Water

Seite 1

Water is the most abundant molecules in most cells - 60% to 90% of the mass of the cell. Macromolecules components of cells assume their characteristic shapes in response to water. Lots of metabolic machinery of cell operates in the presence of water because it is an essential solvent. Physical properties of water allows it to act as a solvent for ionic & polar substances and it's chemical properties allow it to form weak bonds with other compounds and also itself. Water is also related to the function of macromolecules.

Important ! --> Water is not just an inert solvent, it's also a substrate for many cellular reactions.

1) The water molecule is polar.

It is V shaped, angle between 2 covalent bonds O-H is 104.5 degrees

8 electrons, 2 in inner shell 4 in outer shell. Outer shell can accommodate 4 electrons in 1 s orbital & 3 p orbital. Electrons in outer shell is assumed to occupy 4 sp3 hybrid orbitals.

If the electron orbitals sp3 really point out to be a tetrahedral, the original angle would be 109.5 but because of the strong repulsion between the lone pairs pushes the covalent orbitals together ---> 104.5 degrees.

Oxygen is more electronegative than Hydrogen

Oxygen nucleus attract electron more strongly compared to single neutron in Hydrogen nucleus causing an uneven distribution of charge in the covalent bond O-H.

O = Partial negative charge, H = Partial positive charge. === DIPOLE, the bond is polar.

Angled arrangement of polar O-H bonds of water create PERMANENT DIPOLE for the molecule as a whole.

2) Hydrogen Bonding in water

Polar, water molecules attract each other.

1 hydrogen, slightly positive, is attracted to electron pairs in one of the sp3 orbitals, slightly negative, creating a hydrogen bonds.

Hydrogen, remains covalently bonded to it's oxygen atom = Hydrogen donor. It's attracted to another oxygen atom = Hydrogen acceptor.

Distance, hydrogen bond, twice the length of covalent bond.

20 kJ mol-1 of energy is required to break a H bond between water molecules.

Covalent bond, O-H 460 kJ mol-1, C-H 410 kJ mol-1, H bond 20 kJ mol -1. Less than 5% compared to covalent bond. H bonds are weak interactions.

Orientation, H bonds most stable when the atoms are aligned. In line.

Water can form 4 H bonds. Receives 2 H atoms from 2 other water molecule and donate 2 hydrogen to 2 other water molecules. 1 hydrogen, participate only in 1 H bond.

ICE

Every molecule participate in H bonds creating a tetrahedral shape. Bond angles are 109.5 degrees. Polarity is cancelled by the presence of hydrogen bonds.

Energy to break H bond in ice = 23 kJ mol-1, stronger than liquid water.

Ice has high melting point due to the strength of H bonds and the amount of H bonds in ice. More energy needed to disrupt the H bonded lattice of ice.

WATER

Most can form up to 4 H bonds but mostly at a given time it only forms 2 or 3. Structure of liquid water is less ordered than ice.

Water, fluidity = constant fluctuating pattern of H bonds, break and reform. Some have 2,3,4 bonds. Some have only 1, or none.

DENSITY

Like many other, density of water increase as it cool up to 1.000g ml-1 at 4 celsius. After that water expands.

Expand, caused by open H bonded ice crystal. Each molecule bonded to 4 others. Ice = 0.924 g ml-1, less dense than liquid. Causes it to float. Biological implication = layer of ice insulates the creatures below from extreme cold.

SPECIFIC HEAT

amount of heat required to raise the temp of 1 g of subs by 1 celsius.

Water, high heat capacity. More energy required to break multiple H bonds in order to increase the kinetic energy of water molecules. Water in body maintains because temp fluctuation is minimised.

HEAT OF VAPORISATION

Large amount of heat, required to convert water liquid to gas. H bonds needs to be broken. Water molecules needs to dissociate from each other.

Seite 2

WATER - AN EXCELLENT SOLVENT.