Chemistry — Introduction to Equilibrium + 6 more topics

Equilibrium in physical processes means that a system has reached a state where the rates of opposing physical changes are equal. This results in no net change in the observable properties of the system, like temperature, pressure, or concentration. It doesn't mean nothing is happening; it means dynamic processes are perfectly balanced.

Physical equilibrium can occur in several scenarios:

Chemical equilibrium is a dynamic state where the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant. Le Chatelier's Principle states that if a change of condition (stress) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.

Let's break down the main factors that cause these shifts:

Chemical reactions don't always go to completion; many are reversible, meaning products can react to re-form reactants. When a reversible reaction reaches equilibrium, the rate of the forward reaction (reactants forming products) becomes equal to the rate of the reverse reaction (products forming reactants). At this point, the concentrations of reactants and products no longer change, even though both reactions are still happening. It's a dynamic state.

You know that pure water is essential for life, but even pure water isn't just H₂O molecules. A tiny fraction of water molecules actually break apart, or dissociate, into ions. This process is called autoionization of water.

H₂O(l) ⇌ H⁺(aq) + OH⁻(aq)

Or, more accurately, since H⁺ doesn't exist alone in water, it forms a hydronium ion:

2H₂O(l) ⇌ H₃O⁺(aq) + OH⁻(aq)

For simplicity, we often use H⁺ to represent H₃O⁺.

In chemistry, equilibrium is all about reversible reactions. Most reactions don't just go one way; products can often react to re-form the reactants. We show this with a double arrow: A + B <=> C + D.

When we talk about acids and bases, we usually refer to the Brønsted-Lowry definition. An acid is a substance that donates a proton (H⁺), and a base is a substance that accepts a proton.

For example, when hydrochloric acid (HCl) dissolves in water:
HCl(aq) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq)
Here, HCl donates a proton to H₂O, making HCl the acid and H₂O the base. The H₃O⁺ is called the hydronium ion.

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