Geology Reference
In-Depth Information
APPENDIX B: sIMPlE solutIoN
chEMIstry
Acids and bases
the other hand, is a
polyprotic
acid,
4
because each mol-
ecule has more than one H
+
to donate.
A
base
is a substance that accepts or absorbs H
+
ions,
thereby
depleting
the host solution in free H
+
and mak-
ing it
less
acidic. The reaction between KOH (a base)
and H
2
SO
4
(sulfuric acid) may be written:
The simplest definition of an
acid
is 'a substance capa-
ble of contributing hydrogen ions to a solution or
reaction' - that is, an H
+
donor. For example, hydrochlo-
ric acid (HCl) when dissolved in water undergoes
ionization:
(
)
++
(
)
→+
(
)
+
+
−
+
−
+
−
KOHHHSOK HSOHO
base
+
4
4
2
+
−
HCl HCl
→+
acid
showing that KOH is behaving as a base whose net
effect is to convert H
+
into H
2
O.
Other reactions between acids and bases show the
same pattern:
In fact this is a simplification. Each ion is actually sur-
rounded in solution by a shell of water molecules
attracted electrostatically by the charge of the ion
(
hydration
- Box 4.1). One can represent these hydrated
ions as H
+
aq
and Cl
−
aq
.
An acid like HCl, which contributes only one hydro-
gen ion (H
+
= proton) per acid molecule to the solution,
is said to be
monoprotic
. Phosphoric acid (H
3
PO
4
), on
NaOH HCl NaCl HO
base
+ → +
2
acid
salt
water
Sometimes such acids are alternatively described as
monobasic
and
polybasic
respectively.
4
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