Question

Which of the following is correct regarding solutions of sodium metal in liquid ammonia? (1) The solutions are generally blue coloured, good conductors of electricity and conductivity decreases as temperature rises. (2) The solutions are paramagnetic but paramagnetic character decreases as concentration of solution is increased. (3) When \(\mathrm{NH}_{4} \mathrm{Cl}\) solid is added to such a solution, \(\mathrm{NaCl}\) is precipitated. (4) All the above are correct.

Short answer

All of the above are correct.

Step-by-step solution

Evaluate statement 1

Statement 1 says that solutions of sodium metal in liquid ammonia are blue colored, good conductors of electricity, and their conductivity decreases as temperature rises. This is true because the solvated electrons in the solution impart a blue color and are responsible for its high electrical conductivity, which decreases with the rise in temperature due to increased thermal motion of particles.

Evaluate statement 2

Statement 2 mentions that the solutions are paramagnetic and that the paramagnetic character decreases as the concentration increases. This is also correct. In dilute solutions, the unpaired electrons cause paramagnetism. As concentration increases, electrons pair up, reducing paramagnetic nature.

Evaluate statement 3

Statement 3 states that adding \(\text{NH}_4Cl\) to such a solution results in the precipitation of \(\text{NaCl}\). This is accurate because \(\text{NH}_4Cl\) dissociates into \(\text{NH}_4^+\) and \(\text{Cl}^-\), which then reacts with sodium to form \(\text{NaCl}\) precipitate.

Conclusion

Since statements 1, 2, and 3 are correct, statement 4 which says all of the above are correct is accurate.

Key concepts

Solution Conductivity

When sodium metal dissolves in liquid ammonia, the resulting solution shows some interesting properties, including high electrical conductivity. The solvent molecules surround the sodium atoms, freeing electrons that move through the solution. These free electrons make the solution an excellent conductor of electricity.
The solution’s characteristic blue color also comes from these free-moving solvated electrons. However, as the temperature rises, the ions and electrons in the solution move more rapidly, causing collisions that disrupt the flow of electricity, hence reducing conductivity.
The relationship between conductivity and temperature can be expressed by stating that conductivity decreases as thermal motion of particles increases. This interplay of electrons and thermal motion is a key concept in understanding the behavior of this solution.

Paramagnetic Character

Paramagnetism is a form of magnetism that occurs in materials with unpaired electrons. Solutions of sodium metal in liquid ammonia exhibit paramagnetic properties because the solvated electrons are unpaired and freely moving.
In dilute solutions, these unpaired electrons dominate the magnetic behavior, resulting in strong paramagnetism. However, as the concentration of the solution increases, the likelihood of these electrons pairing up also increases, which diminishes the solution’s paramagnetism. More concentrated solutions have fewer unpaired electrons, thus showing reduced paramagnetic character.
Understanding this concept helps in recognizing how magnetic properties can change with concentration and the role of electron pairing.

Precipitation Reaction

A precipitation reaction occurs when two dissolved substances react to form an insoluble solid, or precipitate. When \(\text{NH}_4Cl\) (ammonium chloride) is added to a solution of sodium metal in liquid ammonia, it dissociates into \[ \text{NH}_4^+ \] and \[ \text{Cl}^- \] ions. The chloride ions \(\text{Cl}^-\) react with the sodium ions \(\text{Na}^+\) in the solution to form solid sodium chloride (NaCl), which precipitates out.
The reaction can be summarized as: \[ \text{Na}^+_{(aq)} + \text{Cl}^-_{(aq)} \rightarrow \text{NaCl}_{(s)} \] This is a classic example of a precipitation reaction, where the formation of a solid from a solution signals the end of the reaction.
Understanding the conditions and outcomes of precipitation helps in various applications, including purification processes and chemical analysis.