Chemistry Class 11 NCERT Solutions: Chapter 10 the s Block Elements Part 4

Q: 11. Beryllium and magnesium do not give colour to flame whereas other alkaline earth

metals do so. Why?

Answer

When an alkaline earth metal is heated, the valence electrons get excited to a higher energy level. When this excited electron comes back to its lower energy level, it radiates energy, which belongs to the visible region. Hence, the colour is observed. In Equation , the electrons are strongly bound. The energy required to excite these electrons is very high. Therefore, when the electron reverts back to its original position, the energy released does not fall in the visible region. Hence, no colour in the flame is seen.

Q: 12. Discuss the various reactions that occur in the Solvay process.

Answer

Solvay process is used to prepare sodium carbonate.

When carbon dioxide gas is bubbled through a brine solution saturated with ammonia, sodium hydrogen carbonate is formed. This sodium hydrogen carbonate is then converted to sodium carbonate.

Step 1: Brine solution is saturated with ammonia.

Equation

This ammoniated brine is filtered to remove any impurity.

Step 2: Carbon dioxide is reacted with this ammoniated brine to result in the formation of insoluble sodium hydrogen carbonate.

Equation

Equation

Step 3: The solution containing crystals of Equation is filtered to obtain Equation .

Step 4: Equation is heated strongly to convert it into Equation .

Equation

Step 5: To recover ammonia, the filtrate (after removing Equation ) is mixed with Equation and heated

Equation

The overall reaction-taking place in Solvay process is Equation

Q: 13. Potassium carbonate cannot be prepared by Solvay process. Why?

Answer:

Solvay process cannot be used to prepare potassium carbonate. This is because unlike sodium bicarbonate, potassium bicarbonate is fairly soluble in water and does not precipitate out.

Q: 14. Why is Equation decomposed at a lower temperature whereas Equation at higher temperature?

Answer:

As we move down the alkali metal group, the electropositive character increases. This causes an increase in the stability of alkali carbonates. However, lithium carbonate is not so stable to heat. This is because lithium carbonate is covalent. Lithium ion, being very small in size, polarizes a large carbonate ion, leading to the formation of more stable lithium oxide.

Equation

Therefore, lithium carbonate decomposes at a low temperature while a stable sodium carbonate decomposes at a high temperature.

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