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Chapter 10: Problem 9

Explain the following: (a) Aqueous solutions of \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) is alkaline. (b) Alkali metals dissolve in liquid \(\mathrm{NH}_{3}\) to give blue-coloured solutions. (c) \(\mathrm{NaHCO}_{3}\) exists as a solid while LiHCO \(_{3}\) exists only in aqueous solution.

Short Answer

Expert verified
\(\mathrm{Na}_{2} \mathrm{CO}_{3}\) solutions are alkaline due to the release of \(\mathrm{OH}^{-}\) ions from weak acids. Alkali metals give a blue colour in \(\mathrm{NH}_{3}\) as they donate electrons that absorb energy in the red light spectrum, making the solution appear blue. \(\mathrm{NaHCO}_{3}\) exists as a solid due to its high lattice energy, while LiHCO \(_{3}\) exists in aqueous solution due to the high hydration enthalpy of the Li+ ion.

Step by step solution

01

Explain why aqueous solutions of \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) is alkaline

When \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (Sodium Carbonate) is dissolved in water, it disassociates into Sodium ions \(\mathrm{Na}^{+}\) and Carbonate ions \(\mathrm{CO}_{3}^{-}\). Carbonate ions are derived from a weak acid (Carbonic Acid) and they can react with water molecules to produce hydroxide ions \(\mathrm{OH}^{-}\), making the solution alkaline.
02

Explain why alkali metals dissolve in liquid \(\mathrm{NH}_{3}\) to give blue-coloured solutions

When alkali metals dissolve in ammonia, they donate their valence electrons to the ammonia molecules. Those free electrons can move freely in the solution, absorbing energy in the red part of the light spectrum. The fact that red light is absorbed makes the solution appear blue.
03

Explain why \(\mathrm{NaHCO}_{3}\) exists as a solid while LiHCO \(_{3}\) exists only in aqueous solution

This is a result of the difference in hydration enthalpy and lattice energy between these two compounds. In the case of \(\mathrm{NaHCO}_{3}\), the lattice energy is sufficiently high for it to exist as a solid, while for LiHCO \(_{3}\), the high hydration enthalpy of the Li+ ion means that it's more energetically favourable for it to exist in aqueous solution.

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Most popular questions from this chapter

Write complete balanced equation for the reaction of the following compounds with water. (a) \(\mathrm{Li}_{2} \mathrm{O}\) (b) NaH (c) \(\mathrm{NaNH}_{2}\) (d) \(\mathrm{C}_{2} \mathrm{H}_{2}\) (e) \(\mathrm{Na}_{2} \mathrm{O}_{2}\)

What are crown ethers and cryptates?

Give reason for the following: (a) Group 1 elements are strong reducing agents but poor complexing agents. (b) Caesium is used in photoelectric cells. (c) Sodium metal is kept under kerosene oil. (d) Lithium shows anomalous behaviour. (e) LiF is insoluble in water.

Justify the following: (a) Lithium forms the normal oxide whereas potassium forms the super oxide an burning in air. (b) The melting point gap between lithium and sodium is maximum.

Explain why lithium is the strongest reducing agent in spite of its high ionisation energy.
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