CBSE Class 12 Chemistry Chapter 9: Surface Chemistry NCERT Solutions

Solution:

Chemisorption, also known as chemical adsorption, is characterized by the formation of chemical bonds between the adsorbent and the adsorbate. Here are two key characteristics:

1. High Specificity: Chemisorption is highly specific. It occurs only when there is a strong possibility of forming chemical bonds between the surface of the adsorbent and the molecules of the adsorbate. For example, oxygen is adsorbed on metals like tungsten and iron due to the formation of metal oxides.
2. Surface Area Dependence: Similar to physisorption, chemisorption also increases with an increase in the surface area of the adsorbent. A larger surface area provides more sites for chemical bonding to occur.

Solution:

Physisorption is an exothermic process, meaning it releases heat when adsorption occurs. According to Le-Chatelier's principle, if a change of condition (like temperature) is applied to a system in equilibrium, the system will shift in a direction that relieves the stress. In the case of physisorption, increasing the temperature adds heat to the system. To relieve this stress, the equilibrium shifts in the reverse direction (desorption), causing the extent of physisorption to decrease. Therefore, physisorption is more favorable at lower temperatures.

Solution:

Powdered substances are more effective adsorbents because they possess a significantly larger surface area compared to their crystalline counterparts. When a substance is broken down into fine powder, its total surface area exposed to the surrounding medium increases dramatically. Since the extent of adsorption (both physisorption and chemisorption) is directly proportional to the surface area of the adsorbent, powdered substances exhibit greater adsorbing power.

Solution:

In the Haber's process for the synthesis of ammonia ($N\_2\; +\; 3H\_2\; \backslash rightleftharpoons\; 2NH\_3$), an iron catalyst is used. Carbon monoxide (CO) is a common impurity that can poison the iron catalyst. CO adversely affects the activity of the iron catalyst by strongly adsorbing onto its surface, blocking the active sites required for the adsorption and reaction of nitrogen and hydrogen. Therefore, it is essential to remove CO to maintain the efficiency and longevity of the catalyst.

Solution:

The hydrolysis of an ester in the presence of an acid is represented as:

Ester + Water $\backslash xrightarrow\{H^+\}$ Acid + Alcohol

Initially, the concentration of the acid catalyst is low (or zero if starting with neutral ester and water), leading to a slow rate of hydrolysis. However, the reaction itself produces an acid (e.g., acetic acid from ethyl acetate hydrolysis). This acid product then acts as a catalyst for the ongoing reaction, increasing the rate of hydrolysis. Substances that are produced during a reaction and also catalyze that same reaction are called autocatalysts. Thus, the ester hydrolysis becomes faster after some time due to autocatalysis by the acid formed.

Solution:

In heterogeneous catalysis, the reaction mechanism often involves the adsorption of reactant molecules onto the surface of the solid catalyst. After the reaction occurs on the surface, the product molecules are formed. Desorption is the process by which these product molecules detach from the catalyst's surface. The role of desorption is crucial because it frees up the active sites on the catalyst surface. This allows fresh reactant molecules to adsorb onto the surface, enabling the catalytic cycle to continue efficiently. If desorption is slow, the surface can become blocked by product molecules, hindering further catalysis.

Solution:

The Hardy-Schulze law states that the flocculating power of an ion is directly proportional to the magnitude of the charge on the ion. For example, for the coagulation of a negative sol, the flocculating power increases in the order , and for the coagulation of a positive sol, it increases in the order .

A possible modification or extension to this law could consider the polarizing power of the ion, which is related to its size and charge. While charge is the dominant factor, the size of the ion also plays a role. Smaller ions with the same charge tend to have higher polarizing power. Therefore, the law could be stated as: 'The greater the charge and the appropriate polarizing power of the flocculating ion, the greater is its power to cause precipitation (coagulation)'. However, the primary emphasis in the standard Hardy-Schulze law remains on the magnitude of the charge.

Solution:

When a precipitate is formed in a solution, it may adsorb some soluble impurities from the mother liquor (the solution from which it precipitated). If these impurities are not removed, they will contribute to the measured mass of the precipitate during quantitative estimation, leading to inaccurate results. Washing the precipitate with a suitable solvent, usually water (if the precipitate is insoluble in water), helps to remove these adsorbed soluble impurities. This ensures that the mass being measured is primarily that of the desired precipitate, leading to a more accurate quantitative determination.