Question

This problem is a long one, but fairly conceptual in nature.

a. For each of the following salts, write the reaction that occurs when it dissociates in water: NaCl(s), NaCN(s), KClO2(s), NH4NO3(s), KBr(aq), NaF(s).

b. Consider each of the reactions that you wrote above, and identify the aqueous ions that *could* be proton donors (acids) or proton acceptors (bases). Briefly *explain how you decided which ions to choose*.

c. For each of the acids and bases that you identified in part b, write the chemical reaction it can undergo in aqueous solution (its reaction with water).

d. Are there any reactions that you have written above that you anticipate will occur to such an extent that the pH of the solution will be affected? As part of your answer, be sure to explain how you decided.

e. Assume that in each case, above, 0.01 mol of the salt was dissolved in enough water at 25 deg Celsius to make 1.0L of solution. In each case, what additional information would you need in order to calculate the pH? If there are cases where no additional information is required, be sure to state that as well.

f. Say you take 0.01 mol of NH4CN and dissolve it in enough water at 25 deg Celsius to make 1.0L of solution. Using chemical reactions and words, explain how you would go about determining what effect this salt will have the pH of the solution. Be sure to list any additional information you would need to arrive at an answer.

Answer 1

a. NaCl(s) ----> Na+(aq) + Cl-(aq)

NaCN(s) ----> Na+(aq) + CN-(aq)

KClO2(s) ----> K+(aq) + ClO2(aq)

NH4NO3(s) ---> NH4+(aq) + NO3-(aq)

KBr(aq) ---> K+(aq) + Br-(aq)

NaF(s) ---> Na+(aq) + F-(aq)

b. In the above reaction,

Proton donors are : NH4+

Proton acceptor are : Cl-, CN-, ClO2-, NO3-, Br-, F-

Only NH4+ has an extra proton that it can donate among all the species shown above.

For proton acceptor, most of the anions accept a proton when in aqueous medium.

c. NH4+(aq) + H2O(l) ---> NH3(aq) + H3O+(aq)

Cl-(aq) + H2O(l) ---> HCl(aq) + OH-(aq)

CN-(aq) + H2O(l) ---> HCN(aq) + OH-(aq)

NO3-(aq) + H2O(l) ---> HNO3(aq) + OH-(aq)

Br-(aq) + H2O(l) ----> HBr(aq) + OH-(aq)

F-(aq) + H2O(l) ---> HF(aq) + OH-(aq)

d. pH of the reaction will be affected by strong acids like reaction of water with Cl-, Br-, NO3-. To some extent even HCn, NH4+ dissciation affects the pH.

e. In each case we would need Kb or Ka for conjugate acids or bases to calculate pH.

f. NH4CN ---> NH4+ + CN-

NH4+ + H2O <==> NH3 + OH-

So the resulting solution will have excess OH- in it. This would raise the pH of the solution and be more basic.