Chemical Equilibrium and Le Chatelier’s Principle Vocabulary

Fundamental vocabulary and concepts relating to chemical equilibrium, equilibrium constants, and Le Chatelier's Principle as described in the lecture notes.

Stoichiometric reaction

A "one-way," "complete" reaction characterized by a large negative $\Delta G$ in the forward direction and a large activation energy barrier in the reverse direction.

Reversible reaction

A "two-way" reaction where both forward and reverse processes occur simultaneously, resulting in a mixture of reactants and products where $\Delta G$ is small and $E_a$ is similar in both directions.

Reactants

A human labeling convention referring to whatever substances are written on the left side of the reaction arrow.

Products

A human labeling convention referring to whatever substances are written on the right side of the reaction arrow.

Dynamic chemical equilibrium

A state achieved when the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in concentration.

Equilibrium Constant ($K$)

The ratio of $[products]/[reactants]$ at equilibrium where the coefficients of the balanced equation become exponents; it is derived by setting forward and reverse rates equal.

$$K_c$$

The equilibrium constant written in terms of molarity (concentration) of aqueous or gas species.

$$K_p$$

The equilibrium constant written in terms of partial pressures ($atm$) of gas species only.

$$Ka, Kb, Kw, Ksp, Kf$$

Special-case examples of $K_c$ for reaction types such as acid ionization, base ionization, water autoionization, solubility, and complex-ion formation.

$K_p–K_c$ relationship equation

The mathematical relationship defined as $K_p = K_c(RT)^{\Delta n(gas)}$.

$$\Delta n(gas)$$

The moles of GAS on the product side minus the moles of GAS on the reactant side in a balanced equation.

Large K (e.g., $K > 10$)

Indicates that the equilibrium lies to the right and mostly products are present at equilibrium.

Small K (e.g., $K < 0.10$)

Indicates that the equilibrium lies to the left and mostly reactants are present at equilibrium.

Reaction quotient ($Q$)

Calculated using the same expression form as $K$ ($products/reactants$) but using activity/concentration values at any point in time, not necessarily at equilibrium.

$$Q < K$$

A condition where the system must shift RIGHT (forward, toward products) to reach equilibrium.

$$Q > K$$

A condition where the system must shift LEFT (reverse, toward reactants) to reach equilibrium.

ICE table

A mnemonic for "Initial," "Change," and "Equilibrium" rows used to track concentrations of species during a reaction based on stoichiometry.

5% assumption check

A validity test for the "x is small" approximation calculated as $(\frac{x}{\text{initial concentration}}) \times 100$; the assumption is valid if the result is less than $5\%$.

Le Chatelier's Principle

The principle stating that when a stress is applied to a system at equilibrium, the system responds in a way that counters or partially opposes that stress.

Stress (Le Chatelier's)

Any change that temporarily takes a system out of equilibrium, such as changes in concentration, volume, pressure, or temperature.

Inert gas stress

Addition of a non-reactive species; it has no effect in a rigid container but shifts equilibrium toward the side with more moles of gas in a flexible (variable-volume) container.

Catalyst (Equilibrium Effect)

A substance that speeds up the rate of reaching equilibrium but does NOT shift the position of equilibrium or change the value of $K$.

Endothermic reaction

A reaction where $\Delta H$ is positive and heat is treated as a reactant; increasing temperature shifts the equilibrium to the right and increases $K$.

Exothermic reaction

A reaction where $\Delta H$ is negative and heat is treated as a product; increasing temperature shifts the equilibrium to the left and decreases $K$.