Preparation of Dihydrogen ($H_2$)
Mastering Lab Methods, Industrial Syngas, and High-Purity Electrolysis.
Because molecular Hydrogen ($H_2$) is incredibly light and easily escapes Earth's gravity, very little of it exists freely in our atmosphere. However, it is locked abundantly in water ($H_2O$) and organic compounds. The preparation of Dihydrogen is fundamentally about reducing the $H^+$ ions present in these compounds to gaseous $H_2$.
1. Laboratory Preparation
In the laboratory, $H_2$ is usually prepared in a Kipp's apparatus using moderately reactive metals.
A. Action of Dilute Acids on Metals
Metals lying above hydrogen in the electrochemical series (like $Zn, Mg, Fe$) displace $H_2$ from dilute mineral acids (like $HCl$ or $H_2SO_4$). Zinc is the standard laboratory choice.
Why is granulated (impure) zinc preferred over highly pure zinc?
Pure zinc reacts extremely slowly with dilute acids because the initial bubbles of $H_2$ stick to the metal, forming a protective blanket.
In granulated zinc, impurities (like Copper) act as tiny localized cathodes. This creates thousands of local micro-galvanic cells, massively accelerating the rate of hydrogen evolution.
B. Action of Aqueous Alkalis on Amphoteric Metals
Amphoteric metals like Zinc ($Zn$), Aluminum ($Al$), Tin ($Sn$), and Lead ($Pb$) react with boiling aqueous alkalis (like $NaOH$ or $KOH$) to liberate hydrogen gas and form soluble complex salts.
$2Al + 2NaOH + 2H_2O \rightarrow \underbrace{2NaAlO_2}_{\text{Sod. Aluminate}} + 3H_2 \uparrow$
2. Commercial Preparation: Electrolysis
Electrolysis is the process of using electrical energy to force the non-spontaneous decomposition of water. Pure water is a terrible conductor, so a small amount of acid or base is added to facilitate the flow of current.
A. Electrolysis of Acidulated Water:
Using platinum electrodes, water containing a little dilute $H_2SO_4$ is electrolyzed. Hydrogen gas is liberated at the Cathode (negative electrode), and oxygen at the Anode.
If an exam asks for the preparation of High-Purity ($>99.95\%$) Dihydrogen, the standard acidulated water answer is WRONG.
Ultra-pure hydrogen is exclusively obtained by the electrolysis of a warm aqueous solution of Barium Hydroxide ($Ba(OH)_2$) between Nickel electrodes.
C. By-product of Chlor-Alkali Industry:
Massive amounts of $H_2$ are generated globally as a free by-product during the industrial electrolysis of Brine (concentrated $NaCl$ solution) to produce Sodium Hydroxide ($NaOH$) and Chlorine gas ($Cl_2$).
3. Large Scale Production from Hydrocarbons
Today, the vast majority ($\sim 77\%$) of the world's industrial hydrogen is produced from petrochemicals (hydrocarbons) and coal. The sequence of reactions is heavily tested in JEE.
Figure 1: The industrial sequence for producing Hydrogen from Hydrocarbons.
Step 1: Steam Reforming (Production of Syngas)
Hydrocarbons (like Methane) or Coke (Carbon) are reacted with steam at very high temperatures in the presence of a Nickel catalyst.
$C_{(s)} \text{ (Coke)} + H_2O_{(g)} \xrightarrow{1270\text{K}} \underbrace{CO_{(g)} + H_{2(g)}}_{\text{Coal Gasification}}$
The mixture of $CO$ and $H_2$ is called Water Gas. Because it is used to synthesize methanol and other hydrocarbons, it is widely referred to as Synthesis Gas or Syngas.
Step 2: The Water-Gas Shift Reaction
To extract even more hydrogen and eliminate the toxic Carbon Monoxide, the Syngas mixture is reacted with more steam in the presence of an Iron Chromate ($FeCrO_4$) catalyst (often written as Iron oxide mixed with Chromium oxide) at $673\text{K}$.
This "shifts" the oxygen from water onto the CO, creating $CO_2$ and freeing up the remaining hydrogen.
Step 3: Scrubbing (Purification)
The resulting mixture is mostly $H_2$ and $CO_2$. The $CO_2$ is easily removed ("scrubbed") by passing the gas mixture through water under high pressure, or more commonly in exams, by scrubbing with a solution of Sodium Arsenite.
4. Unique & Historical Methods
A. Lane's Process
A cyclic historical industrial process where superheated steam is passed over iron filings at $1023\text{K} - 1073\text{K}$. The iron oxidizes to magnetic iron oxide ($Fe_3O_4$), releasing hydrogen.
B. Action of Water on Hydrides (Hydrolith)
Ionic hydrides of s-block metals (like $NaH$ or $CaH_2$) react violently with water to liberate $H_2$. Calcium hydride ($CaH_2$) is commercially known as Hydrolith. Because it is a compact solid, Hydrolith was historically carried by the military and balloonists as a portable, instantaneous source of hydrogen gas just by adding water.
Mastery Check: Hydrogen Preparation
15 High-Yield Questions to test your JEE/NEET Preparation
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