Laboratory and Industrial Methods — Revision Notes

⚡ 30-Second Revision

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: $ext{C} + \text{H₂O} xrightarrow{\text{1270 K}} \text{CO} + \text{H₂}$

Key Points: — Reactivity series, $ext{HNO₃}$ is oxidizing, pure water is non-conductive, CO is catalyst poison.

2-Minute Revision

Dihydrogen (H₂) preparation methods are categorized into laboratory and industrial scales. For the lab, common methods include reacting active metals like zinc or magnesium with dilute non-oxidizing acids such as $ext{HCl}$ or $ext{H₂SO₄}$.

Remember the reactivity series: metals above hydrogen displace it. Amphoteric metals like zinc and aluminum also produce H₂ when reacted with strong alkalis like $ext{NaOH}$. A very clean lab method is the electrolysis of acidified water, where a small amount of acid is added to increase electrical conductivity, yielding pure H₂ at the cathode.

Industrially, the most significant method is the steam reforming of hydrocarbons (e.g., methane) with steam over a nickel catalyst at high temperatures, producing 'water gas' ($ext{CO} + \text{H₂}$). This is followed by the water-gas shift reaction, where $ext{CO}$ reacts with more steam over an iron-chromium catalyst to produce additional H₂ and $ext{CO₂}$.

Another industrial source is the electrolysis of brine (chlor-alkali process), where H₂ is a byproduct along with $ext{Cl₂}$ and $ext{NaOH}$. High-purity H₂ can be obtained by electrolysis of warm aqueous $ext{Ba(OH)₂}$ solution.

Always be mindful of specific catalysts, temperatures, and byproducts for each method.

5-Minute Revision

Dihydrogen, H₂, is a versatile gas prepared via various methods depending on the required scale and purity. Laboratory methods are for small-scale production. The most common involves active metals reacting with dilute acids.

For instance, zinc reacts with dilute hydrochloric acid: $ext{Zn(s)} + \text{2HCl(aq)} \rightarrow \text{ZnCl₂(aq)} + \text{H₂(g)}$. Metals like Mg and Fe also work. Crucially, metals must be above hydrogen in the reactivity series.

Nitric acid is generally avoided due to its oxidizing nature, except for very dilute $ext{HNO₃}$ with Mg or Mn. Another lab method is the reaction of amphoteric metals with strong alkalis, e.g., $ext{2Al(s)} + \text{2NaOH(aq)} + \text{6H₂O(l)} \rightarrow \text{2Na[Al(OH)₄](aq)} + \text{3H₂(g)}$.

The electrolysis of acidified water is a key lab method for high-purity H₂: $ext{2H₂O(l)} xrightarrow{\text{electrolysis}} \text{2H₂(g)} + \text{O₂(g)}$. A small amount of acid (like $ext{H₂SO₄}$) is vital to increase water's electrical conductivity.

Industrial methods focus on large-scale, cost-effective production. The predominant method is steam reforming of hydrocarbons, typically methane from natural gas: $ext{CH₄(g)} + \text{H₂O(g)} xrightarrow{\text{Ni catalyst, 1000 K}} \text{CO(g)} + \text{3H₂(g)}$.

This 'water gas' mixture then undergoes the water-gas shift reaction to maximize H₂ yield and remove CO: $ext{CO(g)} + \text{H₂O(g)} xrightarrow{\text{Fe₂O₃/Cr₂O₃ catalyst, 673 K}} \text{CO₂(g)} + \text{H₂(g)}$.

The $ext{CO₂}$ is then removed. Another significant industrial source is the electrolysis of brine (chlor-alkali process), where H₂ is a valuable byproduct alongside $ext{Cl₂}$ and $ext{NaOH}$: $ext{2NaCl(aq)} + \text{2H₂O(l)} xrightarrow{\text{electrolysis}} \text{2NaOH(aq)} + \text{Cl₂(g)} + \text{H₂(g)}$.

For very high purity H₂, electrolysis of warm aqueous $ext{Ba(OH)₂}$ solution with nickel electrodes is employed. Remember the specific catalysts, temperatures, and byproducts for each industrial process.

Prelims Revision Notes

Dihydrogen Preparation: NEET Quick Recall

I. Laboratory Methods (Small Scale, High Purity, Controlled)

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Metals + Dilute Acids:

* Principle: More reactive metals (above H in reactivity series) displace H from non-oxidizing acids. * Reagents: Zn, Mg, Fe with dilute HCl or dilute H₂SO₄. * Reactions: * $ext{Zn(s)} + \text{2HCl(aq)} \rightarrow \text{ZnCl₂(aq)} + \text{H₂(g)}$ * $ext{Mg(s)} + \text{H₂SO₄(aq)} \rightarrow \text{MgSO₄(aq)} + \text{H₂(g)}$ * Avoid: Noble metals (Cu, Ag) – less reactive than H.

Concentrated H₂SO₄ – acts as oxidizing agent. Nitric acid (HNO₃) – strong oxidizing agent, produces oxides of N, not H₂ (exception: very dilute HNO₃ with Mg/Mn).

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Metals + Strong Alkalis:

* Principle: Amphoteric metals react with strong bases. * Reagents: Zn, Al with conc. NaOH or KOH. * Reactions: * $ext{Zn(s)} + \text{2NaOH(aq)} + \text{2H₂O(l)} \rightarrow \text{Na₂[Zn(OH)₄](aq)} + \text{H₂(g)}$ * $ext{2Al(s)} + \text{2NaOH(aq)} + \text{6H₂O(l)} \rightarrow \text{2Na[Al(OH)₄](aq)} + \text{3H₂(g)}$

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Electrolysis of Acidified Water:

* Principle: Electrical decomposition of water. * Setup: Inert electrodes (Pt) in acidified water. * Role of Acid: Pure water is poor conductor; acid (e.g., H₂SO₄) increases conductivity. * Reactions: * Cathode: $ext{2H₂O(l)} + \text{2e⁻} \rightarrow \text{H₂(g)} + \text{2OH⁻(aq)}$ * Anode: $ext{2H₂O(l)} \rightarrow \text{O₂(g)} + \text{4H⁺(aq)} + \text{4e⁻}$ * Overall: $ext{2H₂O(l)} xrightarrow{\text{electrolysis}} \text{2H₂(g)} + \text{O₂(g)}$ * Purity: Yields very pure H₂.

II. Industrial Methods (Large Scale, Cost-effective)

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Steam Reforming of Hydrocarbons (Bosch Process):

* Raw Material: Natural gas (CH₄), naphtha. * Step 1: Reforming: $ext{CH₄(g)} + \text{H₂O(g)} xrightarrow{\text{Ni catalyst, 1000 K}} \text{CO(g)} + \text{3H₂(g)}$ * Step 2: Water-Gas Shift Reaction: Converts CO to more H₂. * $ext{CO(g)} + \text{H₂O(g)} xrightarrow{\text{Fe₂O₃/Cr₂O₃ catalyst, 673 K}} \text{CO₂(g)} + \text{H₂(g)}$ * CO Removal: CO is a catalyst poison; removed by absorption (e.g., ammoniacal cuprous chloride).

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Electrolysis of Brine (Chlor-alkali Process):

* Raw Material: Concentrated aqueous NaCl. * Products: NaOH, Cl₂, and H₂ (byproduct). * Reactions: * Cathode: $ext{2H₂O(l)} + \text{2e⁻} \rightarrow \text{H₂(g)} + \text{2OH⁻(aq)}$ * Anode: $ext{2Cl⁻(aq)} \rightarrow \text{Cl₂(g)} + \text{2e⁻}$

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From Coal (Coal Gasification):

* $ext{C(s)} + \text{H₂O(g)} xrightarrow{\text{1270 K}} \text{CO(g)} + \text{H₂(g)}$ (Water Gas) * Followed by water-gas shift reaction.

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Electrolysis of Warm Aqueous Ba(OH)₂: — For very high purity H₂ (using Ni electrodes).

Vyyuha Quick Recall

To remember the key industrial steps for H₂ from hydrocarbons: Steam Works Cool.

Steam: Steam Reforming ($ext{CH₄} + \text{H₂O} xrightarrow{\text{Ni}}$)
Works: Water-Gas Shift Reaction ($ext{CO} + \text{H₂O} xrightarrow{\text{Fe₂O₃/Cr₂O₃}}$)
Cool: CO Removal (e.g., by absorption)