Science: Acids, Bases and Salts: Role of Water in Dissociation of Acids and Bases (For CBSE, ICSE, IAS, NET, NRA 2022)

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Role of Water in Dissociation of Acids and Bases

When an acid like sulphuric acid or a base like sodium hydroxide is dissolved in water, the solution that is formed is hotter. It shows that the dissolution process is exothermic. A part of the thermal energy which is released during the dissolution process is used up in overcoming the forces holding the hydrogen atom or hydroxyl group in the molecule of the acid or the base in breaking the chemical bond holding them and results in the formation of free H + (aq) and OH – (aq) ions.

Dissociation of Acids and Bases

Many bases are ionic compounds and consist of ions even in the solid state. For example, sodium hydroxide consists of Na + and OH – ion. These ions are held very tightly due to the strong electrostatic forces between the oppositely charged ions. Presence of water as a medium (solvent) weakens these forces greatly and the ions become free to dissolve in water.

Self-Dissociation of Water

Water Helps in the dissociation of acids and bases resulting in the formation of H + (aq) and OH – (aq) ions, respectively. Water itself undergoes dissociation process which is called ′ self-dissociation of water.

Water dissociates into H + (aq) and OH – (aq) ions as: H2O (l) H + (aq) + OH – (aq)

  • pH is defined as + log [H +] or – log [H +] , likewise pOH = – log [OH –] and pKw = – log Kw In pure water or in any aqueous solution pH + pOH = pKw = 14 at 25°C. In pure water [H +] = [OH –] .
  • It is also true in any neutral aqueous solution. In terms of pH, pH = pOH = 7 in water and any neutral solution.
  • In acidic solution [H +] > [OH –] and pH < pOH. Also, pH < 7 at 25°C.
  • In basic solutions [H +] < [OH –] and pH > pOH. Also, pH > 7 at 25°C

PH Scale

we use the pH scale for denoting the concentration of hydrogen ions. pH notation was devised by the Danish biochemist Soren Sorensen in 1909. The term pH means “power of hydrogen” .

Importance of pH

  • Human body works at a pH of about 7.4.
  • Stomach has a pH of about 2 due to presence of hydrochloric acid in it. It is needed for the activation of pepsin protein required for protein digestion.
  • When we eat food containing sugar, then the bacteria present in our mouth breakdown the sugar to form acids. This acid lowers the pH in the mouth. Tooth decay starts when the pH of acid formed in the mouth falls below 5.5. This is because then the acid becomes strong enough to attack the enamel of our teeth and corrode it. This sets in tooth decay. The best way to prevent tooth decay is to clean the mouth thoroughly after eating food.
  • Many animals and plants protect themselves from enemies by injecting painful and irritating acids and bases into their skin.
  • When honeybee stings a person, it injects an acidic liquid into the skin. Rubbing with mild base like baking soda solution on the stung area of the skin gives relief.
  • When a wasp stings, it injects an alkaline liquid into the skin. Then rubbing with a mild acid like vinegar on the stung area of the skin gives relief.
  • Soil pH and plant growth: Most of the plants grow best when the pH of the soil is close to 7. If the soil is too acidic or basic, the plants grow badly or do not grow at all. The soil pH is also affected by the use of chemical fertilizers in the field. Chemicals can be added to soil to adjust its pH and make it suitable for growing plants. If the soil is too acidic then it is treated with materials like quicklime or slaked lime. If the soil is too alkaline then alkalinity can be reduced by adding decaying organic matter.

Salts

Salts are ionic compounds made of a cation other than H + ion and an anion other than OH – ion. When acid and base neutralize, salts are formed. Strong acid and strong base combines to form neutral salt.

NaOH + HCl ⇾ NaCl + H2O

Eq. 1. Formation of Neutral Salt

Strong acid and weak base combine to form acidic salt. For Example, Hydrochloric Acid and ammonium hydroxide combine to form ammonium chloride. Other examples, sodium hydrogen carbonate, sodium hydrogen sulphate etc.

HCl + NH4OH ⇾ NH4Cl + H2O

Eq. 2. Formation of Acidic Salt

Similarly, weak acid and strong base combine to form basic salt. For Example, Acetic Acid and sodium hydroxide combine to form sodium acetate. Other examples are calcium carbonate, potassium cyanide etc.

CH3COOH + NaOH ⇾ CH3COONa + H2O

Eq. 3. Formation of Basic Salt

The most common salt is table salt or sodium chloride (NaCl) .

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