Chemistry Reference
In-Depth Information
OH
H
3
O
)
The presence of excess
(relative to the quantity of
in the solution
is what makes the solution basic.
EXAMPLE 19.4
A solution of ammonium nitrate tests acidic to litmus paper. Explain why such
a solution is acidic.
Solution
The solution contains a feeble base (nitrate ion) and a weak acid (ammonium
ion). The acid is stronger than the base, so the solution is somewhat acidic:
NH
4
(aq)
H
3
O
(aq)
H
2
O(
/
) E NH
3
(aq)
NO
3
(aq)
H
2
O(
/
) £ N.R.
The hydronium ion concentration is greater than the hydroxide ion concentra-
tion, so the solution is somewhat acidic.
Acids and Bases Without Water
The Brønsted theory extends the concepts of acid and base beyond reactions in
aqueous solution. For example, it describes the reaction of ammonia gas with
hydrogen chloride gas to form solid ammonium chloride:
NH
3
(g)
HCl(g) E NH
4
Cl(s)
Here, hydrogen chloride donates its proton to the lone pair of electrons on the
ammonia molecule and is therefore a Brønsted acid. The ammonia molecule is
a base because it accepts that proton. The same pair of substances act as a base
and an acid in aqueous solution.
Snapshot Review
❒
The Brønsted theory expands the definitions of acids and bases to
explain the acidity or basicity of solutions previously regarded as salts
and to explain reactions in other than aqueous solutions.
ChemSkill Builder 18.1
A. Label each substance in the equation for the autoionization of liquid
ammonia as Brønsted acid or base:
NH
3
(
/
) E NH
4
NH
2
NH
3
(
/
)
B. Which of the following equilibrium reactions is expected to proceed
almost to completion?
(a)
(b)
C. Write each of the equations of Problem B in the reverse direction. Which
of
these
reactions is expected to proceed almost to completion?
D. What is the conjugate base of H
3
PO
3
?
OH
(aq) E NO
2
(aq)
HNO
2
(aq)
H
2
O(
/
)
H
2
O(
/
) E NO
2
(aq)
H
3
O
(aq)
HNO
2
(aq)
