CHAPTER 3. REACTION RATES 3.6
forward reaction is highand the rate of the reverse reaction is low. Asthe reaction
proceeds, the rate of theforward reaction decreases and the rate of the reverse reaction
increases, until both occur at the same rate. Thisis called equilibrium.
Although it is not always possible to observe any macroscopic changes,this does not
mean that the reaction has stopped. The forwardand reverse reactions continue to take
place and so microscopic changes still occur in the system. This state is called dynamic
equilibrium. In the liquid-gas phaseequilibrium demonstration, dynamic equilibrium
was reached when there was no observable change in the level of thewater in the
second beaker even though evaporation and condensation continued totake place.
Rate of ReactionTimeequilibrium2HI→H 2 +I 2
H 2 +I 2 →2HI
Figure 3.6: The changein rate of forward and reverse reactions in a closed systemThere are, however, a number of factors that can change the chemicalequilibrium of
a reaction. Changing the concentration, the temperature or the pressure of a reaction
can affect equilibrium. These factors will be discussed in more detaillater in this
chapter.
DEFINITION: Chemical equilibrium
Chemical equilibrium is the state of a chemical reaction, where the
concentrations of the reactants and products have no net change over
time. Usually, this stateresults when the forward chemical reactions
proceed at the same rateas their reverse reactions.