Chemical equilibrium is a fundamental concept in chemistry that describes the state of a chemical reaction where there is no net change in the concentrations of reactants and products. Understanding equilibrium is crucial for predicting chemical behavior and optimizing industrial processes.
Types of Chemical Systems
Open Systems
An open system allows both matter and energy to be exchanged with its surroundings. In these systems:
Particles, molecules, and compounds can move freely in and out
Energy can be transferred between the system and environment
Chemical reactions often go to completion
Example: A boiling kettle is an open system where both steam (matter) and heat (energy) can escape to the surroundings.
Closed Systems
A closed system permits only energy exchange with the surroundings, while matter remains contained within the system. These systems:
Prevent exchange of matter
Allow energy transfer
Enable reversible reactions to reach equilibrium
Example: A sealed coffee thermos allows heat transfer but prevents coffee from escaping.
Understanding Equilibrium States
Static Equilibrium
Static equilibrium occurs when a reaction has completely stopped, with:
Zero forward and reverse reaction rates
Complete conversion of reactants to products
No dynamic forces acting on the system
This can happen when:
All reagents have been consumed
The activation energy barrier is too high under given conditions
Dynamic Equilibrium
In dynamic equilibrium:
Forward and reverse reactions continue at equal rates
No net change in concentrations of reactants and products
Constant molecular-level exchange between reactants and products
The Gibbs free energy change (ΔG) equals zero
Example: Thermal Decomposition of Calcium Carbonate
In an open system:
CaCO3(s)→CO2(g)+CaO(s)
In a closed system:
CaCO3(s)⇌CO2(g)+CaO(s)
Modeling Dynamic Equilibrium
A simple model using water transfer between containers demonstrates key equilibrium concepts:
Total amount of matter remains constant
Rates of transfer become equal
Final levels remain unchanged
Reversible process
Practice Question 1
Consider the following reactions and determine if they're reversible:
1. Reversible in any closed container
2. Typically irreversible due to thermodynamic constraints
3. Irreversible due to open system conditions and thermodynamic constraints