Why are very reactive metals like sodium and potassium not used for the laboratory preparation of dihydrogen?

Very reactive metals such as sodium and potassium react extremely vigorously, even explosively, with water and dilute acids. This high reactivity is due to their low ionization energies, making them readily lose electrons. The reactions are highly exothermic, releasing a significant amount of heat that can ignite the hydrogen produced, posing a severe safety hazard in a laboratory setting. Therefore, moderately reactive metals like zinc or magnesium are preferred for safer and controlled dihydrogen generation.

Why is dilute sulfuric acid preferred over dilute nitric acid for preparing dihydrogen from metals?

Dilute sulfuric acid is a non-oxidizing acid, meaning it primarily acts as a source of $H^+$ ions for reduction to $H_2$. In contrast, nitric acid, even when dilute, is a strong oxidizing agent. When metals react with nitric acid, the hydrogen produced is immediately oxidized by the nitrate ion ($NO_3^-$) to form water. Instead of $H_2$, various oxides of nitrogen (like $NO$, $N_2O$, or $NO_2$) are formed, depending on the concentration of the acid and the reactivity of the metal. Thus, nitric acid is unsuitable for preparing dihydrogen.

What is 'water gas' and how is it related to dihydrogen preparation?

Water gas is a mixture of carbon monoxide ($CO$) and dihydrogen ($H_2$). It is produced industrially by passing steam over red-hot coke (carbon) at very high temperatures (around $1000^circ C$). The reaction is $C(s) + H_2O(g) \xrightarrow{1000^circ C} CO(g) + H_2(g)$. Water gas itself is a valuable industrial fuel and a source of hydrogen. To obtain pure dihydrogen from water gas, the carbon monoxide component is further reacted with steam in the 'water-gas shift reaction' to convert $CO$ into $CO_2$ and generate additional $H_2$, which can then be separated.

Explain the role of a catalyst in the steam reforming of methane.

In the steam reforming of methane ($CH_4 + H_2O \rightarrow CO + 3H_2$), a nickel catalyst is crucial. Catalysts provide an alternative reaction pathway with a lower activation energy, thereby increasing the rate of reaction without being consumed in the process. At the high temperatures ($1000^circ C$) required for this endothermic reaction, the nickel catalyst facilitates the breaking of C-H and O-H bonds and the formation of C-O and H-H bonds, significantly speeding up the production of synthesis gas (CO and $H_2$) from methane and steam, making the process industrially viable.

How is carbon dioxide removed from the hydrogen produced in industrial processes like the Bosch process or steam reforming?

After the water-gas shift reaction, the gas mixture contains hydrogen and carbon dioxide. Carbon dioxide is typically removed by two main methods. One common method is scrubbing with water under pressure, where $CO_2$ dissolves in water more readily than hydrogen. Another effective method involves passing the gas through a solution of potassium carbonate ($K_2CO_3$), which absorbs $CO_2$ to form potassium bicarbonate ($KHCO_3$). Any residual carbon monoxide can be removed by absorption in ammoniacal cuprous chloride solution.

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Preparation of Dihydrogen

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NEET UG

Version 1Updated 22 Mar 2026

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The preparation of dihydrogen ( $H_2$ ) is a fundamental topic in inorganic chemistry, encompassing various methods tailored for laboratory-scale synthesis and large-scale industrial production. These methods leverage distinct chemical principles, primarily redox reactions and electrolysis, to liberate hydrogen from its compounds. Key considerations include the purity of the produced gas, the cost-e…

Quick Summary

Dihydrogen ( $H_2$ ) is prepared through various methods, categorized into laboratory and industrial scales. Laboratory methods typically involve reacting active metals with dilute acids (e.g., zinc with dilute HCl: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$ ) or strong alkalis (e.

g., aluminium with NaOH: $2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2$ ). These methods are simple and suitable for small-scale production. Industrially, dihydrogen is produced in large quantities using more efficient and cost-effective processes.

Key industrial methods include the electrolysis of acidified water ( $2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$ ), which yields high-purity hydrogen but is energy-intensive. Another major method is the steam reforming of hydrocarbons, primarily methane, where methane reacts with steam over a nickel catalyst at high temperatures ( $CH_4 + H_2O \xrightarrow{Ni, 1000^circ C} CO + 3H_2$ ).

The resulting carbon monoxide is then converted to additional hydrogen via the water-gas shift reaction ( $CO + H_2O \rightarrow CO_2 + H_2$ ), and the $CO_2$ is subsequently removed. The Bosch process, using coke and steam, follows a similar principle.

Dihydrogen is also a valuable by-product of the chlor-alkali process (electrolysis of brine). Understanding the specific reagents, conditions, and by-products for each method is crucial for NEET.

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Key Concepts

Reaction of Metals with Acids

This is a classic single displacement redox reaction. Metals that are more reactive than hydrogen (i.e., have…

Electrolysis of Water

Electrolysis is a non-spontaneous process driven by electrical energy. In the electrolysis of water, an…

Water-Gas Shift Reaction (WGSR)

The WGSR is a crucial step in industrial hydrogen production, particularly after steam reforming of…

Lab Methods:

- Active metals + dil. acids: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$ - Amphoteric metals + strong alkalis: $2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2$ - Avoid: Na/K with water/acids (too vigorous), metals with $HNO_3$ (oxidizing acid).

Industrial Methods:

- Electrolysis of acidified water: $2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$ (Pure $H_2$ , energy intensive) - Electrolysis of brine (Chlor-Alkali): $2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electricity}} 2NaOH(aq) + Cl_2(g) + H_2(g)$ (By-product) - Steam Reforming of Hydrocarbons: $CH_4(g) + H_2O(g) \xrightarrow{Ni, 1000^circ C} CO(g) + 3H_2(g)$ (Syngas) - Water-Gas Shift Reaction: $CO(g) + H_2O(g) \xrightarrow{Fe_2O_3/Cr_2O_3, 400^circ C} CO_2(g) + H_2(g)$ (Increases $H_2$ , removes $CO$ ) - Bosch Process: Coke + Steam $\rightarrow$ Water gas $\rightarrow$ Water-Gas Shift $\rightarrow$ $H_2$ (Similar to steam reforming)

To remember the main industrial methods for Dihydrogen:

Every Student Wants Bright Hydrogen

Electrolysis (of water/brine)

Steam reforming (of hydrocarbons)

Water-gas shift reaction (for CO conversion)

Bosch process (from coke)

Hydrogen (the product)

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