Understanding Non-equilibrium Systems in Chemistry

Expert reviewed • 22 November 2024 • 5 minute read

Introduction

Non-equilibrium systems, also known as static equilibrium systems, are fundamental concepts in chemistry where reactions proceed predominantly in one direction. Unlike reversible reactions, these systems don't establish a dynamic equilibrium between reactants and products.

Key Characteristics

A chemical reaction is considered non-equilibrium when:

Products permanently leave the system (open system)
Products are highly stable and don't react with each other
The forward reaction has both negative enthalpy change and positive entropy change

Thermodynamic Principles

The direction of a reaction can be predicted using enthalpy (ΔH) and entropy (ΔS) changes:

Enthalpy Change	Entropy Change	Reaction Type
$\Delta H > 0$ (endothermic)	$\Delta S > 0$ (increase)	Reversible
$\Delta H < 0$ (exothermic)	$\Delta S > 0$ (increase)	Irreversible
$\Delta H > 0$ (endothermic)	$\Delta S < 0$ (decrease)	Not spontaneous
$\Delta H < 0$ (exothermic)	$\Delta S < 0$ (decrease)	Reversible

Examples of Non-equilibrium Systems

1. Combustion Reactions

Combustion reactions are excellent examples of non-equilibrium systems. They are characterized by:

Negative enthalpy change (exothermic)
Positive entropy change (increase in gas molecules)

For example, the combustion of propane:

$2C_3H_8(g) + 7O_2(g) \rightarrow 8H_2O(g) + 6CO_2(g)$

This reaction has:

Standard enthalpy: $\Delta H = -2220.0 \text{ kJ mol}^{-1}$
Entropy change: $\Delta S = +269.91 \text{ J mol}^{-1}K^{-1}$

The reaction produces 14 moles of gas from 9 moles of reactant gases, increasing system entropy.

2. Photosynthesis

Photosynthesis represents a complex non-equilibrium system:

$6CO_2(g) + 6H_2O(l) \rightarrow 6O_2(g) + C_6H_{12}O_6(s)$

Photosynthesis is irreversible because:

It occurs in an open system where products (especially oxygen) leave through plant stomata
It requires specific enzymes that only catalyze the forward reaction
The reverse reaction has prohibitively high activation energy
It involves multiple coordinated reactions occurring in different cellular locations

Thermodynamic Analysis of Photosynthesis

Photosynthesis is non-equilibrium because:

It is endothermic ( $\Delta H > 0$ )
Has decreasing entropy ( $\Delta S < 0$ )
Results in positive Gibbs free energy ( $\Delta G > 0$ )
Occurs through multiple non-spontaneous steps
Takes place in an open cellular system

Chemical Equilibrium: Systems and Dynamics

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Understanding Collision Theory in Chemical Equilibrium

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