Introduction: How Far Does a Reaction Go?
Not all chemical reactions go to completion. Many reactions are reversible , meaning the products can react together to reform the original reactants. This raises a fundamental question: how far does the reaction proceed before it reaches a stable state?
The answer lies in the concept of chemical equilibrium , a dynamic balance between the forward and reverse reactions. Understanding equilibrium allows chemists to predict and control the extent of a reaction, which is crucial in industrial processes, biological systems, and environmental chemistry.
In this subtopic, you will explore:
- What dynamic equilibrium means at the molecular level
- How to express and interpret the equilibrium constant,
- How to predict the response of a system at equilibrium to external changes (Le Châtelier's Principle)
- How to use the reaction quotient, , to determine the direction of change
Think of a busy revolving door at a train station. At peak hours, people enter and exit at the same rate , the door keeps spinning, but the number of people inside stays constant. This is analogous to dynamic equilibrium: reactions keep happening in both directions, but the overall composition stays the same.
Dynamic Equilibrium
Reversible reaction: A reversible reaction is one in which reactants can form products (the forward reaction) while products can simultaneously revert back into reactants (the reverse reaction). It is represented using the double arrow symbol .
Dynamic equilibrium: Dynamic equilibrium is the state reached in a closed system when the rate of the forward reaction equals the rate of the reverse reaction, resulting in constant concentrations of reactants and products over time.
Key Characteristics of Dynamic Equilibrium
- Equal rates: The forward and reverse reactions occur at the same rate , not that they have stopped.
- Constant concentrations: The concentrations of all reactants and products remain unchanged over time. This does not mean they are equal to each other.
- Closed system required: No substances can enter or leave the system. An open system cannot maintain equilibrium because products or reactants would escape.
- Macroscopic properties are constant: Observable properties like colour, pressure, and density do not change at equilibrium.
A very common misconception is that equilibrium means the concentrations of reactants and products are equal. This is incorrect. At equilibrium, concentrations are constant , but they can be very different from each other. Only the rates of the forward and reverse reactions are equal.
Dynamic equilibrium can be reached whether you start with only reactants, only products, or a mixture of both. The same equilibrium state is reached regardless of the starting point (at the same temperature).
