NATURE OF THE REACTANTS
The physical states of reacting substances are important in determining their reactivities.
A puddle of liquid gasoline can burn smoothly, but gasoline vapors can burn explosively.
Two immiscible liquids may react slowly at their interface, but if they are intimately mixed
to provide better contact, the reaction speeds up. White phosphorus and red phosphorus
are different solid forms (allotropes) of elemental phosphorus. White phosphorus ignites
when exposed to oxygen in the air. By contrast, red phosphorus can be kept in open
containers for long periods of time without noticeable reaction.
Samples of dry solid potassium sulfate, K 2 SO 4 , and dry solid barium nitrate, Ba(NO 3 ) 2 ,
can be mixed with no appreciable reaction occurring for several years. But if aqueous solu-
tions of the two are mixed, a reaction occurs rapidly, forming a white precipitate of barium
sulfate.
Ba^2 (aq)SO 42 (aq)88nBaSO 4 (s) (net ionic equation)Chemical identities of elements and compounds affect reaction rates. Metallic sodium,
with its low ionization energy, reacts rapidly with water at room temperature; metallic
calcium has a higher ionization energy and reacts only slowly with water at room temper-
ature. Solutions of a strong acid and a strong base react rapidly when they are mixed
because the interactions involve mainly electrostatic attractions between ions in solution.
Reactions that involve the breaking of covalent bonds are usually slower.
The extent of subdivision of solids or liquids can be crucial in determining reaction
rates. Large chunks of most metals do not burn. But many powdered metals, with larger
surface areas and hence more atoms exposed to the oxygen of the air, burn easily. One
pound of fine iron wire rusts much more rapidly than a solid one-pound chunk of iron.
Violent explosions sometimes occur in grain elevators, coal mines, and chemical plants in
which large amounts of powdered substances are produced. These explosions are exam-
ples of the effect of large surface areas on rates of reaction. The rate of reaction depends
on the surface area or degree of subdivision. The ultimate degree of subdivision would
make all reactant molecules (or ions or atoms) accessible to react at any given time. This
situation can be achieved when the reactants are in the gaseous state or in solution.
16-2
Two allotropes of phosphorus.
White phosphorus (above) ignites
and burns rapidly when exposed to
oxygen in the air, so it is stored
under water. Red phosphorus (below)
reacts with air much more slowly,
and can be stored in contact with air.16-2 Nature of the Reactants 655See the Saunders Interactive
General Chemistry CD-ROM,
Screen 15.3, Control of Reaction Rates
(1): Surface Area.Powdered chalk (mostly calcium
carbonate, CaCO 3 ) reacts rapidly with
dilute hydrochloric acid because it has
a large total surface area. A stick of
chalk has a much smaller surface area,
so it reacts much more slowly.