Dynamic Equilibria

Skills to Develop

• Describe chemical equilibria at the molecular level as a dynamic process.

Simulation of a Chemical Equilibrium

The program illustrates what might be going on in a system that is at equilibrium. At the microscopic level (i.e. looking at the molecular species), the state of equilibrium does not mean that there is no change. However, the equilibrium constant K remains essentially constant. The following point will be illustrated.

• Equilibrium is a dynamic process at the molecular level. There is no net change, but two opposing processes are taking place.
• The equilibrium constant fluctuates slightly due to unequal reaction rates in opposite directions.
• A system moves spontaneously toward a state of equilibrium.
• The driving forces for equilibrium are:
(a) molecules assume the state of lowest energy,
(b) molecules tend to reach a maximum disorder or entropy.

In this system, there are four (abstract) chemical species, each represented by a symbol. They may move around in the enclosed system, changing into one another. At any moment, their numbers may be counted, and the 'equilibrium constant' calculated.

It takes a lot more work to write a simulation involving moving molecules, and here is one that only indicates the changes of molecules. You may observe the equilibrium constant at any time by pressing the key

(K)

during the simulation.

Record 10 such equilibrium constants, and calculate their average value. You should use the appropriate number of significant figures.

Compare the value you have obtained with that obtained by the person sitting next to you and see if the average values are the same. How many significant figures do you think there should be for this system?

After you have read this file, you may press the (Del) key to see one of the pictorial illustrations of chemical equilibrium at the microscopic level.

Questions

1. In a real chemical system there are many more molecules involved. How many molecules are there in 1 mL of air at STP? (Avogadro's number = 6.023e23 molecules /mol; at STP, molar volume of air = 22.4 L)
2. There are two requirements for a system to reach the equilibrium state. One is that the process be reversible. What is the other?
3. In a closed system, all chemical and physical changes tend toward an equilibrium state; is this statement true or false?

Solutions

1. Hint...
Divide Avogadro's number by 24000. Do you understand why this number is used?

Consider...
$$\mathrm{No \times \dfrac{1\: ml}{24000\: ml}}$$; the unit is molecules. This value is known as Luschmidst's number

2. Hint...
Can an equilibrium state be reached in an open system?