oxidation

Hydrogen abstraction reactions dominate chemistry in both stratosphere and troposphere

…….but the radicals that dictate the chemistry are different• Stratosphere: •OH, •O, •Cl, and •Br abstract H atom from stable

molecule such as CH4

• Troposphere: hydroxyl and NOx radicals are the primary reactants

Processes Involving Loosely Bound Oxygen Atoms

A Y-O structure from which O atom can be detached readily

Examples of “Loose O Atom” Reactions

Reaction with atomic oxygen

Y―O → Y + O2

Photochemical decomposition

Y―O + sunlight → Y + O

Reaction with NO

Y―O + NO → Y + NO2

Abstraction of oxygen from Ozone

Y―O + X → Y + XO

O2―O + Cl → O2 + ClO