Chemical Equilibrium Overview

Chemical Equilibrium

• Equilibrium is a state of balance where the rate of the forward reaction equals the rate of the backward reaction.
• It is dynamic because reactions continue to happen but the net change in the reaction mixture is zero.
• There is no change in the concentration of reactants and products over time at equilibrium.
• Chemical equilibrium focuses on the relative amount of reactants and products at equilibrium.

Reaching Equilibrium

• Reaching equilibrium takes time.
• The initial concentration of reactants decreases, and the concentration of products increases until equilibrium is achieved.
• Equilibrium is represented by the equilibrium constant (Kc) for a reversible reaction.

Law of Chemical Equilibrium

• The ratio of the product of the concentrations of products to the product of the concentrations of reactants at equilibrium is a constant.
• The equilibrium constant, K, is a measure of the extent to which a reaction proceeds to completion.

Equilibrium Constant

• Kc is the equilibrium constant in terms of concentrations.
• Kp is the equilibrium constant in terms of partial pressures.
• The value of Kc is affected by temperature only, not by pressure or initial concentrations.
• For a reaction: aA + bB cC + dD, the equilibrium constant is: Kc = [C]^c[D]^d / [A]^a[B]^b.
• Solids and pure liquids are omitted from the Kc expression because their concentrations are essentially constant.

Factors Affecting Equilibrium Position

• Changes in temperature, pressure, or concentration can shift the equilibrium position.
• Le Chatelier's principle: If a change of condition is applied to a system in equilibrium, the system will shift in a direction that relieves the stress.

Le Chatelier's Principle

• Effect of Temperature: If the system is exothermic, increased temperature will shift the equilibrium to the left (toward reactants). If the system is endothermic, increased temperature will shift the equilibrium to the right (toward products).
• Effect of Pressure: Increasing pressure favors the side of the reaction with fewer moles of gas. Decreasing pressure favors the side with more moles of gas.
• Effect of Concentration: Increasing reactant concentration shifts the equilibrium to the right (toward products). Increasing product concentration shifts the equilibrium to the left (toward reactants).

Reaction Quotient

• Q is the reaction quotient which can be calculated at any point in time.
• It is used to predict the direction of a reaction to reach equilibrium.
• Q < K: The reaction will shift to the right to form more products.
• Q > K: The reaction will shift to the left to form more reactants.
• Q = K: The system is in equilibrium.

Le Chatelier's Principle

• Chemical reactions will shift to relieve stress.
• Stress can be changes in:
  • Concentration
  • Pressure
  • Temperature
  • Addition of an inert gas
  • Addition of a catalyst

Concentration Effect

• Increasing the concentration of reactants will shift the reaction forward.
• Decreasing the concentration of reactants will shift the reaction backward.
• Increasing the concentration of products will shift the reaction backward.
• Decreasing the concentration of products will shift the reaction forward.

Pressure Effect

• If pressure is increased, the reaction will shift to the side with fewer moles of gas.
• If pressure is decreased, the reaction will shift to the side with more moles of gas.

Temperature Effect

• For an endothermic reaction (ΔH > 0):
  • Increasing Temperature shifts the reaction forward.
  • Decreasing Temperature shifts the reaction backward.
• For an exothermic reaction (ΔH < 0):
  • Increasing Temperature shifts the reaction backward.
  • Decreasing Temperature shifts the reaction forward.

Catalyst Effect

• A catalyst speeds up the rate of a reaction without affecting the equilibrium position.
• There is no change in the equilibrium position with the addition of a catalyst.

Inert Gas Effect

• The addition of an inert gas (a gas that does not react) at constant volume has no effect on the equilibrium position.
• The addition of an inert gas at constant pressure will shift the reaction to the side with more moles of gas.

Arrhenius Concept

• Acids donate hydrogen ions (H+) in aqueous solutions.
• Bases donate hydroxide ions (OH-) in aqueous solutions.

Brønsted-Lowry Concept

• Acids donate hydrogen ions (H+).
• Bases accept hydrogen ions (H+).

Lewis Concept

• Acids accept electron pairs.
• Bases donate electron pairs.

Electrolytes

• Electrolytes are substances that conduct electricity when dissolved in water.
• Electrolytes contain ions, typically formed through ionic bonds.
• Strong electrolytes dissociate completely in water.
• Weak electrolytes dissociate only partially in water.

Association Constant for Weak Acid

• The association constant (Ka) is a measure of the extent to which a weak acid dissociates in solution.
• The equilibrium constant (Ka) for a weak acid is: Ka = [H+][A-] / [HA].

Dissociation Constant for Polyprotic Acids

• Polyprotic acids have more than one acidic hydrogen.
• Each dissociation step has its own dissociation constant (Ka1, Ka2, etc.).
• The overall dissociation constant (Ktot) for a polyprotic acid is the product of all the individual dissociation constants.
• The first dissociation step has the largest dissociation constant.

pK Values

• pK represents the negative logarithm of the equilibrium constant (K).
• pKa = -log(Ka)
• pKb = -log(Kb)
• pKw = -log(Kw)
• pKa + pKb = pKw

Common Ion Effect

• The common ion effect states that the solubility of a sparingly soluble salt is decreased by the addition of a soluble salt containing a common ion.

Salt Hydrolysis

• Salt hydrolysis is the reaction of a salt with water to produce an acidic, basic, or neutral solution.
• A salt formed from a strong acid and a strong base will produce a neutral solution.
• A salt formed from a weak acid and a strong base will produce a basic solution.
• A salt formed from a strong acid and a weak base will produce an acidic solution.
• A salt formed from a weak acid and a weak base will produce a solution whose pH depends on the relative strengths of the acid and base.

Buffer Solutions

• A buffer solution is a solution that resists changes in pH upon the addition of small amounts of acid or base.
• Buffer solutions are typically composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.
• Buffer solutions are important in biological systems, for example, the blood.

Buffers

• Our body maintains a neutral pH level, neither acidic nor basic. This is achieved by a special solution called a buffer solution that allows us to consume acids and bases without significant shifts in our body's pH.
• Buffer solutions work by minimizing changes to the pH when small amounts of acid or base are added.
• They are composed of a weak acid and its salt formed with a strong base.
• Acidic buffer solutions are created by mixing a weak acid with its salt formed with a strong base. This mixture can resist pH changes when small amounts of base are added.
• Basic buffer solutions are created by mixing a weak base with a salt of a strong acid. This mixture can resist pH changes when small amounts of acid are added.
• Neutral buffer solutions are created by mixing a weak acid with a weak base together.

Solubility Product

• The Solubility Product (Ksp) is a measure of how much a substance can dissolve in a solution.
• It is calculated by multiplying the concentrations of the ions in the solution raised to their respective stoichiometric coefficients.
• For example, if a substance dissolves as A + B-, then the Ksp is A * B. If a substance dissolves as A + 2B, then the Ksp is A * (B)^2.
• The higher the Ksp, the more soluble the substance.
• The lower the Ksp, the less soluble the substance.
• If the concentration of ions reaches a point higher than Ksp, then precipitation occurs, meaning the substance will form a solid and settle out of the solution.
• If the concentration of ions is equal to Ksp, then the solution is saturated, meaning no more substance dissolves or precipitates.

This quiz covers the key concepts of chemical equilibrium, including the dynamic nature of equilibrium, the process of reaching it, and the law governing it. Understand how the concentration of reactants and products changes over time and how the equilibrium constant (Kc) is defined. Test your knowledge of these fundamental principles of chemistry.