Page 271 - Physical chemistry understanding our chemical world

238 ACIDS AND BASES

To understand the acidity of pollutants such as NO and CO 2 ,we
As long ago as the need to appreciate how the gas does not so much dissolve in water
18th century, French as react with it, according to
chemists appreciated
how burning elemen-
+
−
tal carbon, nitrogen CO 2(g) + 2H 2 O (l) −−→ HCO 3(aq) + H 3 O (aq) (6.5)
or sulphur generated
compounds which,
when dissolved in Carbonic acid, H 2 CO 3(aq) , never exists as a pure compound; it only
water, yielded an acidic exists as a species in aqueous solution, where it dissociates in
solution. just the same way as ethanoic acid in Equation (6.1) to form a
solvated proton and the HCO − ion. Note how we form a sol-
3(aq)
vated proton H 3 O (aq) by splitting a molecule of water, rather
+
than merely donating a proton. Carbonic acid is, nevertheless, a
Lowry–Brønsted acid.
The carbonic acid produced in Equation (6.5) is a proton donor,
The nitric acid in acid so the solution contains more solvated protons than hydroxide
rain forms by a more ions, resulting in rain that is (overall) an acid. To make the risk
complicated mecha-
of pollution worse, ‘acid rain’ in fact contains a mixture of sev-
nism: 4NO (g) + 2H 2 O (l)
eral water-borne acids, principally nitric acid, HNO 3 (from nitrous
+O 2(g) −−−→ 4HNO 3(aq)
oxide in water), and sulphurous acid, H 2 SO 3 (an aqueous solution
of sulphur dioxide).
+
In summary, we see how the concentrations of H 3 O and OH −
are the same if water contains no dissolved solutes, but dissolving
‘Hydrolysis’ means to a solute such as NO increases the concentration of H 3 O ;ina
+
split water, the word similar way, the concentration of OH will increase if the water
−
coming from the two contains any species capable of consuming protons.
Greek roots hydro
meaning water, and It is time to introduce a few new words. We say carbonic acid
lysis meaning ‘to cleave forms by hydrolysis, i.e. by splitting a molecule of water. We
or split’. describe the extent of hydrolysis in Equation (6.5) by the following
equilibrium constant:

+
−
[HCO 3 ][H 3 O ]
K = (6.6)
[CO 2 ][H 2 O] 2
We sometimes call Equation (6.6) the hydrolysis constant of carbon
dioxide. In fact, the water term in the ‘denominator’ (the bottom
line) is so large compared with all the other terms that it remains
essentially constant. Therefore, we write Equation (6.6) in a differ-
ent form:
Care:the valuesof K [HCO 3 ][H 3 O ]
+
−

from these equations K = (6.7)
are only meaningful [CO 2 ]
for concentrations at

equilibrium. Note how the two K terms, K in Equation (6.6) and K in Equation
(6.7), will have different values.