25 Equilibria

25.1 Acids and Bases

  1. Conjugate Acid and Conjugate Base:
    1. A conjugate acid is formed when a base gains a proton (H+).
    2. A conjugate base is formed when an acid loses a proton (H+).
  2. Conjugate Acid-Base Pairs:

Conjugate acid-base pairs are two species related by the gain or loss of a single proton. In a reaction, when an acid donates a proton, it forms its conjugate base, and when a base accepts a proton, it forms its conjugate acid.

  1. pH, Ka, pKa, and Kw:
    1. pH: pH is a measure of the hydrogen ion concentration in a solution and is defined as the negative logarithm of the hydrogen ion concentration:

pH = -log [H+]

  1. Ka: Ka is the acid dissociation constant, which represents the strength of an acid in a solution. It is the ratio of the concentration of the products (conjugate base and H+) to the concentration of the acid.

Ka=[H+][A-]/[HA]

where H+ is the concentration of hydrogen ions, A- is concentration of the conjugate base of the acid, and HA is the concentration of chemical species HA.

  1. pKa: pKa is the negative logarithm of Ka and is used to compare the acid strength of different compounds.

pKa = -log Ka

  1. Kw: Kw is the ionic product of water. It is the equilibrium constant for the dissociation of water at 298 K. Its value is 1.00 x 10-14 mol2dm-6.

Kw=[H+][OH-]

Where [H+] is the concentration of hydrogen ions, and [OH-] is the concentration of hydroxide ions. In a neutral solution (pH=7), the concentration of [H+] = [OH-], which means [H+] = 1*10-7 mol dm-3

  1. Calculating [H+] and pH for Different Acids and Bases: 1 Strong Acids:

Strong acids dissociate completely in water, so [H+] is equal to the initial acid concentration, and pH can be calculated directly from [H+].

1 Strong Alkalis:

Strong alkalis are fully dissociated in water, so [OH-] is equal to the initial alkali concentration. pH can be calculated using the relation

pH = 14 - pOH

pOH = -log[OH-]

1 Weak Acids:

Weak acids partially dissociate in water, and their dissociation constant (Ka) is used to calculate [H+] and pH.

  1. Buffer Solutions: 1 Buffer Solution:

A buffer solution is a solution that resists changes in pH when small amounts of acids or bases are added.

(b) Making Buffer Solutions: Buffer solutions are usually prepared by mixing a weak acid (or its conjugate base) with its conjugate base (or weak acid) in a specific ratio.

(c) Controlling pH: Buffer solutions control pH by neutralizing the added acid or base through the reaction with their conjugate acid or base components.

(d) Uses of Buffer Solutions: Buffer solutions are crucial in various applications, such as in biological systems (e.g., blood, which contains bicarbonate ions, HCO3-, to maintain a stable pH).

  1. Calculating the pH of Buffer Solutions: The pH of a buffer solution is calculated using the Henderson-Hasselbalch equation:

pH = pKa + log([A-]/[HA])

where [A-] is the concentration of the conjugate base and [HA] is the concentration of the weak acid.

  1. Solubility Product (Ksp):
    1. Solubility product (Ksp) is the equilibrium constant for a sparingly soluble salt dissolving in water to form its constituent ions.
    2. For a salt, AaBb, the Ksp expression is:

Ksp = [A+]a[B-]b

where [A+] is the concentration of the cation in an aqueous solution, [B-] is the concentration of the anion in an aqueous solution, and a and b are their relative concentrations.

  1. Calculating Ksp:
    1. From Concentrations: Ksp can be calculated if the concentrations of the ions in a saturated solution of the salt are known.
    2. Vice Versa: Given Ksp, the concentrations of the ions in a saturated solution can be determined.
  2. Common Ion Effect:

(a) Solubility: The presence of a common ion (an ion already present in solution) reduces the solubility of a compound in a solution.

(b) Calculations: The common ion effect is used to calculate the solubility of a compound in the presence of a common ion, using the Ksp expression and the initial concentration of the common ion.

25.2 Partition Coefficients:

  1. Partition Coefficient (Kpc):
    1. The partition coefficient is a measure of how a solute distributes itself between two immiscible solvents at equilibrium.
    2. It is defined as the ratio of the concentrations (or activities) of a solute in two different phases

Kpc= (concentration of solute X in solvent A)/(concentration of solute X in solvent B)

  1. Factors Affecting Partition Coefficient: The numerical value of the partition coefficient depends on the polarity of the solute and the solvents used. A higher partition coefficient indicates a stronger affinity of the solute for one solvent over the other, usually based on polarity differences.