Chemical Equilibrium Notes

Vocabulary practice flashcards covering core principles, mathematical relationships, manipulation rules, and Le Chatelier's principle from the Chemical Equilibrium lecture notes.

Dynamic Equilibrium

A condition in a reversible chemical reaction where the forward reaction rate equals the reverse reaction rate ($\text{rate}_{\text{fwd}} = \text{rate}_{\text{rev}}$), resulting in constant concentrations of reactants and products over time.

Equilibrium Constant ($K$)

A unitless value that quantifies the ratio of product concentrations to reactant concentrations at equilibrium, where each species' concentration is raised to the power of its stoichiometric coefficient: $K = \frac{[C]^c [D]^d}{[A]^a [B]^b}$.

Chemical Activity

The thermodynamic basis for the equilibrium constant $K$; because pure solids ($(s)$) and pure liquids ($(l)$) have an activity of $1$, they are excluded from the equilibrium expression.

Reaction Inversion Rule for $K$

A rule stating that if a chemical reaction is reversed, its equilibrium constant is inverted: $K_{\text{rev}} = \frac{1}{K}$.

Coefficient Multiplication Rule for $K$

A rule stating that if a chemical reaction is multiplied by a stoichiometric factor $n$, its equilibrium constant is raised to that power: $K_{\text{new}} = K^n$.

Reaction Addition Rule for $K$

A rule stating that when multiple chemical equations are added together to yield an overall reaction, the overall equilibrium constant is the product of the individual equilibrium constants ($K_{\text{overall}} = K_1 \times K_2$).

$$K_p$$

The equilibrium constant expression written using partial pressures of gaseous products and reactants instead of molar concentrations.

Relationship Between $K_p$ and $K_c$

The mathematical relationship relating pressure-based and concentration-based equilibrium constants: $K_p = K_c(RT)^{\Delta n}$, where $R = 0.08206\,\text{L}\,\text{atm}\,\text{mol}^{-1}\,\text{K}^{-1}$, $T$ is in Kelvin, and $\Delta n = (c + d) - (a + b)$ is the change in moles of gas.

Product-Favored Equilibrium ($K \gg 1$)

A condition in which the equilibrium position lies far to the right, meaning products predominate over reactants at equilibrium.

Reactant-Favored Equilibrium ($K \ll 1$)

A condition in which the equilibrium position lies far to the left, meaning reactants predominate over products at equilibrium.

Reaction Quotient ($Q$)

A ratio of product concentrations or partial pressures to reactant concentrations formulated identically to $K$, but evaluated at any point in time to determine whether a reaction is at equilibrium and which way it will shift.

$$Q < K$$

A state where the reaction quotient is smaller than the equilibrium constant, meaning there are relatively too few products, causing the reaction to shift to the right (toward products).

$$Q > K$$

A state where the reaction quotient is greater than the equilibrium constant, meaning there are relatively too many products, causing the reaction to shift to the left (toward reactants).

Small-$x$ Approximation

An algebraic approximation used when $K$ is sufficiently small that the change in concentration ($x$) is negligible compared to the initial concentration; valid if the calculated change is less than $5\%$ of the initial value.

Le Chatelier's Principle

The principle stating that when a chemical system at equilibrium is disturbed, the system will shift in a direction that minimizes the disturbance.

Effect of Volume and Pressure on Gas Equilibrium

Decreasing vessel volume (increasing pressure) causes the reaction to shift toward the side with fewer moles of gas; increasing volume (decreasing pressure) causes it to shift toward more moles of gas.

Effect of Inert Gas Addition

Adding an inert gas (such as $\text{Ar}$) at constant volume increases the total pressure, but does not alter the partial pressures or concentrations of the reacting species, causing no shift in equilibrium.

Effect of Temperature on Equilibrium

A disturbance that changes the actual value of $K$; increasing temperature shifts an exothermic reaction to the left (decreasing $K$) and an endothermic reaction to the right (increasing $K$).

Catalyst (in Equilibrium)

A substance that increases the rates of both forward and reverse reactions equally, shortening the time needed to reach equilibrium without changing the equilibrium position or concentrations.