Series editors' preface - Wood Tools

standard free energies of starting materials and
products enables us to predict the extent of
the conversion of the former into the latter.
Most of the materials of concern to conservators
are not in their most stable state and
what is therefore of greater interest is how fast
any reaction will occur. For example, G for
the oxidation of cellulose [(C 6H 12O 5)n + 6nO 2
→ 6nCO 2 + 5nH 2O] is large and negative so
that the equilibrium for the reaction lies essentially
completely over in favour of carbon
dioxide and water. While this would appear
to favour spontaneous self-destruction, we
know that a drawing or book can be viewed
in the air for a long time without noticeably
fading away into gaseous products. The rate
of a chemical reaction, which may be defined
as the speed at which the products are
formed, depends on the physical state,
concentration and temperature of the reactants
and on the presence of catalyst. A chemical
reaction cannot begin unless the reactants are
in contact at activation levels of energy and the
reaction cannot proceed if the reactants clog
or prevent fresh molecular collisions from
taking place (e.g. the oxide layers on
aluminium and chromium). Advantage can
sometimes be taken of this in the display
space to make conditions as unfavourable as
possible for reaction.
Molecular freedom of at least one reactant to
move about and mingle with the other(s) is a
prerequisite for chemical reaction. The greater
the effective surface area the faster a reaction
will tend to take place. Solutions are most often
chosen as the medium for carrying out chemical
reactions for just these reasons. Moisture on
the surface of objects and pitted or porous
surfaces both increase the likelihood of
reactions occurring. Chemical reactions take
place when the appropriate atoms, ions or
molecules collide with each other. The number
of collisions will be greater for light molecules
as these travel faster than heavy ones and
should collide more frequently. Larger
molecules should also collide more frequently
than smaller ones. If the number of molecules
in a fixed volume (i.e. the concentration) is
doubled this will result in four times the
number of collisions. An increase in temperature
will also increase the number of collisions
but this is not the main effect of temperature.
Collision between two molecules is a necessary
General review of environment and deterioration 245
but not sufficient condition for reaction. Each
collision requires a certain amount of energy
before reaction can occur and this activation
energy represents a barrier to reaction which
can be represented diagrammatically, as shown
in Figure 6.1. At a particular temperature, not
all particles have the same kinetic energy. Only
collisions involving particles of sufficient
(higher than average) kinetic energy will result
in reaction. The well-known increase in the rate
of reaction that occurs as temperature is raised
is due to the growing proportion of molecules,
as temperature rises, with an energy above the
minimum necessary for reaction (Figure 6.2).
As a very rough guide, a 10 °C rise in temperature
will lead to a doubling of rate of reaction,
though the rate of oxidation of cellulose is
doubled if the temperature rises from 20 °C to
25 °C (Thomson, 1986). It may sometimes be
possible to artificially raise the activation energy
barrier, for example by applying lacquers to
metals. The rise in temperature required to
cause a doubling of rate can be found from the
equation:
log 10 k 1/k 2 = –E a/2.303R [1/T 1 – 1/T 2]
where k 1 and k 2 are rate constants at temperatures
T 1 and T 2, E a is activation energy and R
is the Gas Constant.
Figure 6.1 If they are to react, colliding molecules
must have sufficient kinetic energy to cause chemical
bonds to break or form. The minimum energy required
to initiate a chemical reaction is called the activation
energy. Diagrammatic representation of the path of an
exothermic reaction showing the initial energy of the
reactants, the activation energy (E a) required to achieve
an activated state, the change in free energy (G)
caused by the reaction and the energy of the reaction
products (now lower than the initial energy because this
example is an exothermic reaction – a reaction that
gives out energy in the form of heat)