Modeling equilibrium

Syllabus Dot Point

model static and dynamic equilibrium and analyse the differences between open and closed systems (ACSCH079, ACSCH091)

1. Systems and Equilibrium

To understand equilibrium, we must first classify the system in which a reaction occurs.

System Type	Exchange with Surroundings	Equilibrium Status	Example
Open System	Exchanges Matter and Energy	Cannot be achieved (reactants/products escape)	Boiling water in a saucepan without a lid (steam escapes)
Closed System	Exchanges Energy only (Matter is contained)	Can be achieved	Boiling water in a pressure cooker (steam is trapped)
Isolated System	Exchanges neither Matter nor Energy	N/A (Theoretical/Ideal)	N/A

Static vs. Dynamic Equilibrium:

Static Equilibrium: Occurs when all reaction rates are zero and the system is at rest. There is no movement of reactant or product particles and no further chemical change occurs.
Dynamic Equilibrium: Occurs in a closed system at constant temperature when the rate of the forward reaction equals the rate of the reverse reaction. Although macroscopic properties (concentration, colour, pressure) remain constant, microscopic changes continue indefinitely (bonds break and reform constantly).

2. Modelling Dynamic Equilibrium (The Water Transfer Experiment)

This practical models a reversible reaction ($A \rightleftharpoons B$) using water and graduated cylinders to demonstrate how equilibrium is established from different initial concentrations.

Key Modelling Components:

Model Component	Chemical Analog
Water Volume	Concentration of reactants and products ($[A]$ and $[B]$)
Graduated Cylinders	Reactants (A) and Products (B)
Straws	Reaction Mechanism allowing the conversion
Straw Diameter	Rate Constant ($k$) (Wider straw = larger $k$)
Volume Transferred	Reaction Rate ($r$)

Procedure

The experiment consists of two parts to show that equilibrium can be reached regardless of the starting point.

Part 1: Forward and Reverse Reactions (Initial: $A=25mL, B=0mL$)

Setup: Fill Cylinder A with 25 mL of water. Leave Cylinder B empty.
Transfer: Place a straw into the bottom of each cylinder.
Trap: Place a finger over the top of each straw to trap the water inside.
Exchange: Simultaneously lift the straws and transfer the trapped water to the opposite cylinder (A’s water goes to B; B’s water goes to A).