Page 295 - Physical chemistry understanding our chemical world

262 ACIDS AND BASES

structurally similar sister, ethanoic acid (I), methanoic acid disso-
The naturally occurring ciates in water to yield a solvated proton, H 3 O (aq) .
+
substance histamine Rubbing the site of the sting with a crushed dock leaf is a sim-
causes blood capillaries
ple yet rapid way of decreasing the extent of the pain. In common
to dilate and smooth
muscle to contract. with many other weeds, the sap of a dock leaf contains a mix-
Most cells release it ture of natural amines (e.g. urea (III) above), as well as natural
in response to wound- antihistamines to help decrease any inflammation. The amines are
ing, allergies, and most solvated and, because the sap is water based, are alkaline. Being
inflammatory condi- alkaline, these amines react with methanoic acid to yield a neutral
tions. Antihistamines salt, according to
block the production of
this substance, thereby O O
combating a painful H
swelling. H H +
• − (6.39)
O • N R O H 3 N R
H H

where R is the remainder of the amine molecule. We see how the
process of pain removal involves a neutralization process.

Notice how the lone How do indigestion tablets work?
pair on nitrogen of the
amine attracts a proton Calculations concerning neutralization
from the carboxylic
acid.
Excess acid in the stomach is one of the major causes of indiges-
tion, arising from a difficulty in digesting food. The usual cause
of such indigestion is the stomach simply containing too much hydrochloric acid, or
the stomach acid having too high a concentration (its pH should be about 3). These
failures cause acid to remain even when all the food has been digested fully. The
excess acid is not passive, but tends to digest the lining of the stomach to cause an
ulcer, or reacts by alternative reaction routes, generally resulting in ‘wind’, the gases
of which principally comprises methane.
Most indigestion tablets are made of aluminium or magnesium
Some indigestion hydroxides. The hydroxide in the tablet removes the excess stom-
tablets contain chalk ach acid via a simple acid–base neutralization reaction:
(CaCO 3 ) but the large
volume of CO 2 pro- 3HCl (stomach) + Al(OH) 3(tablet) −−→ 3H 2 O + AlCl 3(aq) (6.40)
duced (cf. Equation
(6.19)) can itself cause The cause of the indigestion is removed because the acid is con-
dyspepsia.
sumed. Solid (unreacted) aluminium hydroxide is relatively insolu-
ble in the gut, and does not dissolve to generate an alkaline solution.
Rather, the outer layer of the tablet dissolves slowly, with just sufficient entering solu-
tion to neutralize the acid. Tablet dissolution stops when the neutralization reaction
is complete.