Lecture on Chemical Equilibrium

Equilibrium

The state when the concentrations of reactants and products remain constant over time.

Dynamic Equilibrium

When species do not stop forming or being destroyed; the rate of formation equals the rate of removal.

Chemical Equilibrium Characteristics

Concentrations of reactants and products do not change over time.

Equilibrium Constant (Kc)

A ratio of the concentrations of products raised to their stoichiometric coefficients to the concentrations of reactants raised to their stoichiometric coefficients.

Kp

An equilibrium constant used when dealing with partial pressures, related to Kc by the equation Kp = Kc(RT)^{Δngas}.

Le Chatelier's Principle

If a system at equilibrium is subjected to a change in concentration, temperature, or pressure, the equilibrium will shift to counteract that change.

Kc Expression

For a reaction aA + bB ⇌ cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b.

Stoichiometric Coefficients

The coefficients in a balanced chemical equation that represent the ratios of moles of reactants and products.

Reactant-Favored Reaction

A reaction where very little product forms and is usually written as 'no reaction' or NR.

Product-Favored Reaction

A reaction where very little reactant remains and can often be written as a forward reaction only.

Reaction Quotient (Q)

The ratio of concentrations of products to reactants at any point in time during a reaction.

Equilibrium Shift

The change in concentrations of reactants or products due to a change in conditions.

Omitting Solids and Liquids in Kc

Pure solids and liquids do not appear in the equilibrium constant expression because their concentrations are constant.

Effect of Temperature on Kc

Kc may increase or decrease when temperature is changed, depending on whether the reaction is exothermic or endothermic.

Ideal Gas Equation

PV = nRT; relates pressure, volume, temperature, and amount of gas.

Kc and Solvent Addition

Kc is not changed by solvent addition or removal, but the equilibrium position may change.

Equilibrium and Catalysts

Catalysts speed up both forward and reverse reactions but do not affect equilibrium concentrations.

Calculating Equilibrium Constants

Kc can be calculated from the equilibrium concentrations of reactants and products.

Kc for Related Reactions

When manipulating stoichiometry, Kc must be adjusted accordingly.

Increasing Volume Effect

Increasing the volume generally favors the side of the reaction with more moles of gas.

Decreasing Volume Effect

Decreasing the volume generally favors the side of the reaction with fewer moles of gas.

Adding Inert Gas Effect

Addition of an inert gas at constant volume does not affect the concentrations of reactants or products.

Kp Calculation from Kc

Kp can be calculated from Kc using Kp = Kc(RT)^{Δngas}, where Δngas is the change in moles of gas.

Q < Kc

Indicates that the reaction must shift forward to reach equilibrium.

Q > Kc

Indicates that the reaction must shift backward to reach equilibrium.

Kc Units

Units for Kc are customarily omitted.

Ice Table Method

Used to calculate changes in concentrations during the establishment of equilibrium.

Equilibrium Concentration Calculation

Using initial concentrations and changes to find equilibrium concentrations.

Effect of Adding Reactants

Adding more reactants will shift the equilibrium toward the product side.

Effect of Removing Products

Removing products will shift the equilibrium toward the product side.

H2 + I2 ⇌ 2HI

An example of a reaction used to demonstrate equilibrium calculations.

Equilibria Involving Pure Liquids

Pure liquids are omitted from Kc because their concentration is constant.

Effect of Heat on Endothermic Reactions

Increased temperature shifts equilibrium right to produce more products.

Effect of Heat on Exothermic Reactions

Increased temperature shifts equilibrium left to produce more reactants.

Equilibrium Shift Due to Concentration Changes

Shifts in equilibrium are changes in concentrations to restore equilibrium.

Constant Temperature Equilibria

Equilibrium will remain consistent if temperature is constant.

Equilibrium Position Change

Adding or removing reactants/products affects the equilibrium position.

Determining Changes in Equilibrium Position

Analyze concentrations and apply Le Chatelier's principle.

Dynamic nature of Equilibrium

Equilibrium is not static; reactions continue to occur at equal rates.

Q = Kc

Indicates that the system is at equilibrium.

Kc = [Products]/[Reactants]

Formula to calculate the equilibrium constant.

Equilibrium Constant Value Interpretation

Value indicates whether reactants or products are favored.

Equilibrium Concentration of Water in Aqueous Solutions

Water concentration is considered constant and omitted from Kc.

Common Misconception about Catalysts

Catalysts do not change the position of equilibrium; they only speed up the rate.

Temperature Increase on Equilibrium

An increase in temperature for an endothermic reaction favors the production of products.

Temperature Decrease on Equilibrium

A decrease in temperature for an exothermic reaction favors the production of products.

Synthesis of Ammonia Reaction

Example of a reaction where Kc is calculated as 5.6 x 10^5.

Effect of Pressure Changes in Reactions

Changes in pressure affect the concentrations of gaseous reactants and products.

Calculation of Kp from Kc Example

Example illustrating how to calculate Kp from Kc at a specific temperature.

Effect of Volume Change on Equilibrium Composition

Changing volume alters concentrations and can shift the position of equilibrium.

Adding Reactants in Equilibrium

Results in an increase of products formed and a shift in direction towards products.

Changing Temperature Effects on Kc of a Reaction

The direction and extent of Kc change depends on whether the reaction is endothermic or exothermic.

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