Problem 142 - A Complete Resource Book in Chemistry for JEE Main

Question

In context with the industrial preparation of hydrogen from water gas \(\left(\mathrm{CO}+\mathrm{H}_{2}\right)\), which of the following is the correct statement? (a) \(\mathrm{CO}\) and \(\mathrm{H}_{2}\) are fractionally separated using differences in their densities (b) \(\mathrm{CO}\) is removed by absorption in aqueous \(\mathrm{Cu}_{2} \mathrm{Cl}_{2}\) solution (c) \(\mathrm{H}_{2}\) is removed through occlusion with \(\mathrm{Pd}\) (d) \(\mathrm{CO}\) is oxidized to \(\mathrm{CO}_{2}\) with steam in the presence of a catalyst followed by absorption of \(\mathrm{CO}_{2}\) in alkali.

Step-by-Step Solution

Answer

Option (d) is correct.

Step 1: Analyzing Option (a)

Option (a) suggests that CO and H2 are separated by exploiting differences in their densities. However, CO and H2 have quite similar densities and this method is not practical or conventionally used in industrial separation processes, making option (a) incorrect.

Step 2: Evaluating Option (b)

Option (b) states that CO is removed by absorption in an aqueous Cu2Cl2 solution. While copper(I) chloride solutions can absorb CO under certain conditions, this method is not typically used in industry for separating CO from H2 in the water gas shift process. Hence, option (b) is also incorrect.

Step 3: Assessing Option (c)

Option (c) mentions that H2 is removed through occlusion with Pd. Palladium can indeed absorb hydrogen, forming a palladium hydride, but this process is not practical for large-scale industrial separation due to cost and inefficiency. Thus, option (c) is not the correct industrial method.

Step 4: Verifying Option (d)

Option (d) describes oxidizing CO to CO2 with steam in the presence of a catalyst, followed by absorption of CO2 in alkali. This is actually a description of the water gas shift reaction, a well-known industrial method for separating CO from H2 by converting CO to CO2, which can then be absorbed using an alkali. Therefore, option (d) is correct.

Key Concepts

Water Gas Shift Reaction

In the production of hydrogen from water gas, the Water Gas Shift Reaction plays a vital role. This reaction involves the conversion of carbon monoxide (CO) and water vapor ( H_2O) into carbon dioxide ( CO_2) and hydrogen ( H_2). The reaction can be represented as follows: \[ ext{CO} + ext{H}_2 ext{O} ightleftharpoons ext{CO}_2 + ext{H}_2\] The Water Gas Shift Reaction is crucial because it increases the yield of hydrogen, which is highly desired in industrial applications. Typically, this reaction occurs in two stages:

• High-Temperature Shift: The reaction is performed at high temperatures (300°C to 450°C) to initially convert most of the CO into CO_2 and H_2.


• Low-Temperature Shift: Following the high-temperature shift, the gas mixture is cooled to about 200°C to 250°C to further enhance the conversion of any remaining CO.

Catalysts are employed to speed up these reactions and achieve higher conversion rates. This process not only garners more hydrogen but also reduces CO to safer levels.

Industrial Separation Techniques

There are various industrial techniques to separate gases based on their physical and chemical properties. In the context of hydrogen production from water gas, several methods are employed to separate gasses like CO, H_2, and CO_2 effectively.

• Absorption: In this technique, gaseous substances are absorbed into a liquid. For instance, CO_2 can be absorbed by alkaline solutions, such as potassium carbonate. This process is essential for removing CO_2 after it has been formed from CO.


• Adsorption: This method involves the adhesion of gas molecules onto solid surfaces. Certain solids, like activated carbon, can selectively adsorb gases based on their size or affinity. While not typically used for CO separation, it is an example of a common industrial technique.


• Membrane Separation: This process uses selective permeability of membranes to separate gases based on size or diffusion rates. Although not always applied to water gas separation, it's a growing area of interest due to its energy efficiency.

Understanding these techniques helps optimize the separation and purification of hydrogen, making the process more efficient and cost-effective.

Catalysis in Chemical Reactions

Catalysts are substances that increase the rate of chemical reactions without undergoing permanent chemical changes. In the Water Gas Shift Reaction, catalysts play a pivotal role, as they allow the reaction to proceed under milder conditions and with improved efficiency.

Importance of Catalysts

Catalysts provide an alternative reaction pathway with lower activation energy. This allows more reactant molecules to reach the necessary energy level to transform into products. In industrial settings, this leads to quicker and more efficient processes.

Types of Catalysts

• Homogeneous Catalysts: These catalysts exist in the same phase (liquid or gas) as the reactants. They are often used in liquid-phase reactions.


• Heterogeneous Catalysts: These are in a different phase than the reactants, typically solid catalysts in gas-phase reactions. They provide a surface for reactions to take place, like metal oxides used in the Water Gas Shift Reaction.

Role in Water Gas Shift Reaction

Specifically, in the Water Gas Shift Reaction, compounds like chromium oxide ( ext{Cr}_2 ext{O}_3) and copper oxide ( ext{CuO}) are often used as catalysts. These materials facilitate the conversion of CO and ext{H}_2 ext{O} into CO_2 and ext{H}_2, thus making the process more efficient.