Overview

We usually think reactions go one way: Wood + Fire -> Ash. You can’t un-burn wood. But many reactions are reversible. Nitrogen + Hydrogen -> Ammonia. But Ammonia also breaks down into Nitrogen + Hydrogen. Eventually, the reaction reaches a stalemate where the forward speed equals the backward speed. This is Equilibrium.

Core Idea

The core idea is Dynamic Balance. It looks like nothing is happening (the levels stay constant), but actually, molecules are reacting back and forth furiously. It’s like walking up a down escalator. If you walk at the same speed as the stairs, you stay in the same place, even though you are moving.

Formal Definition

The state where the rate of the forward reaction equals the rate of the reverse reaction. Le Chatelier’s Principle: If you disturb a system at equilibrium (change heat, pressure, concentration), the system will shift to counteract the disturbance.

Intuition

Imagine two tanks of water connected by a pipe.

  • Start: Tank A is full, Tank B is empty. Water flows A -> B.
  • Equilibrium: Both tanks are half full. Water is still flowing through the pipe, but it flows A -> B and B -> A at the same rate, so the levels don’t change.

Examples

  • Soda Bottle: Inside a closed bottle, CO2 gas is dissolving into the soda, and CO2 bubbles are escaping the soda. They are in equilibrium. When you open the cap (release pressure), you break the equilibrium. The gas rushes out (fizz) to find a new balance.
  • Hemoglobin: In the lungs (high Oxygen), hemoglobin grabs Oxygen. In the muscles (low Oxygen), it releases Oxygen. It relies on the equilibrium shifting based on concentration.
  • Nitrogen Dioxide: $2NO_2$ (Brown) <-> $N_2O_4$ (Colorless). If you put it in ice, it turns colorless. If you put it in hot water, it turns brown. The equilibrium shifts with temperature.

Common Misconceptions

  • Equilibrium means 50/50: No. It just means the rates are equal. You might have 99% products and 1% reactants (Product Favored) or vice versa.
  • Reaction stops: It never stops. It just looks stable.
  • Equilibrium Constant (K): A number that tells you the ratio of products to reactants at equilibrium. Large K = mostly products. Small K = mostly reactants.
  • Homeostasis: Biological equilibrium. (See Biology).

Applications

  • Industrial Chemistry: In the Haber Process (making ammonia), we want to make as much product as possible. We use Le Chatelier’s principle: we squeeze the gas (high pressure) and remove the ammonia as it forms, forcing the reaction to keep shifting right to make more.

Criticism / Limitations

  • Kinetics: Equilibrium tells you what will happen eventually. It doesn’t tell you how long it will take. Diamonds are thermodynamically unstable (they want to turn into graphite), but the reaction is so slow that diamonds are “forever.”

Further Reading

  • Atkins, Peter. Physical Chemistry.
  • Van Ness, H.C. Understanding Thermodynamics.