Part B: Question 5
For the following answer, try using the bullet format.
- 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