unit 2 - water

• Water is the solvent of life. It bathes our cells, dissolves and transports compounds in the blood, provides a medium for movement of molecules into and throughout cellular compartments, separates charged molecules, dissipates heat, and participates in chemical reactions. Most compounds in the body, including proteins, must interact with an aqueous medium in order to function.
• In spite of the variation in the amount of water we ingest each day and produce from metabolism, our body maintains a nearly constant amount of water that is about 50% to 60% of our body weight.
• Composition of water in the body

  • Approximately 60% of the total body water is intracellular and 40% extracellular
  • The extracellular water includes the fluid in plasma (blood after the cells have been removed) and interstitial water (the fluid in the tissue spaces, lying between cells)
  • Transcellular water is a small, specialized portion of extracellular water that includes gastrointestinal secretions, urine, sweat, and fluid that has leaked through capillary walls due to such processes as increased hydrostatic pressure or inflammation
• Water


  A hydride of oxygen due to its elemental composition H2O
• Water has substantially higher boiling point, melting point, heat of vaporization, and surface tension compared to other hydrides that are its nearest neighbors in the periodic table, namely ammonia (NH3), hydrogen fluoride (HF), and hydrogen sulfide (H2S).
• The maximum density of water is found in the in the liquid state, not solid, unlike most matter. This is the reason why solid water (ice) floats on top of liquid water.
• Polarity of water


  The 2 hydrogens of water are covalently linked to the oxygen atom giving a non-linear arrangement, known as bent structure. In the O–H bonds of H2O, oxygen is more electronegative than hydrogen, giving rise to a partial negative charge in the oxygen atom and partial positive charge in the hydrogen atom.
• Hydrogen bond


  A non-covalent interaction (intermolecular force) formed between a hydrogen donor and a hydrogen acceptor. A hydrogen donor is a hydrogen atom covalently bonded to an electronegative atom (F, O, or N). A hydrogen acceptor is a lone pair of electrons on any of those mentioned electronegative atoms.
• Water can be both hydrogen donors and hydrogen acceptors. A water molecule has the potential to form 4 hydrogen bonds (2 donors and 2 acceptors).
• Hydrogen bonding in water is cooperative. A hydrogen bonded water molecule serving as an acceptor is a better hydrogen donor, and a hydrogen bonded water molecule serving as a donor is also a better hydrogen acceptor.
• Consequence of hydrogen bonding in water

  It endows water with its anomalously high boiling point, melting point, heat of vaporization, and surface tension. It also enables water to dissolve many organic biomolecules that contain functional groups that can participate in hydrogen bonding.
• Hydrophilic substances


  Ionic and polar substances that can be dissolved in water
• Ionic substances dissolve in water due to the polarity of water molecules, which can align themselves around ionic substances so that the partially negative oxygen atoms are oriented towards the cations and the partially positive hydrogen atoms are oriented towards the anions.
• Polar substances can be hydrated by water through dipole-dipole interaction, and may also participate in hydrogen bonding, which enhances their solubility in water.
• Hydrophobic substances


  Non-polar substances that do not dissolve in water
• Non-polar substances do not dissolve in water because they are incapable of forming dipole-dipole interaction or hydrogen bond with water, but can interact with each other through hydrophobic interaction.
• Amphipathic molecules


  Molecules that are both hydrophilic and hydrophobic, usually having a non-polar hydrocarbon tail and an ionic or polar end
• When amphipathic molecules are dispersed in water, the hydrophilic head (polar) tends to be hydrated, while the hydrophobic tail (non-polar) tends to be excluded, resulting in the formation of structurally ordered aggregates like micelles and bilayers.
• Bronsted-Lowry definition of acids and bases

  An acid is a substance that can donate a proton (H+), and a base is a substance that can accept a proton (H+)
• Acid-base reactions


  • CH3COOH + H2O ↔ CH3COO- + H3O+
  • NH3 + H2O ↔ NH4+ + OH-
• Water can act as an acid or a base in the presence of other substances, and is referred to as an amphoteric substance.
• Auto-ionization of water


  H2O ↔ H+ + OH-
• Ionization constant of water (Kw)


  Kw = [H+][OH-]
• Reversible reaction


  Reaction is in equilibrium, represented by a double-sided arrow
• Equilibrium expression


  Concentration of reactant in denominator, concentration of product in numerator
• Equilibrium constant (K)


  Describes the extent of a reversible reaction at equilibrium
• Molar concentration


  Concentration in mol/L or M, represented by quantities in square brackets
• Concentration of undissociated H2O in 1L water remains appreciably constant and larger than H+ and OH- during ionization</b>
• Concentrations of H+ and OH- in pure water are equal to 1 × 10-7 M
• Kw = [H+][OH-] = (1 × 10-7 M)(1 × 10-7 M) = 1 × 10-14 M2
• Reciprocal relationship between [H+] and [OH-]

  When [H+] > 1 × 10-7 M, [OH-] < 1 × 10-7 M, and vice versa
• Neutral solution


  [H+] = 1 × 10-7 M
• Acidic solution


  [H+] > 1 × 10-7 M
• Basic solution


  [H+] < 1 × 10-7 M
• pH


  Negative logarithm of hydrogen ion concentration, pH = -log[H+]
• Neutral solution (pure water) has pH = 7
• Solutions with pH < 7 are acidic, solutions with pH > 7 are basic
• Calculating pH of a solution with [H+] = 1.3 × 10-4 M


  pH = -log[1.3 × 10-4] = 3.9, solution is acidic
• Calculating pH of a solution with [OH-] = 1.9 × 10-5 M


  pOH = -log[1.9 × 10-5] = 4.7, pH = 14 - pOH = 9.3, solution is basic
• Strong acid


  Completely dissociates in water, reaction is not reversible