Chapter 1: Chemical Equilibrium Part 4 SABIS Grade 11 (Level M) Chemistry

1.1.5 Equilibrium In Chemical Reactions

Let us examine a chemical reaction to see if these same conditions apply. Suppose we fill two identical bulbs to equal pressures of nitrogen dioxide. Now immerse the first bulb (bulb A) in an ice bath and the second bulb (bulb B) in boiling water, as in Figure 1.6 The gas in bulb A at 0°C is almost colorless; the gas in bulb B at 100°C is reddish-brown. The predominant molecular species in the cold bulb must be different from that in the hot bulb. A variety of experiments shows that the cold bulb contains mostly N2O4 molecules. These same experiments show that the hot bulb contains mostly NO2 molecules. The N2O4 molecules absorb no visible light, so the cold gas is almost colorless. The NO2 molecules do absorb some visible light, so the hot gas is reddish-brown.

Now let us transfer these two bulbs to a water bath at room temperature, as shown in Figure 1.6b. Immediately the color begins to intensify in bulb A. The color shows that a chemical change has occurred, forming NO2 molecules from N2O4:

In bulb A                                              N2O4(g) → 2NO2(g)             (4)

At the same time, the color in bulb B begins to pale, showing that a chemical change has occurred in this bulb as well, forming N2O4 molecules from NO2:

In bulb B                                               2NO2(g) → N2O4(g)            (5)

In each bulb the colors continue to change, bulb A becoming darker and bulb B becoming more pale. Finally, as the two bulbs approach the same temperature, the colors stop changing. A close examination shows that the two colors are now the same!

By direct visual observation we can watch the contents of these two bulbs approach the constancy of macroscopic properties (in this case, color) that indicates equilibrium. In bulb A equilibrium was approached by the dissociation of N2O4, reaction (4); in bulb B, it was approached by the opposite reaction, reaction (5). Here it is clear why the color of each bulb stopped changing at the particular hue characteristic of the equilibrium state at 25°C. The reaction between NO2 and N2O4 can proceed in both directions:

N2O4(g) → 2NO2(g)

2NO2(g) → N2O4(g)

Since N2O4 molecules can dissociate in bulb A, they must also be able to dissociate in bulb B. Surely an N2O4 molecule acts in the same way in bulb A (at 25°C) and in bulb B (at 25°C). The same sort of statements must apply to the combination of two NO2 molecules. If the reaction occurs in bulb B, then it must also occur in bulb A. The net change we see (by observing the changing NO2 color) represents, then, the difference in the rate of production of NO2 and the rate of consumption of NO2. Changes will cease when these two rates are exactly equal. At this time we replace the single arrow in each reaction by a double arrow (⇄), (⇌), or an equals sign (=) to show that equilibrium prevails:

N2O4(g) ⇄ 2NO2(g)    ,        N2O4(g) ⇌ 2NO2(g)

or

N2O4(g) = 2NO2(g)          (6)

For chemical reactions, just as for phase changes, at equilibrium, microscopic processes continue but in a balance which gives no macroscopic changes.

1.1.6 The State Of Equilibrium

It is most important to note that in our description of the equilibrium state we have not implied that at equilibrium the number of moles of N2O4 remaining is the same as the number of moles of NO2 produced. Equation (6) gives us no information concerning the fraction of the nitrogen dioxide present as NO2 at equilibrium. This is easily verified by heating the water surrounding bulbs A and B about 10°. The colors of the gases in both bulbs change to a new equilibrium color (one corresponding to the presence of more NO2). Yet the same expression is applicable:

What does the above equation tell us, then? First, it tells us that equilibrium prevails (the sign tells us that). Next, it tells us that there are two types of molecules present, N2O4 molecules and NO2 molecules. Finally, it tells us that during the approach to equilibrium, two molecules of NO2 are produced (or consumed) for every one molecule of N2O4 dissociated (or formed). It does not tell us whether at equilibrium there will be much or little NO2 compared with the amount of N2O4.

Part 5

1.2 Demonstration of the FeSCN²⁺ System