Acids and bases

Question 1

What does highly pure water do?

Answer 1

~ self-ionizes to a very limited extent driven by the entropy of mixing

~ 2H2O = H3O+ + OH-

Question 2

What is the equilibrium constant for water?

Answer 2

~ k = [H3O+][OH-]/[H2O]^2

~ Kw = k[H2O]^2 = [H3O+][OH-]

~ [OH-] = [H3O+]
~ thus [H3O+] = [OH-] = 1E-7

Question 3

What are the brackets in the equilibrium constant equation?

Answer 3

~ indicate a concentration ratio (unitless) of moles/liter per liter of soln. relative to a standard state conc. of 1mol/liter

Question 4

What’s the definition of pH?

Answer 4

~ pH = log10[H3O+]

~ for deionized water: pH = 7

Question 5

What is [H2O] with dilute additions of acid or base?

Answer 5

~ unchanged

~ Kw = constant = [H3O+][OH-]

Question 6

What happens as acid is added?

Answer 6

~ [H3O+] goes up, so pH goes down
~ [OH-] must decrease

Question 7

What is Le Chatelier’s principle?

Answer 7

equilibria shifts towards reactants to oppose increase in [H3O+], causing a decrease in [OH-]

Question 8

What can salts do?

Answer 8

~ act as acids or bases

~ ex: FeCl3 = Fe3+ + 3Cl-
~ Fe3+(H2O)6 + H2O = Fe2+(H2O)5*OH + H3O+

Question 9

What’s the effect of shifting the pH?

Answer 9

~ can alter the solubility/precipitation tendency of a compound

~ ex: lowering pH will suppress the dissolution of FeCl3

Question 10

What’s the effect of shifting the pH?

Answer 10

~ can alter the solubility/precipitation tendency of a compound

~ ex: lowering pH will suppress the dissolution of FeCl3

Question 11

How is shifting the pH useful for purification?

Answer 11

~ increase pH until compoun of interest dissolves
~ filter out remaining impurity solids
~ decrease pH until solid of interest precipitates; filter out liquid to eliminate still-soluble impurities