The polarity of water
Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen.Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge () near the oxygen atom due the unshared pairs of electrons, and partial positive charges () near the hydrogen atoms.
An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration.
The ability of ions and other molecules to dissolve in water is due to polarity. For example, in the illustration below sodium chloride is shown in its crystalline form and dissolved in water.
An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration.
The ability of ions and other molecules to dissolve in water is due to polarity. For example, in the illustration below sodium chloride is shown in its crystalline form and dissolved in water.
Many other unique properties of water are due to the hydrogen bonds. For example, ice floats because hydrogen bonds hold water molecules further apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. The unique physical properties, including a high heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent properties of water are also due to hydrogen bonding. The hydrophobic effect, or the exclusion of compounds containing carbon and hydrogen (nonpolar compounds) is another unique property of water caused by the hydrogen bonds. The hydrophobic effect is particularly important in the formation of cell membranes. The best description is to say that water "squeezes" nonpolar molecules together.
Thermal and solvent properties in water
Thermal Properties
Cohesive Properties
Solvent Properties
Other Properties
Water undergoes auto-ionization according to the following equation:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
or
2 H2O(l) H3O+(aq) + OH-(aq)
The equilibirum expression for the above reaction is written below and is treated mathematically like all equilibrium expressions.
Kw = [H3O+][OH-]
At 25oC, the value of Kw has been determined to be 1 x 10-14. This value, because it refers to the auto-ionization of water, has been given a special symbol, Kw, but, it is just a special case of Kc.
If one knows the concentration of either the hydronium ions or of the hydroxide ions in a water solution,. the other ion concentration can be determined.
Thermal and solvent properties in water
Thermal Properties
- Water has a high specific heat capacity (the measure of energy required to raise the temperature of 1 g of substance by 1°C)
- Water has a high heat of vaporisation (amount of energy absorbed per gram as it changes from a liquid to a gas / vapour)
- Water has a high heat of fusion (amount of energy required to be lost to change 1 g of liquid to 1 g of solid at 0°C)
- These properties occur as a result of the extensive hydrogen bonding between water molecules - this allows water to absorb considerable amounts of energy with little change in form (H-bonds need to be broken first)
Cohesive Properties
- Water molecules are strongly cohesive (they tend to stick to one another)
- Water molecules will also tend to stick to other molecules that are charged or polar (adhesion)
- These properties occur as a result of the polarity of a water molecule and its ability to form hydrogen bonds with appropriate molecules
Solvent Properties
- Water can dissolve many organic and inorganic substances that contain electronegative atoms (such as fluorine, oxygen and nitrogen)
- This occurs because the polar attraction of large quantities of water molecules can sufficiently weaken intramolecular forces (such as ionic bonds) and result in the dissociation of the atoms
Other Properties
- Water is transparent, allowing light to pass through it (important for photosynthesis)
- Water expands when frozen, becoming less dense / lighter (important for life on earth - oceans don't freeze)
Water undergoes auto-ionization according to the following equation:
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
or
2 H2O(l) H3O+(aq) + OH-(aq)
The equilibirum expression for the above reaction is written below and is treated mathematically like all equilibrium expressions.
Kw = [H3O+][OH-]
At 25oC, the value of Kw has been determined to be 1 x 10-14. This value, because it refers to the auto-ionization of water, has been given a special symbol, Kw, but, it is just a special case of Kc.
If one knows the concentration of either the hydronium ions or of the hydroxide ions in a water solution,. the other ion concentration can be determined.
Example: What is the hydronium ion concentration in a water solution that is 0.050 M NaOH?
The NaOH is a strong base and will 100% dissociate into its component ions. Therefore, the concentration of the hydroxide ions will be 0.050 M. The hydronium ion concentration is then calculated.
[H3O+] = (1 x 10-14)/(0.050) = 2.0 x 10-13 M
Example: What is the hydroxide ion concentration in a water solution that is 4.0 x 10-5 M H3O+?
[OH-] = (1 x 10-14)/(4.0 x 10-5) = 2.5 x 10-10 M
The NaOH is a strong base and will 100% dissociate into its component ions. Therefore, the concentration of the hydroxide ions will be 0.050 M. The hydronium ion concentration is then calculated.
[H3O+] = (1 x 10-14)/(0.050) = 2.0 x 10-13 M
Example: What is the hydroxide ion concentration in a water solution that is 4.0 x 10-5 M H3O+?
[OH-] = (1 x 10-14)/(4.0 x 10-5) = 2.5 x 10-10 M