Chemical Equilibrium and Reversible Reactions

Reversibility of Chemical Reactions

Chemical reactions can proceed in one direction (irreversible) or in both directions (reversible). The reversibility of a reaction depends on the relative stability of reactants and products.

• Reversible Reaction: A reaction that can proceed both forward (reactants to products) and reverse (products to reactants) directions.

• Notation: Reversible reactions are represented by a double arrow ().

• Forward Reaction: The process where reactants are converted to products.

• Reverse Reaction: The process where products are converted back to reactants.

• Example: The dissolution and precipitation of salt in water is a reversible process.

Chemical Equilibrium

Chemical equilibrium occurs when the rates of the forward and reverse reactions are equal, resulting in no net change in the concentrations of reactants and products. This state is dynamic, meaning both reactions continue to occur, but their effects cancel each other out.

• Dynamic Equilibrium: Both forward and reverse reactions continue at equal rates; concentrations remain constant.

• Constant Concentrations: At equilibrium, the concentrations of all species are constant, but not necessarily equal.

• Visual Example: In a saturated salt solution, salt is both dissolving and precipitating at the same rate, so the observable amounts remain unchanged.

Key Characteristics of Equilibrium

• Not Necessarily Equal Concentrations: The concentrations of reactants and products at equilibrium are constant, but they are not usually equal.

• Independence from Initial Conditions: The final equilibrium state is the same regardless of whether the system started with more reactants or more products, provided temperature and pressure are constant.

• Predictability: The extent to which a reaction favors products or reactants at equilibrium is a characteristic property of the reaction under specific conditions.

Example: Decomposition of Dinitrogen Tetraoxide

The decomposition of dinitrogen tetraoxide () to nitrogen dioxide () is a classic example of a reversible reaction reaching equilibrium:

• Reaction:

• Observation: Starting with pure , its concentration decreases as it forms . Over time, the rate of change slows, and both concentrations become constant at equilibrium.

• Equilibrium from Either Direction: Whether starting with or , the system will reach the same equilibrium mixture at a given temperature and pressure.

Summary Table: Key Features of Chemical Equilibrium

Additional info:

• The equilibrium constant () quantifies the ratio of product and reactant concentrations at equilibrium, but was not explicitly mentioned in the provided notes.