Hydrogen: Structure and Preparation

Atomic Structure of Hydrogen

Hydrogen has one proton in its nucleus.
It belongs to Group 1 of the periodic table.
It has one valence electron.
It requires one electron to complete its octet.
Its oxidation number is typically +1 in its compounds.

Preparation of Hydrogen

Metals like potassium, sodium, and calcium react with dilute mineral acids to produce hydrogen gas.
Zinc or aluminum react with hot concentrated solutions of sodium or potassium hydroxide to produce hydrogen gas.
The reaction of zinc with dilute hydrochloric acid liberates hydrogen gas, represented by the equation:
- Zn + 2HCl → ZnCl₂ + H₂↑
Hydrogen gas can be collected over water.
Pure and dry hydrogen gas can be obtained by passing it through calcium chloride, which absorbs moisture.
Hydrogen gas can be collected by upward or downward displacement of air, as hydrogen gas is lighter than air.

Generation, Properties, and Uses of Hydrogen

Reactive metals like sodium and calcium react with cold water to produce hydrogen gas.
Hydrogen can also be produced by the reaction of phosphoric acid and sodium.
Steam reacting with iron at red-hot temperatures yields hydrogen gas, as depicted by the equation:
- 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g)
Production from methane (natural gas) involves reaction with steam in the presence of a nickel catalyst at ~800°C.
The methane-steam reaction produces a mixture of hydrogen and carbon monoxide.
Key Properties of Hydrogen:
- Colorless, odorless, tasteless
- Highly flammable
- Lighter than air
To further process hydrogen gas, it is mixed with excess steam to convert carbon dioxide into carbon monoxide.
Electrolysis of brine produces chlorine and hydrogen.
Metals with high electron affinity do not easily release hydrogen.

Preparation of Hydrogen Gas - Different Methods

Method 1: From Water Gas
- Carbon dioxide is removed from water gas by passing it through an alkaline solution, such as NaOH or KOH.
- Excess steam is passed over iron (III) oxide at 210°C in the Bosch process.
- The carbon monoxide reacts with steam to produce carbon dioxide and hydrogen: CO + H2O → CO2 + H2
- Carbon dioxide is removed from the gas mixture via scrubbing or dissolving in caustic soda solution under pressure (30 atm).
- The remaining gas is predominantly hydrogen. Any remaining unreacted carbon dioxide is absorbed in an ammoniacal solution.
Method 2: Electrolysis of Brine (NaCl)
- Electrolysis of brine in acidified water (H2O) produces hydrogen at the cathode.
Method 3: Cracking of Petroleum Products
- Hydrogen is a byproduct of cracking petroleum products.

Physical Properties of Hydrogen

Colorless, odorless, and tasteless
Lighter than air
Not very soluble in water
Very low boiling point (-253°C)

Chemical Properties of Hydrogen

Oxidation
- Hydrogen burns in air with a pale blue flame, producing water.
- The reaction is represented by: H₂(g) + O₂(g) → H₂O(l)
Reducing Agent
- Hydrogen reduces metal oxides below iron (Cu, Zn, Sn, Pb, As), leaving the metal as a residue.
- Examples:
  - CuO(s) + H₂(g) → Cu(s) + H₂O(g) (black to brown)
  - PbO₂(s) + H₂(g) → Pb(s) + H₂O(g) (yellow to gray)
  - Fe₂O₃(s) + 3H₂(g) → 2Fe(s) + 3H₂O(g) (reddish-brown to gray)
Reaction with Metals
- Hydrogen combines with active metals like sodium and potassium to form hydrides: 2Na(s) + H₂(g) → 2NaH(s)
Reaction with Non-metals
- Hydrogen combines with non-metals to form gaseous hydrides.
- Hydrogen reacts with halogens to form salts, such as HCl(g).
- Equation: H₂(g) + Cl₂(g) → 2HCl(g)
Reaction with Water
- Hydrogen reacts with water to produce hydrogen gas and sodium hydroxide.
- Equation: NaH(s) + H₂O(l) → NaOH(aq) + H₂(g)

Reactions of Hydrogen

Reaction with Chlorine: Reacts in the presence of sunlight, a catalyst. No reaction in the dark.
Reaction with Molten Sulfur: Produces hydrogen sulfide gas (rotten egg smell):
- H₂ + S → H₂S
Reaction with Nitrogen: Forms ammonia under high pressure, low temperature, with a catalyst (finely divided iron):
- N₂ + 3H₂ → 2NH₃
Reaction with Carbon Monoxide: Produces ethanol at 300°C and high pressure:
- CO + 2H₂ → CH₃OH

Properties of Hydrogen

Colorless gas
Burns in air

Isotopes of Hydrogen

Protium (¹H)
Deuterium (²H or D)
Tritium (³H or T)

Test for Hydrogen

Insert a lighted splint into a test tube containing the unknown gas.
If the gas is hydrogen, the splint will burn with a pop sound.

Additional Information on Hydrogen

The chemical properties of hydrogen are similar to those of Protium.
Protium is more reactive due to the presence of one neutron.
Hydrogen forms oxides with some non-metals like oxygen.
Hydrogen reacts with water to form hydrogen gas and sodium hydroxide:
- Na + H₂O → NaOH +H₂

Hydrogen and Oxygen

Properties of Hydrogen:
- Burns with a pale blue flame, producing a pop sound with air.
Uses of Hydrogen:
- Ammonia synthesis (Haber process)
- Hydrochloric acid production
- Welding and cutting metal
- Margarine production
- Soap, candle, and plastic manufacture
- Coal conversion to crude oil
- Rocket fuels
- Reducing agent for metal extraction (e.g., copper and lead)
- Filling balloons (due to its low density)
Properties of Oxygen:
- Non-metallic gaseous element
- Exists as diatomic molecules (O₂)
- Makes up ~21% of Earth's atmosphere
Uses of Oxygen:
- Laboratory Preparation:
  - Thermal decomposition of potassium chlorate (KClO₃)
  - Electrolysis of water
- Industrial Preparation:
  - Liquefaction or fractional distillation of air
  - Electrolysis of water
- Combustion:
  - Supports combustion
- Other Uses: Necessary for respiration and various industrial processes.

Preparation of Oxygen

Decomposition of Hydrogen Peroxide:
- MnO₂ (manganese (IV) oxide) acts as a catalyst.
- The reaction occurs above MnO₂ 's melting point (around 368°C).
- Oxygen gas can be collected over water, concentrated sulfuric acid (H₂SO₄), or calcium chloride (CaCl₂).
Decomposition of Potassium Chlorate:
- Potassium chlorate (KClO₃) decomposes to potassium chloride (KCl) and oxygen in the presence of manganese(IV) oxide (MnO₂) as a catalyst.
- Equation: 2KClO₃(s) → 2KCl(s) + 3O₂(g)
- Oxygen gas is collected over water or via downward displacement of air.
Industrial Preparation:
- Liquefaction of air and fractional distillation: Air is liquefied and separated based on boiling points. Oxygen is a product.
- Electrolysis of water: Electrolysis of slightly acidic water produces oxygen and hydrogen.

Additional Notes on Oxygen Preparation

The decomposition of hydrogen peroxide requires manganese(IV) oxide.
Decomposition of potassium chlorate is faster at higher temperatures.
Pure oxygen is denser than air.

Fractional Distillation of Air

Oxygen is obtained industrially from air.
Air is passed through caustic soda (NaOH) to remove carbon dioxide and dust.
The air is compressed (200 atm) and cooled.
Compressed air is released into an expansion chamber, causing cooling due to rapid expansion.
This process is repeated until the air becomes liquid at -200°C.
Liquid air is fractionally distilled to separate nitrogen and oxygen.

Nitrogen and Oxygen Separation

Liquid air is primarily composed of nitrogen and oxygen.
Nitrogen boils at -196°C, making it more volatile and collected first.
Oxygen, with a boiling point of -183°C, is collected later.

Oxygen Preparation by Electrolysis

Oxygen can be obtained from liquid oxygen (obtained via fractional distillation) by heating.
Liquid oxygen is compressed to -180 atm for storage purposes.
Electrolysis of acidified water (H₂SO₄) can also produce oxygen under pressure and 180 °C.
Oxygen is generated at the anode.

Chemical Reactions involving Oxygen

Incomplete Reactions from the Text:
- The text mentions reactions with pyrogallal, sodium carbonate, and metals but provides incomplete chemical equations.

Physical Properties of Oxygen (O₂)

Colorless, tasteless, and odorless
Neutral to litmus paper
Slightly soluble in water
Denser than air
Diatomic in nature
Liquidifies at -183°C
Solidfies at -218.3°C

Chemical Properties of Oxygen (O₂)

Good oxidizing agent
Absorbed in alkaline solutions
Forms a brown compound with pyragallol
White water solutions of oxygen are typically brown due to its sensitivity to oxygen.
Dissolves in water.

Reactions of Oxygen

1. Reaction with Pyragallol:
- C₆H₄(OH)₂ + O₂ → C₆H₂O₂(OH)₂ + H₂O + 2O₂ (quinone)
2. Reaction with Metals:
- Oxygen forms basic oxides with metals: 4Na(s) + O₂(g) → 2Na₂O(s)
- Basic oxides dissolve in water to form alkalis: Na₂O(s) + H₂O(l) → 2NaOH(aq)
- Metals like sodium, potassium, calcium, zinc, and magnesium burn brightly in oxygen.
- Iron, tin, lead, and copper react slowly with oxygen.

Reactions of Oxygen with Elements and Compounds

Oxidation Reactions:
- Reactions of oxygen with elements and compounds are oxidation reactions.
- Combustion is an oxidation process with heat and light production (e.g., burning wood, coal, and fuel).
Reactions of Metals with Oxygen:
- Silver, gold, and platinum do not react with oxygen when heated.
- Examples:
  - 2Mg + O₂ → 2MgO (Magnesium oxide)
  - Cu + O₂ → CuO (Copper oxide; solid to black solid)
Reactions of Non-metals with Oxygen:
- Sodium burns with a golden-yellow flame to form white sodium oxide. (Na + O₂ → Na₂O)
- Other non-metals like carbon, phosphorus, and sulfur form oxides.
Acidic Oxides (or Acid Anhydrides):
- Carbon, phosphorus, and sulfur burn in air/oxygen to form oxides that dissolve in water and yield acids.
- Examples:
  - H₂ + O₂ → H₂O
  - 3O₂ + C →CO₂(carbon dioxide)
  - CO₂ + H₂O → H₂CO₃ (carbonic acid)
  - 2S + 3O₂ →2SO₃(sulfur trioxide) (reacts with water to form sulfuric acid)
  - P₄ + 5O₂ → P₄O₁₀
  - P₄O₁₀ + 6H₂O → 4H₃PO₄ (phosphoric acid)
Basic Oxides (or Base Anhydrides):
- Basic oxides have basic properties.
- Examples:
  - CaO + H₂O → Ca(OH)₂ (calcium hydroxide – a base)
  - Na₂O + H₂O → 2NaOH (sodium hydroxide – a base)

Notes on Oxygen Reactions

Copper oxide is not an alkali.
Alkali + O₂ → Alkali metal oxide
Sodium + Oxygen → Sodium oxide
Calcium Oxide + Water → Calcium Hydroxide
Formation of Water:
- Hydrogen burns violently with oxygen (blue flame) to form water.
- Equation: 2H₂ + O₂ → 2H₂O.