Acidic, Basic and Neutral materials of Class 7
What is Neutralisation
We have learnt that acids turn blue litmus red and bases turn red litmus blue. Let us see what happens when an acid is mixed with a base. We are going to use an indicator you have not used so far.
When a acid reacts with base a salt and water produce
Example of Neutralisation
HCl(aq) + NaOH (aq) NaCl(aq) + H2O(l)
Basically Neutralisation is the combination between H+ ions of the acid with OH- ions of the base to form H2O.
H+(aq) + Cl−(aq) + Na+(aq) + OH−(aq) Na+(aq) + Cl−(aq) + H2O(l)
H+(aq) + OH−(aq) H2O(l)
Neutralisation reaction involving an acid and base is of exothermic nature. Heat is evolved in all neutralisation reactions. If both acid and base are strong, the value of heat energy evolved remains same irrespective of their nature.
Strong acids and strong bases are completely ionised of their own in the solution, NC energy is needed for their ionisation. Since the cation of base and anion of acid on both sides of the equation cancels out completely, the heat evolved is given by the following reaction
H+(aq) + OH−(aq) H2O(l) + 57.1 kJ
If either the acid or base is weak, some energy is needed for its ionization. As a result, heat evolved is comparatively less.
In this case heat energy needed for the ionization of per mole of NH4OH is (57.1 − 51.5) = 5.6 kJ
NH4OH (aq) →
Applications of neutralisation:
People particularly of old age suffer from acidity problems in the stomach which is caused mainly due to release of excessive gastric juices containing HCI. The acidity is neutralised by antacid tablets which contain sodium hydrogen carbonate (baking soda), magnesium hydroxide etc.
The stings of bees and ants contain formic acid. Its corrosive and poisonous effect can be neutralised by rubbing soap which contains NaOH (an alkali).
The stings of wasps contain an alkali and its poisonous effect can be neutralised by an acid like acetic acid (present in vinegar).
Farmers generally neutralize the effect of acidity in the soil caused by acid rain by adding slaked lime (Calcium hydroxide) to the soil.
Neutralisation in everyday life
Indigestion : Our stomach contains hydrochloric acid. It helps us to digest food, But too much of acid in the stomach causes indigestion. Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid.
Ant sting : The sting of an ant contains formic acid. When an ant bites, it injects the acidic liquid into the skin. The effect of the sting can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.
Soil treatment : Soil treatment Excessive use of chemical fertilisers makes the soil acidic. Plants do not grow well when the soil is either too acidic or too basic. When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter is added to it. Organic matter releases acids which neutralises the basic nature of the soil.
Factory wastes : The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances.
Questions on Neutralisation
Question1. A reaction between acid and base present in aqueous solution to form salt and water is known as………………….
Question2. Heat energy needed for the ionization of per mole of NH4OH is ………………..
Question3. ………………. in the stomach which is caused mainly due to release of excessive gastric juices containing HCI.
Question4. The sting of an ant contains …………………
Solution: 1. Neutralisation
Solution: 2. 5.6 kJ
Solution: 3. Acidity
Solution: 4. Formic Acid