Chemical reactions in aqueous solution

• Adding reactants → Q < K → shift right (more products formed).

• Adding products → Q > K → shift left (more reactants formed).

• Removing reactants → Q > K → shift left; Removing products → Q < K → shift right.

• Equilibrium constant (K) stays the same unless temperature changes.

How does a system respond to changes at equilibrium? (Le Chatelier’s Principle)

• Changing pressure by changing volume affects equilibrium only if gas moles differ on each side.

• Increasing volume (↓ pressure) → shifts toward side with more gas molecules.

• Adding inert gas (e.g. He) at constant volume → no effect on equilibrium (not in Q).

How does pressure or inert gas addition affect equilibrium?

• Solubility (s) = amount of solute dissolving in 1 L solvent; saturation = dynamic equilibrium.

• Ksp = equilibrium constant for dissolving ionic solids (like Kc).

• Qsp < Ksp → no precipitate; Qsp = Ksp → equilibrium; Qsp > Ksp → precipitate forms.

What is solubility and how is Ksp related to it?

• Presence of a common ion decreases solubility of a salt (shifts equilibrium left).

• New solubility: solve using s(c + s), where c = conc. of common ion already present.

What is the common ion effect and how does it affect solubility?

• Ka = [H₃O⁺][A⁻]/[HA]; Kb = [OH⁻][BH⁺]/[B]; both << 1 for weak acids/bases.

• Larger Ka or Kb → stronger acid/base (more dissociation).

• Stronger acid = weaker conjugate base and vice versa.

What are Ka and Kb, and how do they relate to acid/base strength?

• Ka·Kb = Kw at 25°C; pKa + pKb = 14.

• Smaller pKa = stronger acid; smaller pKb = stronger base.

• Conjugate acid/base strength inversely related.

What is the relationship between Ka, Kb, pKa, and pKb?

• Metal complexes like [Fe(H₂O)₆]³⁺ act as weak acids by donating protons.

• Binding to metals activates H₂O → forms OH⁻ faster (e.g. in enzymes like carboxypeptidase).

How do transition metal complexes behave in water?

• Buffer = weak acid + conjugate base (or vice versa), resists changes to pH on addition of H₃O⁺ or OH⁻.

• Acid neutralizes base; base neutralizes acid using reversible equilibria.

• Maintains pH even with dilution or addition of moderate acid/base.

What is a buffer solution and how does it resist pH change?

• Buffer capacity = amount of acid/base the buffer can absorb before pH changes significantly.

• Once either component is used up, buffer no longer resists pH change.

• E.g., 0.1 M CH₃COOH + 0.1 M NaOH forms buffer until acid is exhausted.

How does buffer capacity work and when is it exceeded?

• pH = pKa + log([A⁻]/[HA]); when [A⁻] = [HA], pH = pKa.

• Can also use mole ratio: pH = pKa + log(n_A⁻/n_HA).

How is the pH of a buffer solution calculated?

• Blood pH (7.4) is maintained by H₂CO₃/HCO₃⁻, phosphate buffer, and hemoglobin systems.

• Hyperventilation → ↓ CO₂ → ↑ pH (alkalosis); breathing into a bag restores CO₂ and pH.

• Intracellular pH is stabilized by phosphate buffers and proteins.

How do biological systems use buffers?

• Polar: has dipoles or charges, interacts with water (hydrophilic), dissolves in polar solvents.

• Non-polar: lacks dipoles/charges, hydrophobic, dissolves in non-polar solvents.

• “Like dissolves like”; alcohols are polar but don’t ionize.

What determines whether a molecule is polar or non-polar, and how does this affect solubility?

• Water's polarity enables hydrogen bonding and electrostatic interactions (hydration).

• Biological molecules (DNA, proteins) are stabilized by hydrogen bonds from water.

How does water solvate ions and polar molecules?

• pH = pKa → 50% ionized; pH < pKa → mostly protonated; pH > pKa → mostly deprotonated.

• Example: CH₃COOH ↔ CH₃COO⁻ changes with pH; pH influences charge and solubility.

How does pH affect the ionisation of functional groups in biological molecules?

• Alanine: pKa of COOH = 2.4, NH₃⁺ = 9.8.

• pH < 2.4: COOH protonated, NH₃⁺ protonated → net positive.

• 2.4 < pH < 9.8: COO⁻ and NH₃⁺ → zwitterion.

• pH > 9.8: COO⁻ and NH₂ → net negative.

How does pH affect the ionisation of amino acids?