|
Water makes up about 60-95% of fresh mass of an organism. Water is an important component in cells, act as a solvent an is often a reactant in metabolism and provides an aqueous environment for ma organisms. Water molecule has a "V" shape and is a dipolar molecule. There is uneven charge distribution within the molecule. The oxygen atom has a partial negative charge while the hydrogen atoms have partial positive charge. Hydrogen bond are formed between the negatively changed oxygen atom and the positively charged atoms of other water molecules in their liquid state. These hydrogen bonds form, break and reform with great frequency. A single water molecule can be boned to four other water molecules by hydrogen binds. At 37°C, about 15% of water molecules are bonded to each other by hydrogen bonds. In ice, all the water molecule bonded by hydrogen bonds; hence ice have a slight larger volume than the water forming it.
|
solvent properties of water
The great solvent power of water results from its polarity and H-bonding ability:
- It dissolves ionice compounds through ion-dipole forces that separate the ions from the solid and keep them in solution
- It dissolves polar nonionic substances, such as ethanol (CH3CH2OH) and glucose (C6H12O6), by H bonding.
- It dissolves non polar atmospheric gasses to a limited extend through dipole-induced dipole and dispersion forces.
thermal properties of water
When a substance is heated, some of the added energy increases average molecular speed, some increases molecular vibration and rotation, and some is used to overcome intermolecular forces.
- Specific Heat Capacity. Because water has so many strong H bonds, its specific heat capacity is higher than any common liquid, With oceans covering 70% of Earth's surface, daytime energy from the Sun causes relatively small changes in temperature, allowing life to survive. On the waterless, aimless Moon, temperatures range from 100°C to -150°C during a complete lunar day. Even in Earths deserts, day-night temperature differences of 40°C are common.
- Heat of vaporisation. Numerous strong H bonds give water a very high heat of vaporisation. The average adult has 40kg of body water and generates about 10000kJ of heat a day from metabolism. If this heat were used to increase the average Ek of water molecules in the body, the rise in body temperature of tens of degrees would mean immediate death. Instead, the heat is converted to Ep as it breaks H bonds and evaporate sweat, resulting in a stable body temperature and minimal loss of body fluid.
- Latent heat of fusion. Water has a high latent heat of fusion. Aquatics habitats are slow to freeze. A high heat of fusion means that a lot of heat has to be expelled from the water at 0°C before it could freeze. Content of cell in living things are also slow to freeze.
auto ionisation of water
Water behave both as an acid and a base: in aqueous solution a small fraction of the water molecules undergo acid-base reactions with each other to produce the equilibrium:
This is known as the auto-ionisation of water. The equilibrium expression for this reaction can be written:
The concentration of water under these conditions is not changed significantly, since the proportion of water which dissociates into its ions is small. The water concentration can thus be assumed to be constant and it can be incorporated into Kc:
The expression [H3O+][OH-] is known as the ionic product of water and has a value of 1.00 x 10-14 mol2 dm-6 at 25°C. This value is constant at a given temperature. The ionic product product of water s slightly higher at higher temperatures, suggesting that the dissociation is endothermic.
In pure water, the [H3O+] and [OH-] concentrations are equal. It follows that [H3O+][OH-] = [H3O+]2 = Kw.
So [H3O+] = [OH-] = Kw = 1.0 x 10 x -7 mol dm-3.
Any solution in which the concentrations of H3O+ and OH- are equal is said to be neutral.
In pure water, the [H3O+] and [OH-] concentrations are equal. It follows that [H3O+][OH-] = [H3O+]2 = Kw.
So [H3O+] = [OH-] = Kw = 1.0 x 10 x -7 mol dm-3.
Any solution in which the concentrations of H3O+ and OH- are equal is said to be neutral.