Chemistry Reference
In-Depth Information
We know that (pure) liquid water at will freeze if heat is removed from it
and that ice at will melt if heat is added to it. However, if no heat is added
or removed from a mixture of ice and liquid water at both the melting and
freezing processes still continue. In this case, the rate of melting and the rate
of freezing are the same, so the mass of the ice and the mass of the liquid water
do not change. (The shape of the ice might change, however, as some melting
and freezing take place.) When exactly opposite physical processes or chemi-
cal reactions occur together at the
same rate,
a condition of
equilibrium
is said
to be established. When two opposite processes occur at equal rates, nothing
may
appear
to be happening. However, both processes continue at the rates
dictated by the conditions. If the conditions change, a change in the system may
be observed.
Because chemical equilibrium involves rates of reactions, this chapter first
investigates the factors that affect the rate of a reaction (Section 18.1). The
molecular basis of chemical equilibrium and some of its terminology are then
presented in Section 18.2. LeChâtelier's principle, discussed in Section 18.3,
explains qualitatively how to predict what happens to a system at equilibrium
when a change is imposed on the system. Section 18.4 presents the equilib-
rium constant, which allows us to obtain quantitative results for systems at
equilibrium.
0°C
0°C
0°C,
18.1
Rates of Reaction
The
rate
of a chemical reaction is defined as the change in the concentration
of a reactant or product per unit time. For example, the rate may be described
as the disappearance of 0.300 mol/L of a certain reactant per hour or the appear-
ance of 0.00100 mol/L of a certain product per second.
Any of six factors can affect the rate: (1) the nature of the reactants, (2) the
temperature, (3) the presence of a catalyst, (4) the concentration of reactants in
solution, (5) the pressure of gaseous reactants, and (6) the state of subdivi-
sion of solid reactants. For a reaction to occur, the atoms, molecules, or ions
must come into contact with one another with enough energy to rearrange
chemical bonds in some way. Increased concentration, gas pressure, or sur-
face area of a solid tends to get the particles to collide more frequently, and
increased temperature tends to get them to collide more frequently and with
greater energy to accomplish more effective collisions. Catalysts work in very
many different ways.
Some reactions, such as an explosion of nitroglycerine, inherently tend to
proceed rapidly. Others, such as the reaction of solid limestone with carbon
dioxide in water to form soluble calcium hydrogen carbonate, tend to proceed
slowly. It may take centuries for underground streams to form caverns by reac-
tion with limestone (see Figure 8.9). For a given reaction, chemists have no
control over the nature of the reactants.
Raising the temperature generally increases the rate of a chemical reaction.
A general approximation is that a rise in temperature doubles the rate of
a reaction. The actual increase for any given reaction will not follow this rule
exactly, of course, but a great many reactions follow it approximately.
A
catalyst
increases the rate of a chemical reaction (Section 8.3). The thou-
sands of different catalysts work in many different ways. For example,
10°C
SO
2
and
