CBSE Class 12 Chemistry Chapter 12: Aldehydes, Ketones and Carboxylic Acids NCERT Solutions

Solution: Hydrometallurgy involves the use of aqueous solutions and a more reactive metal to displace a less reactive metal. The feasibility of this process depends on the standard reduction potentials of the metals involved.

Copper has a higher standard reduction potential (+0.34 V) compared to zinc (-0.76 V) and iron (-0.44 V). This means that copper ions are more easily reduced to copper metal than zinc or iron ions are.

In hydrometallurgy, a metal with a lower reduction potential (more reactive) can displace a metal with a higher reduction potential from its salt solution. For example, iron (Fe) can displace copper (Cu²⁺) from a copper sulfate solution because iron is more reactive than copper:

$$Fe\_\{(s)\}\; +\; Cu^\{2+\}\_\{(aq)\}\; \backslash longrightarrow\; Fe^\{2+\}\_\{(aq)\}\; +\; Cu\_\{(s)\}$$

However, to displace zinc from its salt solution (e.g., Zn²⁺), a metal with a reduction potential even lower than that of zinc is required. Metals like magnesium (Mg), calcium (Ca), or potassium (K) are more reactive than zinc. But these highly reactive alkali and alkaline earth metals react vigorously with water, producing hydrogen gas and their respective hydroxides, rather than displacing zinc from an aqueous solution:

$$2K\_\{(s)\}\; +\; 2H\_2O\_\{(l)\}\; \backslash longrightarrow\; 2KOH\_\{(aq)\}\; +\; H\_\{2(g)\}$$

Because these highly reactive metals cannot be used in aqueous solutions for displacement, zinc cannot be effectively extracted by hydrometallurgy using common displacement methods. Copper, on the other hand, can be readily extracted from its ore solutions using a more reactive metal like iron.

Solution: In the froth flotation process, a depressant is a chemical substance added to selectively prevent one of the valuable minerals from floating, thereby allowing the other mineral to be separated in the froth. This is crucial when separating two or more sulphide ores that have different affinities for the collector or air bubbles.

For instance, when separating zinc sulphide (ZnS) and lead sulphide (PbS), sodium cyanide (NaCN) is often used as a depressant. Sodium cyanide selectively reacts with zinc sulphide, forming a soluble complex ion, sodium tetracyanozincate(II) ([Zn(CN)₄]^2-), which remains in the solution and does not attach to the air bubbles. Lead sulphide, however, does not form such a stable complex and can attach to the collector and float to the surface with the froth.

The reaction showing the complex formation with ZnS is:

$$4NaCN\; +\; ZnS\; \backslash longrightarrow\; Na\_2[Zn(CN)\_4]\; +\; Na\_2S$$

Thus, the depressant ensures that only the desired ore (in this case, PbS) is collected in the froth, leading to a purer separation.

Solution: The ease of extraction of a metal by reduction depends on the Gibbs free energy of formation (Δ_fG) of its oxide or sulphide. A more negative Δ_fG indicates a more stable compound, making it harder to reduce.

For copper extraction from its sulphide ore (e.g., Cu₂S), the Gibbs free energy of formation of Cu₂S is relatively low (less negative) compared to the Gibbs free energy of formation of the reducing agent's sulphide (like H₂S or CS₂). This means that reducing agents like hydrogen (H₂) or carbon (C) do not have a sufficiently negative Δ_fG for their corresponding sulphides to effectively reduce Cu₂S to copper metal. In simpler terms, Cu₂S is quite stable and not easily reduced by common reducing agents like H₂ or C.

On the other hand, when extracting copper from its oxide ore (Cu₂O), the Gibbs free energy of formation of Cu₂O is less negative than that of carbon monoxide (CO). This relationship allows carbon (C) to act as an effective reducing agent for Cu₂O. The Δ_fG for CO formation is more negative than that for Cu₂O formation, making the reduction thermodynamically feasible:

$$C\_\{(s)\}\; +\; Cu\_2O\_\{(s)\}\; \backslash longrightarrow\; 2Cu\_\{(s)\}\; +\; CO\_\{(g)\}$$

Therefore, the extraction of copper from its oxide ore by reduction with carbon is significantly easier than reducing its sulphide ore.

Solution: Zone refining is a highly effective purification technique used for obtaining ultra-pure metals, particularly those used in semiconductors and electronics. It is based on the principle that impurities are generally more soluble in the molten state of a metal than in its solid state.

The process involves taking a rod of the impure metal and moving a circular heater along its length. This heater creates a narrow molten zone (melt) of the metal. As the heater moves from one end to the other, the molten zone also moves with it. In this molten zone, the impurities, being more soluble, concentrate in the liquid phase. As the molten zone moves away, the pure metal solidifies behind it, leaving the impurities concentrated in the molten zone. This process is repeated multiple times by moving the heater back and forth or by moving the rod through a stationary heater. With each pass, a larger proportion of the impurities gets collected at one end of the rod. Finally, the end of the rod containing the concentrated impurities is cut off, yielding a highly purified metal rod.

This method is particularly useful for purifying elements like silicon, germanium, boron, gallium, and indium.

Solution: Column chromatography is a powerful separation and purification technique used in chemistry to separate components of a mixture based on their differential adsorption onto a stationary phase as a mobile phase moves through it.

The principle behind column chromatography is that different substances in a mixture exhibit varying degrees of adsorption onto the stationary phase and solubility in the mobile phase. The stationary phase, typically a solid adsorbent like alumina (Al₂O₃) or silica gel (SiO₂), is packed into a vertical column. The mobile phase, which can be a liquid solvent or a mixture of solvents, is allowed to flow through the column, carrying the mixture to be separated. The components of the mixture distribute themselves between the stationary and mobile phases. Components that are more strongly adsorbed onto the stationary phase move down the column more slowly, while those that are less strongly adsorbed and more soluble in the mobile phase move down faster.

As the mobile phase flows continuously, the components separate into distinct bands along the column. These separated bands can then be collected individually by eluting them from the column with a suitable solvent and collecting the fractions that emerge from the bottom. This technique is highly effective for purifying elements and compounds, especially when the impurities have chemical properties similar to the desired substance, and it is particularly useful for separating small quantities of substances.