Water and pH

Biochemistry is the most important part of biology, that is important to understand the living creatures. Biochemistry is the study of the chemical substances and vital processes occurring in living organisms. Biochemists focus heavily on the role, function, and structure of bio molecules. The study of the chemistry behind biological processes and the synthesis of biologically active molecules are examples of biochemistry.

So i will be discussing biochemistry now, 

Lets Start with Water and pH

Water

• The physical and chemical properties of water make it remarkably suitable for its numerous functions in living organisms.
• The water molecule (H2O) is composed of two atoms of hydrogen and one of oxygen.
• Each hydrogen atom is linked to the oxygen atom by a single covalent atom.
• The oxygen atom bears a slight negative charge and each of the two hydrogen atoms bears a slightly positive charge. There is a separation of charge within a molecule.
• The electron distribution is oxygen-hydrogen bonds described as polar or asymmetrical.
• Water molecule is bent; the bond angle is 104.5 degree.
• Molecules such as water in which charges are separated, are called dipoles. Such molecules have opposite charges on two ends.
  
  
  

Properties of water

• Water is a polar molecule.
• Water dissolves almost anything. More substances dissolve in water than in any other liquid. For this reason, water is often called the universal solvent. The reason for water's excellent dissolving capability relates to its polarity; water offers positive and negative charges to which other atoms of molecules can attach.
• Water exists in three physical states. Water can be a liquid, a gas vapour, or a solid (ice). Water turns into a vapour at 100°C and turns into ice at 0°C at standard atmospheric pressure.
• Water has a neutral pH. Pure water has a pH of 7, which is neutral.
• Water has the highest latent heat of vaporization. The heat needed to change water from a liquid to a gas is called the latent heat of vaporization. To raise the temperature of one gram of water from 0 degrees to 100 degrees would require 100 calories of heat. However-, to change one gram of water from a liquid to a gas requires 540 calories. During this process there is no change in temperature; that is only a change in the physical state of the water as it turns from a liquid to a gas.
• Water has a high latent heat of fusion. The heat removal required to change water into ice is called the latent heat of fusion. For one gram of water 80 calories of heat must be removed.
• Water has a high heat capacity. Water has the highest heat capacity than any liquid, except ammonia. The heat capacity of a substance is defined as the amount of heat that is required to raise the temperature of 1 gram of a substance by one degree.
• Water has a high surface tension

Ionization of water

Liquid water molecules have a limited capacity to ionize to form a hydrogen ion (H') and a hydroxide ion (OH-). (H' does not actually exist in aqueous solution. In water, a proton combines with a water molecule to form the hydrated hydrogen ion, H3O+, commonly called a hydronium ion. For convenience, however, hydrated proton is usually represented as H').

The dissociation of water,

H2O 🡪 H+  + OH-

It may be expressed according to the concepts of the law of mass action as follows:

K = [H1 [OH]

[1-1,0]

Where Keg is the equilibrium constant for the reaction. Since the molar concentration of pure water (55.5 8) is considerably larger than that of any solutes, it too is considered a constant. (The concentration of water is obtained by dividing the number of grams in 1 litre of water, 1000g, by the molecular weight of water, 18 g/mole). After this value is substituted into above equation, it may be rewritten as follows:

Keq x 55.5 = [H-] [OH-)

The term Keg x 55.5 is called the ion product of water (Kw). Since the equilibrium constant for the reversible ionization of water is equal to 1.8 x 10'6 M (at 25°C), the above relationship yields

Kw = (1.8 x 10-16) (55.5) = [H'] [OH-) = 1.0x10-"

This means that the ionic product of [H'] and [OH-] in any water solution (at 25°C) is always lx10-.4. Since [H'] is equal to [OH-) when pure water dissociates,

[H'] = [OH-] = 1x10-7M

Thus the hydrogen ion concentration of pure water is equal to 1x10-7 M.

When a solution contains equal amounts of hi+ and OH-, it is said to be neutral. When an ionic or polar substance is dissolved in water, it may change the relative numbers of H' and OH-. Solutions with an excess of H. are acidic, while those with a greater number of OH- are basic. Hydrogen ion concentration varies over a very wide range: commonly between 10° and 10-".

The Arrhenius Theory of acids and bases 

• Acids are substances which produce hydrogen ions in solution.
• Bases are substances which produce hydroxide ions in solution.

The Bronsted-Lowry Theory of acids and bases

• An acid is a proton (hydrogen ion) donor.
• A base is a proton (hydrogen ion) acceptor.

The Lewis Theory of acids and bases

• An acid is an electron pair acceptor.
• A base is an electron pair donor.

A weak acid is an acid that is partially dissociated into its ions in an aqueous solution or water. In contrast, a strong acid fully dissociates into its ions in water. The conjugate base of a weak acid is a weak base, while the conjugate acid of a weak base is a weak acid. At the same concentration, weak acids have a higher pH value than strong acids.

The pH scale conveniently expresses hydrogen ion concentration. pH is defined as the negative logarithm of the concentration of the hydrogen ions.

pH=-log[H+]

On pH scale , neutrality is defined as 7.

Solutions with pH less than 7, are acidic and those with more than 7 are basic or alkaline.

Henderson-Hasselbalch equation is,

pH = pKa + log([A-]/[HA])

In the next article we will discuss about Buffers.

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