SIR Q MODELS REAL REACTIONS
Here’s SIR Q - the next toolbox. It provides one with ten gas reactions, with five temperatures each. The equilibrium constants are legitimate, within the ideal approximation. (The kinetic behaviour is pure fantasy, designed to fail to reach equilibrium with no catalyst and reach equilibrium with maximum catalyst.)
One may choose any reaction, from very simple to quite complicated, and choose any one of its five temperatures. One may choose any starting partial pressures, in 0.1 atm steps from 0 to 1.5 atm. One may choose the amount of catalyst (including none). One may or may not show the current value of the reaction quotient, Q, as the reaction runs, and one may or may not choose to superimpose the previous reaction run.
Here’s how one might coach the
class to discover the law of chemical equilibrium.
Start with the simplest reaction, the
isomerization of pentene, at a reasonable temperature (here, 750° ).
It doesn’t really matter what starting pressure of 1-pentene one uses. The previous
reaction, shown in faint colour, was uncatalyzed. It was still going at the end of the
run.
Would it go to completion if one waited long enough? The current run has lots of catalyst to speed it up, and appears to reach a steady stste.
(You might explain that catalysts affect the rates of reactions – usually by speeding them up, and you might take bets on whether the catalyst will cause the reaction to run to completion.)
The reaction seems to stop at 0.400 atm of 1-pentene and 0.200 atm of cyclopentane. There’s also a strong indication that the catalyst didn’t affect the place to which the reaction was going.
What do you think would happen if we started with cyclopentane?
Evidently the reaction is heading for
some final state, which is not 100% product (cyclopentane).
Some things to try: other stoichiometrically equivalent starting states, such as 0.3 atm 1-pentene and 0.3 atm cyclopentane, with and without catalyst.
Conclusion: This reaction is reversible; it has a characteristic final (equilibrium) state; catalysts change the rate of approach to the equilibrium state, but don’t alter its value.
Click here to see how to discover the law of mass action using SIR Q