(BQ) Part 2 book "Elements of environmental chemistry" has contents: CO2 equilibria, fates of organic compounds, toxic environmental compounds, water solubility, fish partition coefficients, problem set, polluted rain. | CHAPTER 4 CO2 EQUILIBRIA One motivation for studying CO2 equilibria is to understand the effect that trace gases in the atmosphere have on the acidity (pH) of rain. Acid rain has been a problem of national and international scope with major economic consequences. Our approach will be to set up several equations for various reactions and use them to find the pH of rain (or of a lake, etc.). First, we need to remember the definition of pH. pH ¼ À log½Hþ where log refers to the common (base 10) logarithm, and anything in square brackets refers to molar (moles per liter) concentration units. The use of the lowercase ‘‘p’’ here refers to the power of 10. It is true that pANYTHING ¼ Àlog[ANYTHING] For example, pKa of acetic acid is , which means that Ka is 10À4:76 ¼ 1:75 Â 10À5 (verify this for yourself). Remember that the equilibrium constant for the reaction A þ B $ C þ D is K¼ ½C ½D ½A ½B Elements of Environmental Chemistry, by Ronald A. Hites Copyright # 2007 John Wiley & Sons, Inc. 108 CO2 EQUILIBRIA This means that when the reaction between A and B is at equilibrium (., when the reaction has come to completion and when neither A or B is being lost nor C or D is being formed), the ratio of the concentrations of the products times each other divided by the concentrations of the reactants times each other is constant. If K is very small, then there are relatively low product concentrations compared to the reactant In fact, most interesting K values are usually small; hence, we use the pK notation. Also remember for pure water, the reaction H2 O $ Hþ þ OHÀ has an equilibrium constant of 10À14:00 at room temperature, or in our notation, pKw ¼ 14:00. Finally, let us define the Henry’s law constant, which is the ratio of the equilibrium concentration of a compound in solution to the equilibrium concentration of that compound in the gas phase over that solution. It is usually given as KH . This constant can be given in one of two ways: with .
