of reactant, they are said to be in equilibrium. In writing an equation, we indicate
equilibrium by drawing arrows pointing in opposite directions
( ------> )
( <------- )
As an example of an equilibrium reaction, consider the dissociation of a compound into
2Na+ + CO3 -2
Sodium carbonate in solution dissociates into sodium ions and carbonate ions. Some of
the ions come back together to form sodium carbonate. Thus, an equilibrium is
EXTERNAL CONDITIONS AFFECTING CHEMICAL REACTIONS
External conditions that affect reactions are usually types of energy that are put
into a reaction, such as heat or light. Chemical reactions are always accompanied by
an energy change. Either energy is released or it is acquired. When the amount of
energy is changed, so is the amount of matter. This is called the Law of Conservation of
Matter and Energy. However, ordinary chemical reactions involve such small matter
changes that they go undetected and may be ignored.
a. Heat. Generally, heat is the form of energy we are most concerns us most. It
may affect a reaction in one of two ways.
(1) Exothermic reactions. If a reaction gives off heat, it is called an
exothermic reaction. External heat, if supplied to this type of reaction, will slow down
the rate of reaction.
(2) Endothermic reactions. If a reaction takes in heat, it is an endothermic
reaction. lf heat is added to an endothermic reaction, the rate of reaction will increase.
This may be of value in the preparation of medicinal products.
b. Light. Light is a form of energy that may cause many chemicals to
decompose. For this reason, it is necessary to protect some drugs from contact with
light by placing them in dark-colored or opaque containers. These containers prevent
most or all of the outside light from coming into contact with the drug.
It has already been emphasized that all reactions occur on an atom-to-atom
level. This presents a small problem to us, since we cannot hold an atom in our hand,
or count out a specific number of atoms to put into a reaction. How then do we
measure amounts of material that will react together? Chemists long ago solved this