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Lecture 8

CHMB16Fall2012 Lecture 8 Notes.docx

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Kagan Kerman

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CHMB16Fall2012 Lecture 8: Acid-Base Titrations (continued) Activity and Equilibria o What happens to the EQ below if KNO3 is added? o o from the EQ constant as it is written, you would predict that there would be no effect o o the EQ constant is not constant when expressed in terms of concentration o why does solubility increase as an inert salt is added? o Has to do with their effect on the ionic atmosphere o An anion or cation in solution will attract the opposite charged ions o As a result the region immediately surrounding the ion will have a net opposite charge o This region is called the ionic atmosphere o When molecules enter each other’s ionic atmospheres then they begin to react because then they will collide (collision theory)  if they are not close enough to one another then they will not react Ion Size o o Ion size (α) is a reflection of the diameter of the hydrated ion o Small highly charged ions can bind solvent more strongly than larger or less highly charged ions o Result: Li+ has a larger ion size than Rb+, even though its atomic radii is smaller o So lithium is more reactive because it has a greater ionic atmosphere (so larger area in which to react with other molecules compared to rubidium) Ionic strength o The effect of the ionic atmosphere can be described numerically by calculating ionic strength (μ) of a solution o o the ionic strength measures the total concentration of ions in solution (the more charged an ion the more it counts) o calculating the real ionic strength of solutions can be difficult since many ions with charged >2 do not completely dissociate o this causes errors when using this formula 1 o Dr. Keerman uses 100mM NaCl to balance the ionic strength of a buffer solution  i.e. to bring the ionic strength closer to physiological serum ionic strength Ionic strength and EQ constants o We can account for the affect of ionic strength on EQ constant by using activities (a) instead of concentrations o o activity coefficient measures the deviation of X from it ideal behavior o so the correct EQ constant equation is now as follows: o o o Dissociation of water (Ion-Product Constant) o water undergoes autoprotolysis o o o so at 25 degrees C, the pH of water is 7 (neutral) but 2 o o so the pH is 6.98 instead of 7 because activity numbers can change depending on the temperature of solutions  the value of Kw depends on the solution’s temperature o dissociation increases with increasing temperature ** Buffers o buffered solutions will resist changes in pH upon the addition of acid or base or the dilution of the solution o the buffer resists change in pH by consuming any added H+ or OH- until its buffering capacity is exceeded o to produce a buffer, an acid and its conjugate base are mixed together o to act as a buffer, the concentrations of the acid and its conjugate base must be comparable (within a factor of 10) o buffers are important as many reactions are pH dependant  especially biological reactions o o figure shows pH dependence of an enzyme catalyzed reaction o bufferes are found widely in nature to control pH o bufferes = controlled environment for reaction to take place  i.e. maintains optimal conditions for the reaction o o o pH = pKa where the concentrations of the acid-base pairs are the same o 3 o o o Properties of Buffers o effect of dilution: the pH of a buffer is independent of dilution until the concentration becomes low enough that the approximation [HA]=[A-] breaks down o extreemes of pH: the above approximation also breaks down when pH is extremely low or high o ionic strength: the correct H-H equation should include activities  changing the ionic strength of a buffer can have a marked impact on its pH o temperature: magnitude of Ka depends on temperature so the pH of a buffer is also dependent on temperature (varies
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