ACIDBASE SPECIATION CALCULATIONS

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ACID-BASE SPECIATION CALCULATIONS
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MONOPROTIC ACIDS

• What are the pH and concentrations of all species
  in a 0.1 mol L-1 HF solution?
• 1) Write out important species H, OH-, HF0, F-.
• 2) Write out all independent reactions and their
  equilibrium constants
• HF0 ? H F-
• H2O(l) ? H OH-

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• 3) Write out mass-balance expressions
• 0.1 mol L-1 ?F F- HF0
• 4) Write out the charge-balance expression
• H F- OH-
• 5) Make reasonable assumptions
• HF is an acid, so H gtgt OH- the
  charge-balance becomes
• H ? F- X
• and the mass-balance becomes
• HF0 0.1 - X

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• 6) Solve quadratic equation

a -1 b -10-3.2 -6.31x10-4 c 10-4.2
6.31x10-5
X1 -0.00825 X2 0.00765 H F-
7.65x10-3 mol L-1 pH -log H 2.12 HF0
0.1 - 0.00765 0.0924 mol L-1
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• 7) Check assumption 1.318x10-12 ltlt 7.65x10-3, so
  OH- ltlt H.
• What if we assumed HF0 gtgt F-, i.e., HF0 ?
  0.1? This might be valid because HF is a weak
  acid.
• 10-3.2 X2/0.1
• X2 10-4.2
• X 10-2.1 0.00794
• H F- 7.94x10-3 mol L-1 pH 2.10
• HF0 0.1 - 0.00794 0.092 mol L-1
• The above answer is only 8 different from 0.1.
  It seems in any case where KA lt 10-3.2, the above
  assumption should be good!

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POLYPROTIC ACID

• What is the pH and concentration of all species
  in a 0.1 mol L-1 solution of H3PO4?
• 1) Species H, OH-, H3PO40, H2PO4-, HPO42-,
  PO43-
• 2) Mass action expressions
• H3PO40 ? H2PO4- H
• H2PO4- ? HPO42- H
• HPO42- ? PO43- H

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• H2O(l) ? H OH-
• 3) Mass-balance
• 0.1 mol L-1 H3PO40 H2PO4- HPO42-
  PO43-
• 4) Charge-balance
• H H2PO4- 2HPO42- 3PO43- OH-
• 5) Assumptions
• a) Because H3PO40 is an acid H gtgt OH-
• b) Because H2PO4- and HPO42- are very weak acids
  and H3PO40 is only moderately weak
• H3PO40 gt H2PO4- gtgt HPO42- gtgt PO43-
• so, 0.1 H3PO40 H2PO4-
• and H H2PO4- X.

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• 10-3.1 - 10-2.1X - X2 0

X1 0.0245 X2 -0.0324 H H2PO4-
0.0245 mol L-1 pH 1.61 H3PO40 0.1 - 0.0245
0.0755 mol L-1
So HPO42- 10-7.0 mol L-1
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• PO43- 10-17.79 1.62x10-18 mol L-1
• OH- 10-14/10-1.61 10-12.39 4.07x10-13 mol
  L-1

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THE pH OF NORMAL RAINWATER

• The pH of normal, unpolluted rainwater is
  controlled by carbonic acid equilibrium
• CO2(g) ? CO2(aq) ? H2CO30
• What is the pH of rainwater in equilibrium with
  atmospheric CO2 (PCO2 10-3.5 atm)?
• 1) Species CO2(g), H2CO30, HCO3-, CO32-, H,
  OH-.
• 2) Mass action
• CO2(g) H2O(l) ? H2CO30

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• H2CO30? HCO3- H
• HCO3- ? CO32- H
• H2O(l) ? H OH-
• 3) Instead of a mass-balance we have PCO2
  10-3.5 atm
• 4) Charge balance
• H HCO3- 2CO32- OH-
• 5) Assumptions
• a) H2CO30 is an acid so H gtgt OH-
• b) HCO3- is a weak acid so HCO3- gtgt CO32-
• Therefore, H HCO3- X

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H2CO30 (10-1.5)(10-3.5) 10-5 mol L-1
X2 (10-6.3)(10-5.0) 10-11.3 X H
HCO3- 10-5.65 2.24x10-6 mol L-1 pH
5.65 So the pH of pure, normal rainwater is
acidic! Check assumption OH- 10-14/10-5.65
4.47x10-9 so H gtgt OH-
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pH OF RAIN ACIDIFIED WITH SO2

• The concentration of SO2 in the atmosphere is
  usually less than that of CO2, but SO2 is more
  soluble in water and forms a stronger acid.
• What is the pH of a rain droplet in equilibrium
  with an atmosphere with PSO2 5x10-9 atm?
• We proceed as in previous example, but we assume
  that the effect of CO2 can be neglected because
  SO2 produces a stronger acid.
• 1) Species SO2(g), H2SO30, HSO3-, SO32-, H, OH-

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• 2) Mass action
• SO2(g) H2O(l) ? H2SO30
• H2SO30 ? HSO3- H
• HSO3- ? SO32- H
• H2O(l) ? H OH-
• 3) Instead of mass-balance PSO2 5x10-9 atm
• 4) Charge-balance H HSO3- 2SO32-
  OH-

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• 5) Assumptions
• a) Because H2SO30 is an acid H gtgt OH-
• b) HSO3- is a weak acid so HSO3- gtgt SO32-
• Therefore, H HSO3- X
• H2SO30 KHPSO2 2.0 x 5x10-9 10-8 mol L-1
• K1 2x10-2 X2/10-8
• X2 2x10-10
• X H HSO3- 1.414x10-5 mol L-1 pH
  4.85
• OH- 10-14/10-4.85 7.07x10-10 mol L-1
• Note that only a very small amount of SO2 in the
  atmosphere can result in significant
  acidification of rain!

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EFFECT OF OXIDATION OF SO2

• What would the pH of a droplet of rain be if all
  the SO2 in the atmosphere were oxidized to H2SO4?
• Assume that 1 m3 of atmosphere contains 0.001 dm3
  of liquid water and the temperature is 15C.
• Because H2SO4 is a strong acid, we can assume
  that it is totally dissociated. Thus, we can
  solve this problem simply by calculating the
  concentration of SO2 in moles L-1.
• If PSO2 5x10-9 atm, then 1 m3 of atmosphere
  contains 5x10-9 m3 of SO2. To convert to moles,
  we must know how much volume a mole of gas
  occupies at 15 C and 1 atm.

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USE IDEAL GAS LAW

• PV nRT
• P 1 atm n 1 mol R 0. 0820575 atm L
  moles-1 K-1 T 288.15 K
• V
• (1 mol)(0.0820575 atm L moles-1 K-1)(288.15 K)/1
  atm
• 23.65 L 23.65x103 cm3 (1 m/100 cm)3 0.02365
  m3
• Thus, 1 m3 of air would contain 5x10-9/0.0237
  2.11x10-7 moles of SO2. If all this SO2 is
  oxidized and removed into a droplet of H2O of
  volume 0.001 dm3, this would result in 2.11x10-4
  mol L-1 of H2SO4. This yields 4.22x10-4 mol L-1
  H or pH 3.37.

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URBAN ATMOSPHERE

• Primary pollutant a pollutant compound directly
  released to the atmosphere (e.g., smoke, CO, CO2,
  etc.).
• Secondary pollutant a pollutant compound formed
  as a product of chemical reactions in the
  atmosphere.

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LONDON SMOGPRIMARY POLLUTION

• Urban air pollution was primarily the product of
  combustion of fuels.
• The rapid development of pollution coincided with
  the transition to fossil fuel burning.
• Normal, complete fuel combustion is described by
• 4CH 5O2(g) ? 4CO2(g) 2H2O(g)
• Neither CO2 nor H2O are particularly toxic.
  However, during incomplete combustion we get CO
• 4CH 3O2(g) ? 4CO(g) 2H2O(g)

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LONDON SMOG(CONTINUED)

• And also smoke particles
• 4CH O2(g) ? 4C(s) 2H2O(g)
• At low temperatures, we might also get PAHs
  (polycyclic aromatic hydrocarbons) such as
  benzo(?)pyrene, which are carcinogenic.

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LONDON SMOG(CONTINUED)

• Fuel contaminants may also be a problem
• 4FeS2(s) 11O2(g) ? 8SO2(g) 2Fe2O3(s)
• Sulfur is highest in coals and fuel oils.
• Smoke and SO2 are primary pollutants.
• SMOG combination of smoke and fog (water
  droplets).
• SO2(g) H2O(l) ? H HSO3-
• 2HSO3- O2(aq) ? 2H 2SO42-
• Fog droplets containing H2SO4 caused respiratory
  diseases.

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LOS ANGELES SMOGSECONDARY POLLUTION

• Use of greater volatility liquid fuels in motor
  vehicles caused a new type of air pollution.
• The major pollutants are not themselves emitted
  by motor vehicles, but are formed by reactions
  involving primary pollutants.
• Photochemical smog smog whose formation is
  catalyzed by sunlight.
• Fuel is burned in air, not pure O2, which has
  important consequences.

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LOS ANGELES SMOG (CONTINUED)

• O(g) N2(g) ? NO(g) N(g)
• N(g) O2(g) ? NO(g) O(g)
• N2(g) O2(g) ? 2NO(g)
• Next, NO is oxidized in smog to give NO2(g) (see
  Box 3.6 in Andrews for details). NO2 is a
  brownish gas that absorbs light.
• NO2(g) h? ? NO(g) O(g)
• O(g) O2(g) ? O3(g)
• Thus, ozone (a respiratory irritant) is produced
  as a secondary pollutant.

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VOLATILE ORGANIC COMPOUNDS (VOCs)

• VOCs may also be released from fuel combustion.
• VOCs cause two problems
• They aid in NO2 production.
• CH4(g) 2O2(g) 2NO(g) h? ? H2O HCHO(g)
  2NO2(g)
• They lead to formation of aldehydes (eye
  irritants and carcinogens)
• PAN peroxyacetylnitrate - important eye irritant
  in photochemical smog.

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LEADED VS. UNDLEADED GASOLINE

• Leaded gasoline contained tetraethyl lead -
  Pb(C2H5)4 which resulted in Pb pollution of air,
  soils and waters.
• Some unleaded gasoline contains benzene, which is
  a carcinogen and contributor to photochemical
  SMOG.
• Some gasoline contains MTBE (methyl tertiary
  butyl ether) which was added to improve air
  quality as a fuel oxygenate, but now has
  contaminated water supplies where gasoline has
  leaked or spilled.

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EFFECTS OF AIR POLLUTION

• H2SO4 formed in smog can be an important agent of
  corrosion
• H2SO4(aq) CaCO3(s) H2O(l) ? CO2(g)
  CaSO42H2O(s)
• Gypsum causes two problems
• 1) It dissolves in rain.
• 2) Increased volume leads to mechanical stress.
• Ozone attacks rubber, plastics, pigments and dyes.

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REMOVAL PROCESSES

• Wet deposition removal of soluble components in
  rain or snow.
• Oxidation is an important acid-forming process.
• Organic compounds ? carboxylic acids (acetic,
  formic)
• Sulfur compounds ? H2SO4
• Organosulfur compounds ? MSA (methanesulfonic
  acid or CH3SO3H
• Nitrogen compounds ? HNO3
• Rain is usually acidic due to presence of
  dissolved CO2 (pH 5.6). These acids can lower
  pH further.
• Dry deposition Direct removal of gaseous or
  particulate pollutants onto surface of the Earth.