Acids and Bases

Definitions and Theories of Acids and Bases

Acids and bases are fundamental concepts in chemistry, described by several theories. Understanding these definitions is essential for identifying acid-base reactions and predicting their behavior.

• Arrhenius Acid: A substance that increases the concentration of H+ ions in aqueous solution.

• Arrhenius Base: A substance that increases the concentration of OH- ions in aqueous solution.

• Brønsted-Lowry Acid: A proton (H+) donor.

• Brønsted-Lowry Base: A proton (H+) acceptor.

• Lewis Acid: An electron pair acceptor.

• Lewis Base: An electron pair donor.

Example: BCl3, BF3, and BeH2 are Lewis acids because they can accept electron pairs due to their electron-deficient nature.

Conjugate Acid-Base Pairs

When an acid donates a proton, it forms its conjugate base; when a base accepts a proton, it forms its conjugate acid.

• Conjugate Pair: Consists of an acid and its conjugate base or a base and its conjugate acid.

• Example: In the reaction: , NH4+ is the acid, NH3 is its conjugate base.

Nucleophiles and Electrophiles

Nucleophiles and electrophiles are important in acid-base and organic reactions.

• Nucleophile: A species that donates an electron pair (Lewis base).

• Electrophile: A species that accepts an electron pair (Lewis acid).

Amphoteric Compounds

Amphoteric compounds can act as either acids or bases depending on the reaction environment.

• Example: Water (H2O), Aluminum hydroxide (Al(OH)3).

Acid and Base Ionization Calculations

Ionization refers to the process by which acids and bases dissociate in water. The extent of ionization is quantified by equilibrium constants.

• Acid Dissociation Constant (Ka):

• Base Dissociation Constant (Kb):

• ICE Table: Used to calculate equilibrium concentrations (Initial, Change, Equilibrium).

• Approximation Rule: If Ka or Kb is small, the change in concentration is negligible compared to the initial concentration.

Percent Ionization

Percent ionization measures the fraction of acid or base molecules that ionize in solution.

• Formula:

Buffers and Molar Solubility (Ksp)

Buffer Solutions

Buffers are solutions that resist changes in pH when small amounts of acid or base are added. They are crucial in biological and chemical systems.

• Composition: A buffer consists of a weak acid and its conjugate base, or a weak base and its conjugate acid.

• Natural Buffers: Blood (carbonic acid/bicarbonate system), seawater.

• Preparation: Mix a weak acid with its salt (e.g., acetic acid and sodium acetate).

Buffer Action and pH Maintenance

Buffers maintain a nearly constant pH by neutralizing added acids or bases.

• Mechanism: The weak acid neutralizes added base; the conjugate base neutralizes added acid.

• Dilution: Diluting a buffer does not significantly change its pH, but reduces its capacity.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer to the concentrations of its acid and conjugate base.

• Equation:

• For bases:

Acidity and Basicity Relationships

Acidity and basicity are quantified by pH, pKa, Ka, and related measures.

• Acidity: Lower pH, lower pKa, higher Ka, higher [H3O+].

• Basicity: Higher pOH, lower pKb, higher Kb, higher [OH-].

• Relationship: (at 25°C)

• Ion Product of Water:

Interpretation of pKa and pKb

• Low pKa: Strong acid (dissociates more).

• High pKa: Weak acid (dissociates less).

• Low pKb: Strong base.

• High pKb: Weak base.

Calculating pH, pOH, Kw, Ka

• pH:

• pOH:

• K_w:

• K_a:

Solubility Product Constant (Ksp)

Dissociation of Salts and Ksp Expressions

Solubility product constant (Ksp) describes the equilibrium between a solid and its ions in solution.

• Dissociation: Write the equation for the salt dissolving in water.

• Ksp Expression: For :

Interpretation of Ksp

• Low Ksp: Salt is less soluble.

• High Ksp: Salt is more soluble.

Calculating Molar Solubility and Ksp

• Molar Solubility: The number of moles of solute that dissolve per liter of solution.

• Calculation: Use ICE tables and Ksp expressions to solve for solubility.

Ksp and Reaction Quotient (Q)

The reaction quotient (Q) is calculated the same way as Ksp, but with current concentrations.

• If Q < Ksp: Solution is unsaturated; more solute can dissolve.

• If Q = Ksp: Solution is saturated; equilibrium is reached.

• If Q > Ksp: Solution is supersaturated; precipitation occurs.

Predicting Precipitation

• Precipitate Formation: Occurs when Q > Ksp.

• No Precipitate: If Q < Ksp, all ions remain dissolved.

Summary Table: Ksp vs. Q and Solution Types

Example: Mixing solutions of AgNO3 and NaCl: If [Ag+][Cl-] > Ksp for AgCl, a precipitate forms.

Additional info: ICE tables and approximation rules are standard methods for equilibrium calculations in acid-base and solubility problems. The Henderson-Hasselbalch equation is widely used for buffer calculations in both laboratory and biological contexts.