Water, the Solvent of Life
Key Principles The solvent properties of H2O shaped the evolution of living things. The ionization behavior of H2O and of weak acids and bases dissolved in H2O can be represented by one or more equilibrium constants. An aqueous solution of a weak acid and its salt makes a buffer that resists changes in pH in response to added acid or base. Enzymes, which catalyze all of the processes inside a cell, have evolved to function optimally at near-neutral (physiological) pH
Structure and chemical properties of water Water is abundant in living organism. Interstitial [ˌɪntəˈstɪʃəl] between organs and tissues
Molecular bonds of water Water is composed of 2 H atoms covalently bonded to one O atom. The electronic structure of water is tetrahedral. • 2 covalent bonds with H atoms • 2 sets of unpaired electrons Results in a polar molecule [ˌtetrəˈhiːdrəl]
Polar [ˈpəʊlə] A molecule with uneven distribution of charge Electronegativity A measure of the affinity of an atom’s nucleus for its outmost electrons (how strong the nucleus of an atom holds on to its electrons) Electronegativity of the Atoms
Hydrogen bonds
1 Weak interactions in aqueous systems Hydrogen bonding gives H2O its unusual properties ←Covalent bond (strong) Water is made of O and H, they are covalently bound. It is a polar molecule, the electrons are not shared equally between O and H ←positively charged (partially) negatively charged (partially)→
1 Weak interactions in aqueous systems Hydrogen bonding gives H2O its unusual properties • H2O has a higher melting point, boiling point, and heat of vaporization than most of other common solvents. Hydrogen bond is Not within the molecule of H2O • Hydrogen bond = electrostatic attraction between the oxygen atom of one H2O molecule and the hydrogen of another The distance is different. O-H Between molecules, long, weaker (non-covalent) Within molecules, short, strong (covalent)
Water is polar. The atoms have dipoles: partial (δ) charges Due to unequal sharing of e- Dipoles can align to form hydrogen bond. Has some covalent character (directionality)
Strength of hydrogen bonds Hydrogen bonds are relatively weak Bond dissociation energy = ～23 kJ/mol in liquid H2O 10% covalent, 90% electrostatic Hydrogen bonds are fleeting Lifetime of each hydrogen bond is just 1 to 20 picoseconds (10-12) in liquid When one hydrogen bond breaks, another forms