What is Hydrocyanic Acid? Complete Guide for Chemistry Students
HCN acid name is hydrogen cyanide, a highly volatile, colourless, and extremely poisonous liquid. It has a boiling point 26° C and freezing point -14° C. A solution of the compound in water is named prussic acid. It was discovered in 1782 by a Swedish chemist, Carl Wilhelm Scheele, who prepared it from the pigment Prussian blue. Hydrogen cyanide compounds are used for several chemical processes, including fumigation, hardening of iron and steel, electroplating. Therefore the concentration of ores. It also is employed in the preparation of acrylonitrile, which is used in the production of acrylic fibres, synthetic rubber, and plastics.
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Hydrogen cyanide is very toxic because it inhibits cellular oxidative processes. Humans can survive 50–60 parts of the compound per million parts of air for an hour without severe costs, but exposure to concentrations of 200–500 parts per million of air for a half-hour is typically fatal. A method of inflicting execution consists of administering a deadly dose of the compound gas.
Hydrogen cyanide is an excellent solvent for several salts, but not widely used as a solvent due to its toxicity. In pure form, the compound may be stable, but it polymerizes readily within the presence of essential substances, like ammonia or cyanide. The salts used in the extraction of ores, in electrolytic processes, and the treatment of steel. Organic compounds such as aldehydes and ketones, forming cyanohydrins, which function intermediates in many organic syntheses.
HCN Structure
The chemical formula of hydrocyanic acid is HCN. Its molecular formula is written as CHN, and its molar mass is 27.03 g/mol. Hydrogen cyanide may be a simple planar molecule, with a triple bond between the carbon and nitrogen atoms.
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HCN Occurrence
HCN occurs naturally in the pits of certain fruits such as cherries, apples, and apricots, and the fruit pits contain small amounts of cyanohydrins from HCN.
HCN Preparation
HCN is prepared on a laboratory scale by the addition of acids to cyanide salts of alkali metals (such as NaCN, KCN, etc.):
HCl + NaCN → HCN + NaCl
HCN Physical Properties
HCN is found as a pale blue, colourless transparent liquid (hydrocyanic acid) or a colourless gas (hydrogen cyanide). Hydrocyanic acid features a density of 0.687 g/mL, and boils slightly above temperature, at 25.6 °C (78.1 °F). It has a distinct smell of bitter almonds, which is used to identify the presence of this highly poisonous material.
HCN Chemical Name
Its chemical name is hydrogen cyanide, which is a weak acid, and partially ionizes in water to give H+ (or H3O+) and the cyanide anion, CN-.
HCN + H2O → H3O+ + CN-
It reacts with bases to form salts called cyanides.
HCN + KOH → KCN + H2O
Uses of HCN
Despite its toxicity, HCN is a necessary reagent used in the production of a variety of useful industrial chemicals such as sodium cyanide, potassium cyanide, methyl methacrylate (monomer used for making polymers and plastics), chelating agents EDTA and NTA, as well as the polymer Nylon. HCN is also used to prepare pesticides and chemical warfare agents.
FAQs on Hydrocyanic Acid: Structure, Preparation, and Uses
1. What is the chemical formula and structure of Hydrocyanic Acid?
Hydrocyanic acid, also known as prussic acid, has the chemical formula HCN. Its structure consists of a hydrogen atom single-bonded to a carbon atom, which is in turn triple-bonded to a nitrogen atom. The structural representation is H−C≡N. This simple, linear molecule is a key compound in both industrial and organic chemistry.
2. What is the molecular geometry and hybridization of the central carbon atom in HCN?
The molecular geometry of hydrocyanic acid (HCN) is linear. The central carbon atom forms a single bond with hydrogen and a triple bond with nitrogen, leaving no lone pairs on the carbon. Due to the presence of two electron domains (one single bond, one triple bond), the carbon atom undergoes sp hybridization. This results in a straight molecular shape with a bond angle of 180°.
3. How is hydrocyanic acid commercially prepared?
The primary industrial method for producing hydrocyanic acid is the Andrussow process. This process involves the catalytic oxidation of ammonia and methane in the presence of oxygen. The reaction is carried out at high temperatures (around 1200 °C) over a platinum-rhodium catalyst. The balanced chemical equation for this process is:
2CH₄ + 2NH₃ + 3O₂ → 2HCN + 6H₂O
4. What are the most important industrial uses of hydrocyanic acid?
Hydrocyanic acid is a crucial precursor in the chemical industry. Its main applications include:
- Polymer Production: It is used to manufacture adiponitrile, a key intermediate in the production of Nylon 6,6.
- Plastics: It is a reactant in making acetone cyanohydrin, which is converted to methyl methacrylate, the monomer for acrylic glass (Plexiglas).
- Chelating Agents: It is used in the synthesis of chelating agents like ethylenediaminetetraacetic acid (EDTA).
- Mining: It is used in the form of its salts (e.g., sodium cyanide) for gold and silver extraction from ores.
5. Is hydrocyanic acid considered a strong or a weak acid, and why?
Hydrocyanic acid (HCN) is a very weak acid. Its weakness is due to the high stability of the molecule and the relatively strong bond between hydrogen and carbon. In an aqueous solution, it only partially dissociates into a hydrogen ion (H⁺) and a cyanide ion (CN⁻). Its acid dissociation constant (Ka) is very low (approximately 6.17 x 10⁻¹⁰), which confirms its status as a weak acid, much weaker than carboxylic acids like acetic acid.
6. Why is hydrocyanic acid and its salts so toxic to humans?
The extreme toxicity of hydrocyanic acid is due to the cyanide ion (CN⁻). When ingested or inhaled, the cyanide ion binds with high affinity to the ferric (Fe³⁺) ion in cytochrome c oxidase, an essential enzyme in the mitochondrial electron transport chain. This binding action blocks the enzyme, effectively halting cellular respiration. As a result, cells can no longer use oxygen to produce ATP (energy), leading to chemical asphyxiation and rapid cell death.
7. How do the bond polarities in HCN contribute to its overall molecular polarity?
Although HCN is a linear molecule, it is polar. This is because the individual bond polarities do not cancel each other out. The electronegativity values are H (2.20), C (2.55), and N (3.04). The C-N bond is highly polar with the dipole moment pointing towards the more electronegative nitrogen. The H-C bond is slightly polar, also pointing towards carbon. Since both bond dipoles point in the same general direction (from H towards N), they add up, creating a significant net dipole moment for the molecule.
8. What is the significance of hydrocyanic acid in organic synthesis, as per the CBSE syllabus?
In organic chemistry, particularly in the chapter 'Aldehydes, Ketones, and Carboxylic Acids', hydrocyanic acid plays a vital role in carbon chain extension. It undergoes a nucleophilic addition reaction with aldehydes and ketones to form products called cyanohydrins. This reaction is important because the cyanide group (-CN) in the cyanohydrin can be further hydrolysed to a carboxylic acid group (-COOH), creating a new, longer carbon chain with a functional group, which is a key step in many organic syntheses.
