Why is pure water not used for the electrolysis method of dihydrogen preparation?

Pure water is a very poor conductor of electricity because it has a very low concentration of ions. For efficient electrolysis, an electrolyte is required to carry the charge. Therefore, a small amount of acid (like sulfuric acid) or a base (like sodium hydroxide) is added to water. This increases the concentration of ions (H⁺/OH⁻ from acid/base and H⁺/OH⁻ from water itself), making the solution conductive and allowing the electric current to pass through, facilitating the decomposition of water into dihydrogen and dioxygen.

Why is nitric acid generally not preferred for preparing dihydrogen in the laboratory?

Nitric acid ($ ext{HNO₃}$) is a strong oxidizing agent. When it reacts with metals, it tends to oxidize the nascent hydrogen (H) produced to water ($ ext{H₂O}$) instead of allowing it to combine and form dihydrogen gas ($ ext{H₂}$). Instead, the nitric acid itself gets reduced, producing various oxides of nitrogen like $ ext{NO₂}$, $ ext{NO}$, or $ ext{N₂O}$, depending on its concentration and the reactivity of the metal. Only very dilute nitric acid reacting with highly electropositive metals like magnesium or manganese can produce dihydrogen.

What is the significance of the 'water-gas shift reaction' in industrial dihydrogen production?

The water-gas shift reaction ($ ext{CO(g)} + ext{H₂O(g)} ightleftharpoons ext{CO₂(g)} + ext{H₂(g)}$) is crucial for two main reasons. Firstly, it increases the overall yield of dihydrogen from the initial steam reforming process. The reforming of hydrocarbons produces a mixture of CO and H₂ (water gas), and this subsequent reaction converts the CO into additional H₂. Secondly, it helps in reducing the concentration of carbon monoxide, which is a potent poison for catalysts used in subsequent processes, such as the Haber-Bosch synthesis of ammonia. Removing CO is essential for the efficiency and longevity of these catalysts.

Which industrial method yields dihydrogen as a byproduct, and what are its primary products?

The electrolysis of brine, also known as the chlor-alkali process, yields dihydrogen as a valuable byproduct. The primary products of this industrial process are chlorine gas ($ ext{Cl₂}$) and sodium hydroxide ($ ext{NaOH}$). The overall reaction is $ ext{2NaCl(aq)} + ext{2H₂O(l)} xrightarrow{ ext{electrolysis}} ext{2NaOH(aq)} + ext{Cl₂(g)} + ext{H₂(g)}$. This method is highly efficient as it produces three commercially important chemicals simultaneously.

Why are very reactive metals like sodium or potassium not used for laboratory preparation of dihydrogen with acids?

Very reactive metals such as sodium (Na) and potassium (K) react extremely vigorously, even explosively, with dilute acids and even with water. The reactions are highly exothermic, releasing a large amount of heat that can ignite the hydrogen gas produced, leading to dangerous explosions. For example, $ ext{2Na(s)} + ext{2HCl(aq)} ightarrow ext{2NaCl(aq)} + ext{H₂(g)}$ is too violent for safe laboratory preparation. Therefore, less reactive but still sufficiently reactive metals like zinc or magnesium are preferred for controlled laboratory production.

Laboratory and Industrial Methods

Chemistry

NEET UG

Version 1Updated 22 Mar 2026

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Dihydrogen, represented by the chemical formula H₂, is the simplest and lightest element, existing as a diatomic molecule under standard conditions. Its preparation methods are broadly categorized into laboratory and industrial scales, each employing distinct chemical principles and aiming for varying levels of purity and production volume. Laboratory methods typically involve reactions of active …

Quick Summary

Dihydrogen (H₂) is a fundamental chemical, prepared via distinct laboratory and industrial methods. Laboratory methods focus on small-scale, controlled production, typically involving the reaction of active metals (like zinc or magnesium) with dilute non-oxidizing acids (e.

g., $ext{HCl}$ , $ext{H₂SO₄}$ ), or the reaction of amphoteric metals (like zinc or aluminum) with strong alkalis (e.g., $ext{NaOH}$ ). Another key lab method is the electrolysis of acidified water, which yields high-purity dihydrogen.

Industrial methods, conversely, aim for large-scale, cost-effective output. The most prevalent industrial technique is the steam reforming of hydrocarbons (like natural gas), followed by the water-gas shift reaction to maximize dihydrogen yield and remove carbon monoxide.

Other industrial sources include the electrolysis of brine (where H₂ is a byproduct alongside $ext{Cl₂}$ and $ext{NaOH}$ ) and coal gasification. The choice of method depends on the required quantity, purity, and economic considerations, with applications spanning from fuel and reducing agents to the synthesis of ammonia and methanol.

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Key Concepts

Redox in Metal-Acid Reaction

Many laboratory methods for dihydrogen production involve redox reactions where a more reactive metal…

Electrolysis of Acidified Water

This method uses electrical energy to split water molecules. Water itself is a poor conductor, so a small…

Bosch Process (Steam Reforming + Water-Gas Shift)

The Bosch process is a series of industrial reactions to produce dihydrogen from hydrocarbons or coal. It…

Lab Methods:

- Metals + Dilute Acids: $ext{Zn} + \text{2HCl} \rightarrow \text{ZnCl₂} + \text{H₂}$ (Mg, Fe also) - Metals + Strong Alkalis: $ext{2Al} + \text{2NaOH} + \text{6H₂O} \rightarrow \text{2Na[Al(OH)₄]} + \text{3H₂}$ (Zn also) - Electrolysis of Acidified Water: $ext{2H₂O} xrightarrow{\text{electrolysis}} \text{2H₂} + \text{O₂}$ (acid for conductivity)

Industrial Methods:

- Steam Reforming: $ext{CH₄} + \text{H₂O} xrightarrow{\text{Ni, 1000 K}} \text{CO} + \text{3H₂}$ - Water-Gas Shift: $ext{CO} + \text{H₂O} xrightarrow{\text{Fe₂O₃/Cr₂O₃, 673 K}} \text{CO₂} + \text{H₂}$ - Electrolysis of Brine: $ext{2NaCl} + \text{2H₂O} xrightarrow{\text{electrolysis}} \text{2NaOH} + \text{Cl₂} + \text{H₂}$ - Coal Gasification: $ext{C} + \text{H₂O} xrightarrow{\text{1270 K}} \text{CO} + \text{H₂}$