Study guide: Water, pH and Buffer Solutions

📋 Course Outline

1. Water Structure and Polarity
2. Hydrogen Bonds in Water
3. Hydrophilic and Hydrophobic Interactions
4. Water Ionization and pH
5. Acids, Bases and Conjugate Pairs
6. Buffer Solutions and pKa

📖 1. Water Structure and Polarity

🔑 Key Concepts & Definitions

• Water molecule: Contains two hydrogen atoms and one oxygen atom linked by covalent bonds, with an H–O–H angle of 104.5°.

📝 Essential Points

• The water molecule has a globally neutral charge, but oxygen is more electronegative than hydrogen, giving each hydrogen a partial charge of +0.41 and oxygen a partial charge of −0.82.

💡 Memory Hook

A bent water molecule with two partially positive hydrogens and one partially negative oxygen

📖 2. Hydrogen Bonds in Water

🔑 Key Concepts & Definitions

• Hydrogen bond: An interaction in which the partial positive charge of one molecule interacts with the partial negative charge of another molecule.

★ Must-know

📌 In ice, four stable hydrogen bonds per water molecule form a crystalline network; in liquid water, hydrogen bonds are fewer and last 10−8 to 10−11 seconds; in water vapor, intermolecular interactions are absent.

Further detail

• A water molecule can theoretically form four hydrogen bonds, each with an energy of about 19 kJ·mol−1, whereas a covalent bond has an energy of about 460 kJ·mol−1.

💡 Memory Hook

Covalent bonds are strong and internal, whereas hydrogen bonds are weaker and intermolecular

📖 3. Hydrophilic and Hydrophobic Interactions

★ Must-know

• Hydrogen bonds between water molecules and ions organize oppositely charged ions in a salt crystal and promote crystal dissolution into ions.

• The water-interacting functional groups are:

  • hydroxyl
  • carbonyl
  • carboxyl
  • amine

📌 Hydrophilic molecules can form hydrogen bonds with water and tend to be soluble, whereas hydrophobic molecules are nonpolar, do not form such bonds and separate from water.

Further detail

• Hydrophobic interactions occur in:
  • oils
  • biological membranes made of phospholipids
  • nonpolar solvents
  • micelles
  • liposomes or bilayers

💡 Memory Hook

Hydrophilic substances interact with water, whereas hydrophobic substances separate from it

📖 4. Water Ionization and pH

📝 Essential Points

📐 Formula — Water ionization is represented by $\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$, and the ionic product of water is $K_e=[\mathrm{H^+}][\mathrm{OH^-}]=10^{-14}\,(\mathrm{mol/L})^2$.

📐 Formula — In pure water, $[\mathrm{H^+}]=[\mathrm{OH^-}]=10^{-7}\,\mathrm{mol/L}$ and $\mathrm{pH}=-\log_{10}[\mathrm{H^+}]=7$.

📌 A solution is neutral when [H+] equals [OH−], acidic when [H+] is greater than [OH−], and basic when [H+] is lower than [OH−].

💡 Memory Hook

Water ionizes into H+ and OH−, then pH measures H+ concentration

📖 5. Acids, Bases and Conjugate Pairs

★ Must-know

📌 A strong acid or base dissociates completely, whereas a weak acid–base pair dissociates incompletely and contains acid and base species simultaneously.

📐 Formula — For a weak acid–base pair, $K_a=\frac{[\mathrm{H^+}][\mathrm{base}]}{[\mathrm{acid}]}$ and $\mathrm{pH}=\mathrm{p}K_a+\log\frac{[\mathrm{base}]}{[\mathrm{acid}]}$, with $\mathrm{p}K_a=-\log K_a$.

• The pKa is the pH at which the concentrations of a weak acid and its conjugate base are equal.

Further detail

• Adding 10−2 mol·L−1 hydrochloric acid produces 10−2 mol·L−1 H+ and gives pH 2 because hydrochloric acid dissociates completely.

💡 Memory Hook

Strong acids and bases dissociate completely, whereas weak conjugate pairs coexist

📖 6. Buffer Solutions and pKa

🔑 Key Concepts & Definitions

• Buffer solution: A solution whose pH is only slightly affected by adding an acid or a base.

📝 Essential Points

• For an acetate/acetic acid buffer containing 50×10−3 mol·L−1 of each component and a pKa of 4.85, the initial pH is 4.85; adding 10−2 mol·L−1 HCl changes the concentrations to 40×10−3 mol·L−1 acetate and 60×10−3 mol·L−1 acetic acid, giving pH 4.67.

📌 A buffer is most effective when its pH is close to the pKa of the chosen weak acid–base pair.

💡 Memory Hook

A conjugate acid–base pair consumes added acid or base, causing limited pH variation

📊 Synthesis Tables

Acid–Base Solution Types