Hazardous Chemicals Handbook (pdf) - CCC

26 GENERAL PRINCIPLES OF CHEMISTRY
electronic level. Thus, the conversion of atomic copper to the oxide produces Cu ++ ions as copper
atoms have lost electrons. The reduction of lead oxide by hydrogen entails removal of electrons
from the oxide to produce neutral metallic lead. Oxidizing agents are therefore chemicals which
attract electrons, and reducing agents chemicals that add electrons to an element or compound.
Using these definitions the presence of either oxygen or hydrogen is not required, e.g. since
chlorine is a stronger oxidizing agent than iodine it displaces iodine from iodides:
Oxidation
Cl2(aq) + 2I – (aq) = 2Cl – (aq) + I2
Reduction
Physical state
Chemicals exist as gases, liquids or solids. Solids have definite shapes and volume and are held
together by strong intermolecular and interatomic forces. For many substances, these forces are
strong enough to maintain the atoms in definite ordered arrays, called crystals. Solids with little
or no crystal structure are termed amorphous.
Gases have weaker attractive forces between individual molecules and therefore diffuse rapidly
and assume the shape of their container. Molecules can be separated by vast distances unless the
gas is subjected to high pressure. Their volumes are easily affected by temperature and pressure.
The behaviour of any gas is dependent on only a few general laws based upon the properties of
volume, pressure and temperature as discussed in Chapter 4.
The molecules of liquids are separated by relatively small distances so the attractive forces
between molecules tend to hold firm within a definite volume at fixed temperature. Molecular
forces also result in the phenomenon of interfacial tension. The repulsive forces between molecules
exert a sufficiently powerful influence that volume changes caused by pressure changes can be
neglected i.e. liquids are incompressible.
A useful property of liquids is their ability to dissolve gases, other liquids and solids. The
solutions produced may be end-products, e.g. carbonated drinks, paints, disinfectants or the
process itself may serve a useful function, e.g. pickling of metals, removal of pollutant gas from
air by absorption (Chapter 17), leaching of a constituent from bulk solid. Clearly a solution’s
properties can differ significantly from the individual constituents. Solvents are covalent compounds
in which molecules are much closer together than in a gas and the intermolecular forces are
therefore relatively strong. When the molecules of a covalent solute are physically and chemically
similar to those of a liquid solvent the intermolecular forces of each are the same and the solute
and solvent will usually mix readily with each other. The quantity of solute in solvent is often
expressed as a concentration, e.g. in grams/litre.
Important common physical properties related to these states of matter are summarized in
Table 3.3.
Acids
Acids and bases (see later) are interrelated. Traditionally, acids are compounds which contain
hydrogen and which dissociate in water to form hydrogen ions or protons, H + , commonly written as: