Cliffs AP Chemistry, 3rd Edition

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Part B: Question 5


For the following answer, try using the bullet format.



  1. 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

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