Hydrogen bonds6-HHEEEEEo+o+5+6?6H福OH8+8+
Hydrogen bonds
1 Weakinteractionsinagueous systemsHydrogen bonding gives H,O its unusual properties-positivelycharged (partially)-Covalentbond(strong)：6negativelycharged (partially)Covalentbond (strong)H68+(a)Water is madeofOand H,theyarecovalentlybound.It isapolarmolecule,theelectronsarenotshared equallybetweenOandH
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 systemsHydrogen bonding gives H,O its unusual propertiesH,Ohasahighermeltingpoint104.5°boiling point, and heat of vaporizationthanmostofothercommonsolventsHydrogenbond isNotwithinthemoleculeof H,OHydrogen bond0.177nmThedistance0isdifferentHydrogenbond=electrostaticattractionCovalentbondbetweentheoxygen atomof oneH,O0.0965nmmoleculeandthehydrogenofanotherH8+O--H(b)(a)Betweenmolecules,long,weaker(non-covalent)Within molecules,short,strong (covalent)
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)
8*WaterispolarHThe atoms have dipoles: partial () chargesO-8HDuetounequal sharing ofeS+Dipoles can align to form hydrogen bondHas some covalentcharacter (directionality
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)
StrengthofhydrogenbondsHydrogen bonds are relatively weakBond dissociation energy= ～23kJ/mol in liquid H,010% covalent, 90% electrostaticHydrogen bonds are fleetingLifetime of each hydrogen bond is just 1 to 20picoseconds (10-12) in liquidWhen one hydrogen bond breaks, another forms
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