Cliffs AP Chemistry, 3rd Edition

Part B: Question 5

For the following answer, try using the bullet format.

5. Restatement: Explain each of the following:
   (a) Water can act as either an acid or a base.

• Water can provide both H+and OH–.
  H 2 O "H++ OH–


• According to Brønsted-Lowry theory, a water molecule can accept a proton, thereby be-
  coming a hydronium ion. In this case, water is acting as a base (proton acceptor).
  H 2 O + H+→H 3 O+


• When water acts as a Brønsted-Lowry acid, it donates a proton to another species, thereby
  converting to the hydroxide ion.


• According to Lewis theory, water can act as a Lewis base (electron pair donor). Water
  contains an unshared pair of electrons that is utilized in accepting a proton to form the hy-
  dronium ion.

(b) HF is a weaker acid than HCl.

• Fluorine is more electronegative than Cl.


• The bond between H and F is therefore stronger than the bond between H and Cl.


• Acid strength is measured in terms of how easy it is for the H to ionize. The stronger the
  acid, the weaker the bond between the H atom and the rest of the acid molecule; measured
  as Kaor, if the acid is polyprotic, Ka 1 , Ka 2 , Ka3,...
  (c) For the triprotic acid H 3 PO 4 , Kalis 7.5 × 10 –3, whereas Ka 2 is 6.2 × 10 –8.


• Ka 1 represents the first hydrogen to depart the H 3 PO 4 molecule, leaving the conjugate base,
  H 2 PO 4 –.


• The conjugate base, H 2 PO 4 – , has an overall negative charge.


• The overall negative charge of the H 2 PO 4 – species increases the attraction of its own con-
  jugate base HPO 4 2–to the departing proton. This creates a strongerbond, which indicates
  that it is a weaker acid.

OH+H+

H

HO

H

H

+

base acid conjugate

base

conjugate

acid

H 2 OH+ 2 O OH− + H 3 O+

Part IV: AP Chemistry Practice Test