Dr. Herrick's  CH223
Extra Stuff



  Aqua Regia A mixture of the two strong acids HCl and HNO3 that dissolves "royal" metals such as gold that are otherwise difficult to put into solution for reactions or purification. The power to dissolve gold is related to the redox reaction
  Au + 4H+ + 4Cl - + NO3- AuCl4- + NO + 2H2O
in which the chloride ion facilitates oxidation of Au to the auric ion Au3+.  We’ll be looking into oxidation-reduction reactions and electrochemistry in Chapter 21. When the solution containing excess HCl is evaporated it crystallizes out hydrated tetrachlorauric acid, HAuCl4.4H2O. The related salt NaAuCl4.2H2O has been used in toning photographic prints.  Aqua regia is commonly used in laboratories simply to clean things.

Temperature Dependence of Kw.  The pH of neutral water at room temperature is approximately 7.00, but this value changes at higher or lower temperatures.  The water ionization reaction
2H2O H3O+ + OH-     
is endothermic, so LeChatelier's principle says the equilibrium shifts to increase the hydronium ion concentration (and hence decrease pKw and the neutral pH) as the temperature is raised:

You can find a more extensive tabulation in the CRC Handbook of Chemistry and Physics.

Acid-Base Distribution Diagrams.   Diagrams for the distribution of acid-base species from a multiprotic acid HnA may be determined as a function of pH by expressing ratios of species in terms of the acid equilibrium constants K1, K2,....   Here are the final expressions for the mole fraction of each species.  You can use these to make accurate plots of their relative amounts in aqueous solution.

1)  monoprotic acid HA (K1)

2)  diprotic acid H2A (K1, K2)

3)  triprotic acid H3A (K1, K2, K3)



 Role of Kw in pH of weak acid or base solutions.   See frames 6-9 of the April 9 lecture.  Kw may be neglected when the pH is less than 6 or greater than 8. 

  Profiles:  Linus Pauling, Maria Mayer (& picture),    
  Nobel Prizes in Chemistry
  amino acid structures and pKa values
  accurate values of physical constants:
  National Chemistry Week
  Ann Herrick: Books for Kids & Teens

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