Question
Comment on the statement: Equilibrium can be approached from either direction.
Solution
This can be proved experimentally by considering the equilibrium between nitrogen tetraoxide N2O4 ( a colourless gas) and nitrogen dioxide NO2 (a reddish brown gas).
It is observed that:
(i) At 298K, the two gases exist as an equilibrium having a pale brown colour.
(ii) At 273K, N2O4 is stable and only exists as a colourless gas.
(iii) At 373K, N2O4 decomposes completely into NO2 which is reddish brown in colour.
Take two identical bulbs A and B with equal amounts of NO2 gas and seal them. Place bulb A in an ice bath and bulb B in boiling water. After some time, the gas in bulb A (placed in an ice bath) becomes colourless showing complete conversion of NO2 into N2O4. The gas in bulb B (placed in boiling water) has reddish brown colour showing completely NO2 gas.
After some time when both the bulbs attain the temperature of the water bath (i.e. 298K), the colour in the two bulbs become identical and no further change in colour takes place.
It is observed that:
(i) At 298K, the two gases exist as an equilibrium having a pale brown colour.
(ii) At 273K, N2O4 is stable and only exists as a colourless gas.
(iii) At 373K, N2O4 decomposes completely into NO2 which is reddish brown in colour.
Take two identical bulbs A and B with equal amounts of NO2 gas and seal them. Place bulb A in an ice bath and bulb B in boiling water. After some time, the gas in bulb A (placed in an ice bath) becomes colourless showing complete conversion of NO2 into N2O4. The gas in bulb B (placed in boiling water) has reddish brown colour showing completely NO2 gas.
Now take out the bulbs A and B and transfer them to a water bath at 298K (at room temperature). The colour of the gas in bulb A starts changing into brown colour indicating the gradual conversion of N2O4 into NO2.
On the other hand, the colour of the gas in bulb B beings to fade and becomes pale indicating the gradual change of NO2 to N2O4.
On the other hand, the colour of the gas in bulb B beings to fade and becomes pale indicating the gradual change of NO2 to N2O4.
After some time when both the bulbs attain the temperature of the water bath (i.e. 298K), the colour in the two bulbs become identical and no further change in colour takes place.
This consistency of colour shows that the equilibrium has been attained in both the cases and both the bulbs contain a mixture of NO2 and N2O4 having the same composition. Thus, equilibrium can be approached from either direction.
