Physicochemical profile
In its free state, the element exists as a diatomic molecule — dihydrogen. This substance possesses unique physical characteristics. The gas is entirely devoid of color and specific odor, and its density is the lowest among all known gaseous substances. Solubility in water is negligible (the solubility constant is 0.0221 at zero temperature). The boiling point of the liquid phase is one of the lowest in nature.
Specialized steel reservoirs are used for the safe storage and transportation of compressed gas. They are easily recognized by their strict color coding: the cylinder is painted green and marked with two bright red stripes.
Synthesis in Kipp's apparatus
The classic laboratory method is based on a substitution reaction between a metal and an acid. Granulated zinc is most commonly used, treated with a solution of hydrochloric acid or diluted sulfuric acid (in a 1:8 ratio). The ionic equation of the process is as follows: Zn + 2H⁺ = H₂↑ + Zn²⁺
The process is carried out in a special glass device — Kipp's apparatus. Its design allows for automatic regulation of the chemical reaction:
- Loading reagents: Zinc rods or small metal ingots are placed in the middle reservoir of the device. The lower sphere and part of the upper sphere are filled with the acid solution.
- Activation (stopcock open): Under pressure, the liquid rises from the lower compartment into the middle one. Contact between the acid and zinc begins, resulting in a vigorous reaction with the release of the target gas.
- Shutdown (stopcock closed): The evolving gas accumulates, and its excess pressure forces the liquid acid back down into the lower reservoir. Contact between the reagents is interrupted, and the chemical process completely stops.
Purification of the resulting gas
The gas synthesized in Kipp's apparatus is not entirely pure. If hydrochloric acid was used as a reagent, volatile hydrogen chloride vapors are present in the gas mixture. Furthermore, the product inevitably contains water vapor.
A system of washing bottles is used to obtain a pure substance:
- Passing through water effectively removes soluble hydrogen chloride impurities.
- Passing through concentrated sulfuric acid provides deep drying of the gas, as the acid avidly absorbs water vapor.
Alternative laboratory methods
In addition to using zinc, hydrogen can be obtained through the interaction of active compounds with water or alcohols. These methods include:
- Hydride hydrolysis: Upon contact of sodium hydride with water, gas is vigorously released and an alkali is formed.
- Interaction with alcohols: Metallic sodium actively reacts with ethyl alcohol. This results in the formation of sodium ethoxide and the release of hydrogen gas.
Industrial production and applications
Industrial scale requires huge volumes of raw materials, making laboratory methods cost-ineffective. In factories, synthesis is carried out through the following pathways:
- Thermal decomposition of methane or its steam reforming.
- Coal gasification: Treatment of red-hot coal with water at a temperature of 800 °C.
- As a byproduct during the electrolysis of aqueous solutions.
Any industrially obtained gas must undergo thorough purification from accompanying components.
Where hydrogen is in demand:
- Chemical industry: Used as a basic raw material for the synthesis of ammonia, hydrogen chloride, and numerous organic compounds.
- Energy sector: Used as an effective fuel gas for high-temperature torches (flame temperature exceeds 3100 °C).
- Space industry: In liquid form, it serves as a powerful and eco-friendly component of rocket fuel.
To remember the cylinder labeling, imagine a green lawn (eco-friendly fuel) with two red warning ribbons lying on it (flammability hazard).
Class and exam questions
Why does gas evolution stop when the stopcock of a Kipp's apparatus is closed?
The gas accumulating in a closed system creates increased pressure. This pressure mechanically displaces the acid from the middle reservoir downward, depriving it of contact with pieces of metal, which stops the chemical reaction.
Why is hydrogen passed through concentrated sulfuric acid?
Concentrated sulfuric acid has high hygroscopicity. Passing the gas through a washing bottle with this acid removes water vapor (drying).
Which acids are suitable for the reaction with zinc in Kipp's apparatus?
Usually, hydrochloric acid or diluted sulfuric acid in a 1:8 ratio is used.
What else to study on this topic
- Detailed design and operating rules of Kipp's apparatus
- The chemism of the methane steam reforming process
- Thermodynamics of coal gasification at 800 °C
- Properties of liquid hydrogen as a component of rocket fuel
- Reaction mechanism of alkali metals with aliphatic alcohols
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Other topics in "Chapter 8. Hydrogen"
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