Dictionary of Chemistry [6th Ed.]

the dissociation constant Kais given

by

Ka= α^2 n/(1 – α)V

where αis the degree of dissociation,

n the initial amount of substance (be-

fore dissociation), and V the volume.

If αis small compared with 1, then

α^2 = KV/n; i.e. the degree of dissocia-

tion is proportional to the square

root of the dilution. The law wasÜrst

put forward by Wilhelm Ostwald to

account for electrical conductivities

of electrolyte solutions.

outer-sphere mechanism See

electron-transfer reaction.

overpotential(overvoltage)A po-

tential that must be applied in an

electrolytic cell in addition to the

theoretical potential required to lib-

erate a given substance at an elec-

trode. The value depends on the

electrode material and on the cur-

rent density. It is a kinetic effect oc-

curring because of the signiÜcant

activation energy for electron trans-

fer at the electrodes, and is particu-

larly important for the liberation of

such gases as hydrogen and oxygen.

For example, in the electrolysis of a

solution of zinc ions, hydrogen (EŠ=

0.00 V) would be expected to be liber-

ated at the cathode in preference to

zinc (EŠ= –0.76 V). In fact, the high

overpotential of hydrogen on zinc

(about 1 V under suitable conditions)

means that zinc can be deposited in-

stead. The relation between the cur-

rent and the overpotential is given by

the *Butler–Volmer equation.

overtonesSee harmonic.

oxalateA salt or ester of *oxalic

acid.

oxalic acid (ethanedioic acid)A

crystalline solid, (COOH) 2 , that is

slightly soluble in water. Oxalic acid

is strongly acidic and very poisonous.

It occurs in certain plants, e.g. sorrel

and the leaf blades of rhubarb.

oxaloacetic acidA compound,

HO 2 CCH 2 COCO 2 H, that plays an inte-

gral role in the *Krebs cycle. The

anion, oxaloacetate, reacts with the

acetyl group from acetyl coenzyme A

to form citrate.

oxazoleA heterocyclic compound

having a nitrogen atom and an oxy-

gen atom in aÜve-membered ring,

C 3 H 3 NO.

outer-sphere mechanism 392

o

O

N

Oxazole

oxfuelA liquid fuel containing

added alcohols or ethers to act as an

additional source of oxygen during

combustion of the fuel. It has been

claimed that such additives help to

lower the concentration of carbon

monoxide in engine emissions.

oxidantSee oxidizing agent.

oxidationSee oxidation–

reduction.

oxidation number (oxidation

state)See oxidation–reduction.

oxidation–reduction (redox) Orig-

inally, oxidationwas simply regarded

as a chemical reaction with oxygen.

The reverse process – loss of oxygen

• was called reduction. Reaction with
  hydrogen also came to be regarded as
  reduction. Later, a more general idea
  of oxidation and reduction was devel-
  oped in which oxidation was loss of
  electrons and reduction was gain of
  electrons. This wider deÜnition cov-
  ered the original one. For example,
  in the reaction
  4Na(s) + O 2 (g) →2Na 2 O(s)
  the sodium atoms lose electrons to