NEET Chemistry Notes Chemical Equilibrium-Acids and Bases
Acids and Bases
Acids and Bases
An acid is that whose aqueous solution tastes sour, turns blue litmus red, neutralises bases and so on. On the other hand, the aqueous solution of a base tastes bitter, turns red litmus blue, neutralises acid and so on.
When dissolved in water, the substances which release H+ ions are called acids and which release OH– ions are called bases.
Acids are proton donors while bases are proton acceptors.
Acids are the substances which accept a pair of electrons to form a coordinate bond and bases are the substances which donate a pair of electrons to form coordinate bond.
Relative Strengths of Acids and Bases
The extent to which the acidic property is shown by an acid is a measure of its strength. The strength of an acidic solution does not depend upon its concentration but depend upon the number of H+ ions present. Thus, to measure the relative strength of the two acids, the measurements of their H+ ions concentration, i.e. the degree of dissociation of their equinormal solutions is used.
where, K1 and K2 are the dissociation constants of two acids at the same concentration C.
are their degree of dissociation.
Di- and Polybasic Acids and Di- and Polyacidic Bases
Acids which have more than one ionisable proton per molecule of acids are known as polybasic or polyprotic acids, e.g. oxalic acid (HOOC—COOH), H2S04 and H3P04. . The ionisation reactions for a dibasic acid, H2X are represented as
The corresponding equilibrium constants are as
(Ka1 Ka2 and are called first and second ionisation constants, respectively, of the acid H2X). Similarly, for tribasic acids such as H3P04, there are three ionisation constants.
It is found that Ka1 of polyprotic acid is greater than Ka2 or Ka3 (i.e. Ka1 >Ka2 >Ka3), because it is more difficult to remove a positively charged proton from a negative ion due to electrostatic forces. Similarly, it is more difficult to remove a proton from a doubly charged anion as compared to single charged anion. The primary reaction involves the dissociation of in the solution comes mainly from the first dissociation step.
Ionic Product of Water (Kw)
Pure water is a weak electrolyte and is ionised according to following equation.
where, K = ionisation constant.
Value of Kw depends upon temperature. If temperature increases, value of Kw also increases
It is used to express and compare the acidic and basic strength of a solution. It is defined as the negative logarithm of H30+ ion concentration
pH Scale Range
pH scale range is 0 to 14 and it depends upon the value of Kw. As temperature increases, value of pH decreases at 25°C. pH scale range will be
pH of very dilute acids or bases is nearly 7 but not 7 (i.e. not simply – log [acid or base]) due to ionisation of water.
pH of strong acids with concentration > 1 M is never negative, it is zero only.
pH of Mixtures of Acids and Bases
The rules for determining the pH of mixtures of acids and bases are as follows:
- If strong acid or strong base remains un used, calculate the concentration or molarity of left in the solution and then calculate the pH or pOH accordingly.
- If weak acid or weak base is left behind or remains unused, a buffer (acidic or basic) is formed. Calculate the concentration of salt formed (mmoles of salt formed/volume of solution) and the concentration of weak acid or weak base left behind.
- Use the buffer equation to calculate the pH of the solution.
- If acids or bases are completely neutralised, then salt is formed.
- Calculate the concentration of the salt formed and use the hydrolysis equation to calculate the pH of the solution.
Common Ion Effect
It states that if to the solution of a weak electrolyte, a solution of strong electrolyte is added which furnishes an ion common to that furnished by the weak electrolyte, the ionisation of the weak electrolyte is suppressed.
If NH4Cl or NaOH is added to NH4OH solution, the above equilibrium will shift to the left due to high concentration of common ion and therefore, the ionisation of NH4OH is further suppressed.
In Ilnd group of qualitative analysis, H2S is passed in the presence of HCl. This is due to the fact that HCl suppresses the ionisation of weakly dissociated H2S. Due to this only sulphides of II group radicals are precipitated. Sulphides of III, IV etc., groups are not precipitated because of their high solubility product.