QUESTIONS • Oxygen has a constant mixing ratio in the atmosphere. How would you expect its number density in surface air to vary between day and night? • Give a rough order of magnitude for the number of molecules present in a typical 1 micrometer aerosol particle. • Does it make sense to talk about the mixing ratio of aerosolparticles in air? To express the concentration of soot aerosol in units of ppbv?
THE PERIODIC TABLE First drafts of Mendeleev’s periodic table, 1869 photos from Mendeleev museum, St. Petersburg, 2007
CHEMICAL BONDS Bond 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) Destruction of a bond corresponds to a release of energy. Generally double or triple bond energies are higher than for single bonds. Ionic bonds: electron attraction between positive and negative ions e- transfer Convalent bonds: sharing of paired electrons Polar convalent bonds: When 2 atoms from different elements share e- unequally
OXIDATION STATE Oxidation State describes positive or negative character of atoms, or degree of oxidation Ionic Molecules: oxidation state is the same as the charge on the ion example: Na+1Cl-1 Ca+2Br2-1 Note: sum of oxidation numbers must equal zero Covalent Molecules: more arbitrary, based on electronegativity scale example: CO2: C+4O2-2 oxidation: C oxidation state has increased from -IV to +IV (the opposite would be reduction) Atmosphere is generally an oxidizing medium.
ORGANIC MOLECULAR NOMENCLATURE Alkanes (C-C single bonds) Alkenes (C-C double bonds) CnH2n+2 CnH2n ethane ethene Alkynes (C-C triple bonds) Aromatic compounds CnH2n-2 CnH2n-6 ethyne Benzene Oxygenated hydrocarbons: Aldehydes, alcohols, ketones, etc… acetaldehyde methanol Acetic acid
COMMON IONS Ammonium 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-
CHEMICAL THERMODYNAMICS Enthalpy: Thermodynamic potential of the system Heat of reaction (ΔHrxn)= change of enthalpy depends on T, is independent of path ΔHf = heat of formation (per mole) by definition = 0 for elements Exothermic Endothermic Gibbs Free Energy: calculated ΔG in same way as enthalpy change ΔG < 0 forward reaction spontaneous ΔG > 0 reverse reaction spontaneous ΔG = 0 reaction is at equilibrium S = entropy
REACTION RATES: BASICS A balanced chemical reaction does not represent the actual steps of thereaction pathway or mechanism Rate-determining step: the slowest step which determines the max rate of overall rxn Rate of an elementary reaction: A + B C Reaction Rate = k [A][B] Rate of reactions generally increase with temperature: Catalysts decrease the energy of activation increases the rate of forward and reverse reactions k=rate constant If A ≠ f(T) = Arrhenius form E = activation energy a,b correspond to reaction order General Reaction Rates: aA + bB + … gG + hH …. , k Reaction Rate = k[A]a[B]b…
CHEMICAL KINETICS For multi-step reactions, need to sum the individual reaction rates: A + B C k1 A + D B k2 Biomolecular Reaction: A + B C + D Collision of 2 reactants (A and B) forms an activated complex (AB*) which decomposes rapidly to the products (C and D) Reaction Rate: Special Case: Self Reaction: A + A B + C k: unit here [cm3/molecule/s]
CHEMICAL KINETICS: THREE-BODY REACTIONS A + B AB* 3 AB* A + B 4 AB* + M AB + M* 5 M* M + heat 6 A + B + M AB + M 7 M = third body (usually inert: O2, N2) stabilizes the excited products AB* In the atmosphere, take [M]=na Rate of formation from 3rd rxn: But assume AB* short lifetime, can use steady state approximation formation rate = loss rate Re-arrange: 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
CHEMICAL EQUILIBRIA Notation: also see kr=k-f A + B C + D, kf A + B ↔ C + D C + D A + B, kr At equilibria (or ss) : 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
PHOTOLYSIS Breaking a chemical bond with an incident photon: AB + hν A + B AB + hν AB* AB + hνluminescence AB + M quenching A + B photodissociation j = photolytic rate constant [s-1] h = Planck constant ν = frequency J = actinic flux [photons/cm2/s] σx = absorption cross-section [cm2/molecule] φx = quantum yield (probability photon abs causes photolysis) [molecules/photon] Defining the photolytic rate constant: For polycromatic radiation: A gas molecule will absorb radiation at a given wavelength only if the energy can be used to increase the internal energy of a molecule Rotational transitions far IR radiation (> 20 µm) Vibrational transitions near IR radiation (0.7-20 µm) Electronic transitions UV radiation (< 0.4 µm)
RADICAL-ASSISTED REACTION CHAINS Radical: chemical species with an unpaired electron in the valence shell example: NO (7 + 8 = 15 e) = radical, HNO3 (1+7+24 = 32 e) = non-radical high free energies, more reactive nomenclature often denotes these with a dot, example: CH3● Radical chain reactions (often called photochemical chain reactions): nonradical + hν radical + radical initiation radical + nonradical radical + nonradical propogation …. radical + radical nonradical + nonradical termination ( OR: radical + radical + M nonradical + M) Example: Hydrogen & Bromine: Br2+H22HBr
ACIDS AND BASES NOTE: OH- (hydroxide ion) OH (hydroxyl radical)! H2O(l)↔ H+(aq) + OH-(aq) pH = -log[H+] the activity of H+ < 7 = acidic > 7 = basic 7 = neutral Note: in atmosphere neutral pH=5-5.7 because pure water takes up CO2 Acid-Base Equilibrium: example ionization of acetic acid
SOLUBILITY AND HENRY’S LAW Solubility Equilibria: Henry’s Law: Distribution of species between aqueous and gas phases Slightly soluble salt Ksp = solubility product HA = Henry’s Law Constant Units here are mol/L/atm (sometimes reciprocal – be careful!) Some Henry’s Law Constants of Atmospheric Relevance: