> See also: > - [[Chemical Reactions]] > - [[Acids and Bases]] > - [[Ionization]] > - [[Reaction Kinetics]] # Chemical Equilibrium The vast majority of [[chemical reactions]] are capable of reversing in the opposite direction and thus can be called *reversible reactions*. The balance between the forward and reverse directions of a reaction is known as **chemical equilibrium**. At equilibrium, the **rates of the forward/reverse reactions are equal** and no changes in reactant or product concentrations will be made, *unless the equilibrium is disturbed* (aka stressed). > [!important]- The Principle of Microscopic Reversibility > While we can classify chemical reactions as reversible or irreversible based on whether they proceed in the forward & reverse directions, it is a fundamental law of physics that *all processes are reversible at a microscopic level*. ## Equilibrium Constants At equilibrium, the molar concentrations of products and reactants will be fixed in *a given ratio* known as the **equilibrium constant**, $K_{eq}$ $aA + bB \leftrightharpoons cC + dD$ $K = \frac{[C]^c[D]^d}{[A]^a[B]^b}$ - If $K_{eq} > 1$, the concentration of the products is **greater** than the reactants - If $K_{eq} < 1$, the concentration of the products is **less** than the reactants --- When This is more generally known as the reaction quotient: $K = \frac{\text{[Products]}}{\text{[Reactants]}}$ Equilibrium constants can be referred to in different ways depending on the chemical reaction occurring. Below is a table of several equilibrium constants and the reaction types they are related to: | Name | Symbol | Description | | | ---- | ---- | ---- | ---- | | Equilibrium Constant | $K_{eq}$ | The general designation for an equilibrium constant | | | | $K_{sp}$ | Solubility product | | | Water Autoionization Constant | $K_w$ | | | | | $K_a$ | | | TODO: - ionization constant vs dissociation constant - ## Ionization & Dissociation Constants **Ionization constants** are *specific to acids, bases, or water* and describe the equilibrium between **Autoionization Constant (Ion-Product)** $K_w = [H_3O^+][OH^-]$ - At room temperature (25 $\degree C$): - $K_w = 1.0 \times 10^{-14}$ [[Acids and Bases]]