Chemical Equilibrium
Reversibility of reactions
Some chemical reactions are clearly nonreversible -ex : burn a piece of paper.
However, under certain circumstances, some reactions are reversible. ex: NO2 tubes.
By alternately heating and cooling the tubes, we are in fact altering the relative
concentrations of NO2 and N2O4. (NO2 is the dark orange colour. N2O4 is colourless.)
The reaction that is occurring in the tube is the following:
In ice water : 2 NO2 ----> N2O4
In hot water : N2O4 ----> 2 NO2
In fact, it can be shown that at room temperature BOTH reactions are occurring at the
same time. This can be shown as the following equation :
N2O4 <====> 2 NO2
The double ended arrows indicate that the reaction is reversible that is it can proceed in
either direction or in both directions at the same time.
There comes a point in time when the reaction proceeding one way or the other
appears to come to an end. We can tell this by the lack of any colour change. When the
system inside the closed container ceases to change, it is said to be at EQUILIBRIUM.
At this point, all observable properties are remaining constant. In this case, the colour
remains constant. It appears that the reaction has stopped.
At the molecular level, however, it is found that this is not the case. By introducing some
radioactive I2 into an equilibrium system as follows :
H2 (g) + I2 (s) <====> 2 HI (g)
it is found that after a period of time, even though the relative amounts of I2 H2 and HI
haven't changed, the relative amounts of RADIOACTIVE HI and I2 have changed. This
clearly shows that at the molecular level, the reaction is still continuing even though the
observable properties remained constant. Because the reaction is continuing at all
times, equilibrium is said to be DYNAMIC, that is the reaction is always proceeding.
Then what exactly is equal about equilibrium anyway ?
Because the observable amounts of reactants and products remain constant, the
forward reaction is not going any faster than the reverse reaction. (If it did, we would
have an increase in the relative amounts of products and a decrease in the relative
amounts of the reactants.) At the same time, the reverse reaction isn't any faster either.
Therefore, the RATE of the forward reaction is the same as the RATE of the reverse
reaction. The rates of the forward and reverse reaction are what are equal in
equilibrium.
Recognizing Equilibrium
We have said that when all observable properties remain constant, a system is at
equilibrium. Well, that is not exactly true. A few other conditions must also be met.
1. The system must be closed. Such as the container was
sealed. If it is not closed, other substances might
interfere with the reaction or react themselves.
2. The temperature was kept constant. We have seen that
the temperature affects the rate of a chemical reaction.
Because we essentially have TWO reactions taking place
here, we can NOT assume the temperature change will
affect both reactions the same way. In fact, just the
opposite is true. The change in temperature will affect the
rates of the forward and reverse reactions differently. We
saw this in the NO2 tubes. By lowering the temperature,
we would expect both of the reactions to become slower.
They did, but one of them slowed down even more than
the other.
3. We must have both reactants and products present.
Without this, one of the reactions would not proceed.
Why? Because we would not have anything to react ! We
can start out with only reactants or products but the
system will proceed to equilibrium, it will not be at
equilibrium to start with.
Concentration Diagrams :
If we take a look at the system we started with today,
N2O4 <====> 2 NO2
at room temperature the each had a certain concentration.
change
heat
cool
add NO2
[NO2]
increase
decrease
increase
[N2O4]
decrease
increase
increase by a ½ as much
How do we know that these happened? When we heated the tubes, they got
darker. That indicates that more NO2 was made so we must have used up some of the
N2O4. Conversely, when we cooled the tubes, they got clearer. More N2O4 and less
NO2.
1. The concentrations of N2O4 and NO2 are constant at equilibrium as shown
by the horizontal lines. Both concentration lines MUST be horizontal at the
same time or else the graph doesn't make sense.
2. The concentrations of N2O4 and NO2 are not necessarily the same at any
time. They can be. In fact the curves could cross at some point in time.
3. The increase in NO2 when heat is added is twice the decrease in [N2O4].
Why? Because of the coefficients in the equation. Whenever one N2O4
molecule is used up, two NO2 molecules are made.
4. The greatest rate of change occurs immediately after the temperature
change. As the system proceeds towards a new equilibrium, the rate of
change is slower because it is closer to the new equilibrium.
By the way, we have all seen the experiment in the tubes before - NO2 is one of
the products in smog. Notice in the summer on hot, windless days an orange haze is
seen over the horizon. This is NO2. In the winter, the smog doesn't go away, it is just
less noticeable. The cooler temperatures lead to more N2O4 and less NO2 which we
can't see as well !!