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What are Equilibrium and Non-equilibrium Systems? // HSC Chemistry
Science Ready
Overview
This video introduces the concepts of chemical systems, equilibrium, and reversible reactions, crucial for understanding Module 5. It differentiates between open and closed systems, explaining how matter and energy exchange affects a reaction's environment. The video then delves into the conditions for reversibility, linking it to enthalpy and entropy changes. Finally, it distinguishes between static and dynamic equilibrium, emphasizing that dynamic equilibrium, occurring in closed, reversible systems, is the primary focus for further study.
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Chapters
- A chemical system is the environment where a reaction occurs.
- Open systems allow both matter and energy to move in and out.
- Closed systems allow energy to move in and out, but matter is contained within the system.
Understanding the type of system is fundamental because it dictates whether matter can be exchanged, which is a prerequisite for achieving equilibrium in many chemical reactions.
A pot of water with a lid is a closed system, while an uncovered pot is an open system where ingredients can be added.
- Reactions can be reversible based on enthalpy (heat change) and entropy (disorder change).
- Exothermic reactions (negative enthalpy) and increased disorder (positive entropy) are generally favorable for a forward reaction.
- A reaction is only considered reversible if it has favorable conditions (either enthalpy or entropy, or both) for the forward reaction, which implies the reverse reaction also has potential drivers.
- If a forward reaction is both enthalpically and entropically favorable, the reverse reaction will not occur because it would be both endothermic and have negative entropy, lacking any driving force.
These thermodynamic factors determine whether a reaction can proceed in both forward and reverse directions, which is essential for reaching a state of equilibrium.
Combustion is exothermic (favorable enthalpy) and increases the number of gas molecules (favorable entropy), making it irreversible because the reverse reaction would be endothermic and decrease disorder.
- Irreversible reactions proceed in one direction only and go to completion.
- These reactions eventually reach a static equilibrium where the rates of both forward and reverse reactions are zero.
- Examples include complete combustion and photosynthesis.
Recognizing irreversible reactions helps distinguish them from reversible ones and understand why they don't establish a dynamic balance.
The complete combustion of propane produces carbon dioxide and water, and this reaction does not readily reverse under normal conditions.
- Dynamic equilibrium occurs in reversible reactions within a closed system.
- At dynamic equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.
- Although the rates are equal, both forward and reverse reactions are still occurring at the molecular level (microscopic change).
- There is no net change in the amounts of reactants and products (macroscopic change) at dynamic equilibrium.
- A system at dynamic equilibrium has a Gibbs free energy of zero.
This is the central concept of the module, representing a state of balance in reversible reactions where reactions continue but without observable change.
Heating calcium carbonate in a sealed container: it decomposes into calcium oxide and carbon dioxide, but the carbon dioxide can react back with calcium oxide to reform calcium carbonate, reaching a dynamic balance.
- A model using two containers of water illustrates dynamic equilibrium.
- Water is transferred between containers until the levels are equal, representing equal rates of forward and reverse processes.
- This model highlights that dynamic equilibrium requires a closed system (total water remains constant) and reversibility (water can move both ways).
- A limitation of this model is that it doesn't visually represent the ongoing molecular activity (microscopic changes) occurring at equilibrium.
This analogy helps visualize the concept of equal rates and balance, making the abstract idea of dynamic equilibrium more concrete.
Pouring water between two containers until the water levels are identical in both, signifying that the rate of water moving from container 1 to 2 equals the rate of water moving from 2 to 1.
Key takeaways
- Chemical systems are classified as open or closed based on the exchange of matter and energy.
- Reversibility of a reaction is influenced by enthalpy (heat) and entropy (disorder) changes.
- Irreversible reactions proceed to completion and reach a static equilibrium where all reactions cease.
- Dynamic equilibrium is a state in reversible reactions where forward and reverse reaction rates are equal and non-zero.
- Dynamic equilibrium requires a closed system and a reversible reaction.
- At dynamic equilibrium, there are no observable macroscopic changes, but molecular processes continue.
- Non-equilibrium often refers to static equilibrium associated with irreversible reactions.
Key terms
Chemical SystemOpen SystemClosed SystemEnthalpyEntropyReversible ReactionIrreversible ReactionStatic EquilibriumDynamic EquilibriumExothermicEndothermicGibbs Free Energy
Test your understanding
- What is the fundamental difference between an open and a closed chemical system regarding matter and energy exchange?
- How do enthalpy and entropy changes influence whether a chemical reaction is likely to be reversible?
- What characterizes a static equilibrium, and what types of reactions lead to it?
- Explain the conditions necessary for a dynamic equilibrium to be established in a chemical reaction.
- How does the concept of 'non-equilibrium' relate to static equilibrium in the context of chemical reactions?