Preparation of Dihydrogen — Revision Notes

⚡ 30-Second Revision

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)

2-Minute Revision

Dihydrogen ($H_2$) preparation is vital for both laboratory and industrial applications. In the lab, it's commonly made by reacting moderately active metals like zinc with dilute non-oxidizing acids, such as hydrochloric acid ($Zn + 2HCl \rightarrow ZnCl_2 + H_2$).

Amphoteric metals like aluminium also produce $H_2$ with strong alkalis ($2Al + 2NaOH + 2H_2O \rightarrow 2NaAlO_2 + 3H_2$). It's crucial to remember that highly reactive metals (Na, K) are too dangerous, and oxidizing acids ($HNO_3$) don't yield $H_2$.

Industrially, the most prevalent method is the steam reforming of hydrocarbons, primarily methane. Methane reacts with steam over a nickel catalyst at high temperatures ($1000^circ C$) to produce synthesis gas ($CO + H_2$).

To maximize $H_2$ yield and remove toxic $CO$, the water-gas shift reaction is employed, where $CO$ reacts with more steam over an iron-chromium catalyst to form $CO_2$ and additional $H_2$. The $CO_2$ is then removed.

Electrolysis of acidified water ($2H_2O \xrightarrow{\text{electricity}} 2H_2 + O_2$) provides very pure $H_2$ but is energy-intensive. Dihydrogen is also a by-product of the chlor-alkali process.

5-Minute Revision

The preparation of dihydrogen ($H_2$) is categorized into laboratory and industrial methods, each with specific reagents and conditions. For laboratory scale, simple and safe reactions are preferred. Active metals like zinc ($Zn$) or magnesium ($Mg$) react with dilute non-oxidizing acids such as hydrochloric acid ($HCl$) or sulfuric acid ($H_2SO_4$).

For example, $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$. Amphoteric metals like aluminium ($Al$) or zinc can also react with strong alkalis like sodium hydroxide ($NaOH$) solution, often with heating, to produce dihydrogen: $2Al(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow 2NaAlO_2(aq) + 3H_2(g)$.

It's important to avoid highly reactive metals like sodium or potassium due to their explosive reactions with water, and oxidizing acids like nitric acid, which produce oxides of nitrogen instead of $H_2$.

Industrial production focuses on large-scale, cost-effective methods. The most common is the steam reforming of hydrocarbons, primarily methane (natural gas). This involves reacting methane with steam at very high temperatures ($~1000^circ C$) in the presence of a nickel catalyst: $CH_4(g) + H_2O(g) \xrightarrow{Ni, 1000^circ C} CO(g) + 3H_2(g)$.

The resulting mixture, known as synthesis gas or syngas, contains carbon monoxide ($CO$) and dihydrogen ($H_2$). To increase the hydrogen yield and remove the undesirable $CO$, the water-gas shift reaction is performed: $CO(g) + H_2O(g) \xrightarrow{Fe_2O_3/Cr_2O_3, 400^circ C} CO_2(g) + H_2(g)$.

The carbon dioxide ($CO_2$) is then removed by scrubbing with water or potassium carbonate solution. Another significant industrial method is the electrolysis of acidified water: $2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$.

This yields very pure hydrogen but is energy-intensive. Dihydrogen is also obtained as a valuable by-product during the electrolysis of brine (chlor-alkali process) for $NaOH$ and $Cl_2$ production: $2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electricity}} 2NaOH(aq) + Cl_2(g) + H_2(g)$.

The Bosch process, using coke and steam, is similar to steam reforming. For NEET, focus on the specific reagents, catalysts, conditions, and by-products of each method.

Prelims Revision Notes

Preparation of Dihydrogen ($H_2$)

I. Laboratory Methods (Small Scale):

1
From Active Metals and Dilute Acids:

* Principle: Metals more reactive than hydrogen displace it from non-oxidizing acids. * Reagents: Zinc ($Zn$), Magnesium ($Mg$), Iron ($Fe$) with dilute $HCl$ or $H_2SO_4$. * Reaction Example: $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$ * Key Point: Very reactive metals (Na, K) are too vigorous. Oxidizing acids ($HNO_3$, conc. $H_2SO_4$) do not produce $H_2$ as they oxidize nascent hydrogen.

1
From Amphoteric Metals and Strong Alkalis:

* Principle: Amphoteric metals react with strong bases. * Reagents: Zinc ($Zn$), Aluminium ($Al$) with concentrated $NaOH$ or $KOH$ solution. * Reaction Example: $2Al(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow 2NaAlO_2(aq) + 3H_2(g)$ * Conditions: Often requires heating.

1
From Water with Active Metals:

* Principle: Highly electropositive metals react with water. * Reagents: Na, K, Ca with cold water (vigorous/explosive); Mg with hot water/steam. * Safety: Not preferred for lab due to high reactivity of Na/K.

II. Industrial Methods (Large Scale):

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Electrolysis of Acidified Water:

* Principle: Electrical decomposition of water. * Reaction: $2H_2O(l) \xrightarrow{\text{electricity}} 2H_2(g) + O_2(g)$ * Product: Very pure $H_2$. * Disadvantage: Energy-intensive, expensive.

1
Electrolysis of Brine (Chlor-Alkali Process):

* Principle: $H_2$ is a by-product during $NaOH$ and $Cl_2$ production. * Reaction: $2NaCl(aq) + 2H_2O(l) \xrightarrow{\text{electricity}} 2NaOH(aq) + Cl_2(g) + H_2(g)$

1
Steam Reforming of Hydrocarbons (Most Common):

* Principle: Reaction of hydrocarbons (e.g., methane) with steam. * Reaction: $CH_4(g) + H_2O(g) \xrightarrow{Ni, 1000^circ C} CO(g) + 3H_2(g)$ (Produces 'Synthesis Gas' or 'Syngas') * Catalyst: Nickel (Ni). * Temperature: High ($~1000^circ C$).

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Water-Gas Shift Reaction:

* Principle: Converts $CO$ in syngas to $CO_2$ and produces more $H_2$. * Reaction: $CO(g) + H_2O(g) \xrightarrow{Fe_2O_3/Cr_2O_3, 400^circ C} CO_2(g) + H_2(g)$ * Catalyst: Iron-chromium oxide ($Fe_2O_3/Cr_2O_3$). * Purpose: Increases $H_2$ yield, removes $CO$ (a catalyst poison).

1
Bosch Process (from Coke):

* Steps: a. Water gas production: $C(s) + H_2O(g) \xrightarrow{1000^circ C} CO(g) + H_2(g)$ b. Water-gas shift reaction (as above). c. $CO_2$ removal (e.g., by scrubbing with water under pressure or $K_2CO_3$ solution).

Key Takeaways for NEET:

Memorize specific reagents, catalysts, and conditions.
Understand the purpose of each step in industrial processes.
Differentiate between lab and industrial methods based on scale, cost, and purity.
Recognize redox reactions involved.

Vyyuha Quick Recall

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)