Page 268 - Physical chemistry understanding our chemical world

PROPERTIES OF LOWRY–BRØNSTED ACIDS AND BASES 235

a substance capable of donating a proton. Therefore, we describe ethanoic acid as a
‘Lowry–Brønsted acid’.

Why is it dangerous to allow water near an electrical
appliance, if water is an insulator?

The solvated proton

We all need to know that electricity and water are an extremely dangerous combi-
nation, and explains why we are taught never to sit in the bath at the same time as
shaving with a plug-in razor or drying our hair. Electrocution is almost inevitable,
and is often fatal.
The quantity we call ‘electricity’ is a manifestation of charge Q
passing through a suitable conductor. The electrical conductivity κ Care: the symbol of
of water (be it dishwater, rainwater, bathwater, etc.) must be rela- conductivity is not K
tively high because electricity can readily conduct through water. but the Greek letter
Nevertheless, the value of κ for water is so low that we class water kappa, κ.
as an insulator. Surely there is a contradiction here?
‘Super-pure water’ has been distilled several times, and is indeed an insulator:
its conductivity κ is low at 6.2 × 10 −8 Scm −1 at 298 K, and lies midway between
−1
classic insulators such as Teflon, with a conductivity of about 10 −15 Scm , and
−1
semiconductors such as doped silicon, for which κ = 10 −2 Scm . The conductivity
−1
6
of metallic copper is as high as 10 Scm .
The value of κ cited above was for super-pure water, i.e. the
product of multiple distillations, but ‘normal’ water from a tap Pure water is a mixture
of three components:
will inevitably contain solutes (hence the ‘furring’ inside a kettle
H 2 O, and its two dis-
or pipe). Inorganic solutes are generally ionic salts; most organic
sociation products, the
solutes are not ionic. The conductivity of super-pure water is low +
solvated proton (H 3 O )
because the molecules of water are almost exclusively covalent,
and the hydroxide ion
with the extent of ionicity being very slight. But as soon as a salt (OH ).
−
dissolves, the extent of ionicity in the water increases dramatically,
causing more extensive water dissociation.
Ignoring for the moment the solute in solution, the water dis- It is safer in many
sociation involves the splitting of water itself in a process called instances to assume
autoprotolysis. The reaction is usually represented as the solvated proton
has the formula unit
+
+ − [H(H 2 O) 4 ] ,withfour
2H 2 O −−→ H 3 O (aq) + OH (aq) (6.2)
water molecules ar-
+
where the H 3 O (aq) species is often called a solvated proton.It also ranged tetrahedrally
has the names hydroxonium ion and hydronium ion. The complex around a central pro-
+
ion H 3 O (aq) is a more accurate representation of the proton respon- ton, the proton being
stabilized by a lone
sible for acidic behaviour than the simplistic ‘H (aq) ’ we wrote in pair from each oxygen
+
Equation (6.1). Note how the left-hand side of Equation (6.2) is
atom.
covalent and the right-hand side is ionic.