Hydrogen cyanide

HCN Molar mass27.0253 g/mol Appearance Colorless liquid or gas Odor bitter almond-like Density0.6876 g/cm Melting point−13.29 °C (8.08 °F; 259.86 K) Boiling point26 °C (79 °F; 299 K)

Solubility in water

Miscible Solubility in ethanolMiscible Vapor pressure100 kPa (25 °C)

Henry's law
constant (k_H)

75 μmol Pa kg Acidity (pK_a) 9.21 (in water),

12.9 (in DMSO)

Basicity (pK_b) 4.79 (cyanide anion) Conjugate acid Hydrocyanonium Conjugate base Cyanide

Refractive index (n_D)

1.2675 Viscosity0.183 mPa·s (25 °C) Structure

Crystal structure

tetragonal (>170 K)
orthorhombic (<170 K)

Point group

C_∞v

Molecular shape

Linear

Dipole moment

2.98 D Thermochemistry

Heat capacity(C)

35.9 J K mol (gas)

Std molar
entropy(S₂₉₈)

201.8 J K mol

Std enthalpy of
formation(Δ_fH₂₉₈)

135.1 kJ mol Hazards GHS labelling:

Pictograms

Signal word

Danger

Hazard statements

H220, H224, H300+H310+H330, H319, H336, H370, H410

Precautionary statements

P210, P261, P305+P351+P338NFPA 704 (fire diamond)

4

2

Flash point−17.8 °C (0.0 °F; 255.3 K)

Autoignition
temperature

538 °C (1,000 °F; 811 K) Explosive limits5.6% – 40.0% Lethal dose or concentration (LD, LC):

LC₅₀ (median concentration)

501 ppm (rat, 5 min)
323 ppm (mouse, 5 min)
275 ppm (rat, 15 min)
170 ppm (rat, 30 min)
160 ppm (rat, 30 min)
323 ppm (rat, 5 min)

LC_Lo (lowest published)

200 ppm (mammal, 5 min)
36 ppm (mammal, 2 hr)
107 ppm (human, 10 min)
759 ppm (rabbit, 1 min)
759 ppm (cat, 1 min)
357 ppm (human, 2 min)
179 ppm (human, 1 hr) NIOSH (US health exposure limits):

PEL (Permissible)

TWA 10 ppm (11 mg/m) [skin]

REL (Recommended)

ST 4.7 ppm (5 mg/m) [skin]

IDLH (Immediate danger)

50 ppm Related compounds

Related alkanenitriles

Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).

verify ( ?)

Hydrogen cyanide (also called prussic acid) is a chemical compound with the formula HCN and structural formulaH−C≡N. It is a highly toxic and flammable liquid that boils slightly above room temperature, at 25.6 °C (78.1 °F). HCN is produced on an industrial scale and is a highly valued precursor to many chemical compounds ranging from polymers to pharmaceuticals. Large-scale applications are for the production of potassium cyanide and adiponitrile, used in mining and plastics, respectively. It is more toxic than solid cyanide compounds due to its volatile nature. A solution of hydrogen cyanide in water, represented as HCN(aq), is called hydrocyanic acid. The salts of the cyanide anion are known as cyanides.

Whether hydrogen cyanide is an organic compound or not is a topic of debate among chemists. It is traditionally considered inorganic, but can also be considered a nitrile, giving rise to its alternative names of methanenitrile and formonitrile.