Formic acid's physical properties

Physical properties of formic acid

It can be freely miscible with water, ethanol, ether and glycerol, and most polar organic solvents. It also has certain solubility in hydrocarbons.

Formic acid solution with high concentration is easy to freeze in winter.

Relative density (20 ℃, 4 ℃): 1.220

Refractive index: 1.3714

Combustion heat: 254.4 kJ / mol

Critical temperature: 306.8 ℃

Critical pressure: 8.63 MPa

Flash point: 68.9 ℃ (open cup)

Density: 1.22 g / cm3

Saturated vapor pressure (24 ℃): 5.33 kPa

Viscosity (MPA · s, 25 º C): 1.966

Viscosity (MPA · s, 30 º C): 1.443

Heat of evaporation (kJ / mol, 25 º C): 19.90

Heat of evaporation (kJ / mol, B.P.): 23.19

Heat of fusion (kJ / mol): 12.69

Heat of formation (kJ / mol, 25 º C, liquid): -425.04

Specific heat capacity (kJ / (kg · K), 26.68 º C, constant pressure): 2.15

Thermal conductivity (w / (m · K), 15 º C): 13.9148

Thermal conductivity (w / (m · K), 30 º C): 13.8456

Thermal conductivity (w / (m · K), 60 º C): 13.7744

Thermal conductivity (w / (m · K), 90 º C): 13.6033

Log value of octanol / water partition coefficient: -0.54

Upper explosive limit (V / V): 57.0%

Lower explosive limit (V / V): 18.0%

molecular structure

Molar refractive index: 8.40

Molar volume (cm3 / mol): 39.8

Isotonic specific volume (90.2 K): 97.5

Surface tension (dyne / cm): 35.8

Polarizability (10-24 cm3): 3.33 [4]

In hydrocarbons and in the gaseous state, formic acid appears as a dimer bound by hydrogen bonds. In the gaseous state, the hydrogen bond leads to a large deviation between the equation of state of formic acid gas and the ideal gas. The liquid and solid formic acid are composed of formic acid molecules that are continuously bonded by hydrogen bonds. Due to the special structure of formic acid, one of its hydrogen atoms is directly connected to the carboxyl group. It can also be regarded as a hydroxyl formaldehyde.

Chemical properties of formic acid

1. Acidity of formic acid

Formic acid is the only fatty acid with hydrogen atom attached to the carboxyl group. The electron repulsion force of the hydrogen atom is far less than that of the hydrocarbon group, making the electron density of the carbon atom of the carboxyl group lower than that of other carboxylic acids. Moreover, due to the conjugation effect, the electrons on the oxygen atom of the carboxyl group are more carbon biased, so the acidity of formic acid is stronger than that of other carboxylic acids in the same series. Formic acid has the same properties as most other carboxylic acids. Under normal circumstances, formic acid does not generate acyl chloride or anhydride. Formic acid is a monobasic weak acid in aqueous solution. The acidity coefficient (PKA) = 3.75 (at 20 ℃), and the pH value of 1% formic acid solution is 2.2.

The following are the properties of formic acid:

React with alkali

Formic acid has the property of acid, so it can neutralize with alkali to form formate and water.

Reaction of formic acid with ammonia: HCOOH + NH3 · H2O = = hcoonh4 + H2O

Formic acid and sodium hydroxide: HCOOH + NaOH = hcoona + H2O

Formic acid and copper hydroxide: 2hcooh + Cu (OH) 2 = Cu (HCOO) 2 + 2H2O

Formic acid and calcium hydroxide: 2hcooh + Ca (OH) 2 = Ca (HCOO) + 2H2O

React with salt

Formic acid can also react with some salts.

Formic acid and sodium carbonate: 2hcooh + Na2CO3 = 2hcoona + H2O + CO2 ↑

Formic acid and calcium carbonate: 2hcooh + CaCO3 = Ca (CH3COO) 2 + H2O + CO2 ↑

Reaction with metal

Due to the nature of weak acids, formic acid is corrosive to many metals, such as magnesium and zinc, which react to generate hydrogen and metal formates.

Reaction of formic acid with sodium: 2hcooh + 2Na = 2hcoona + H2 ↑

Reaction of formic acid with zinc: 2hcooh + Zn = Zn (COOH) 2 + H2 ↑

Reaction of formic acid with magnesium: 2hcooh + Mg = Mg (COOH) 2 + H2 ↑

2. Fade potassium permanganate

Formic acid has similar reducing property to aldehyde, while acidic potassium permanganate has strong oxidizing property. Therefore, oxidation-reduction reaction can take place between the two to make the potassium permanganate solution fade.

3. Decomposition reaction

Formic acid is decomposed into CO and H2O by heating under the catalysis of concentrated sulfuric acid.

4. Silver mirror reaction

Formic acid is a strong reductant. It can act as a silver mirror reaction to reduce silver ions in silver ammoniate ions to metallic silver, and it is oxidized to carbon dioxide and water.  [5]

5. Addition reaction

Formic acid is the only carboxylic acid that can react with olefins. Under the action of acid (such as sulfuric acid and hydrofluoric acid), formic acid reacts rapidly with olefins to form formate. But side reactions similar to Koch reaction can also occur, and the product is higher carboxylic acid.

6. Esterification reaction

Formic acid can be esterified with alcohol under the action of concentrated sulfuric acid and heating. If formic acid and methanol are heated under the catalysis of concentrated sulfuric acid, esterification reaction can occur to produce methyl formate.

7. Reduction reaction

Formic acid can be reduced to formaldehyde by hydrogenation under the action of catalyst.

8. Complete combustion of formic acid

High purity formic acid can be completely burned under oxygen, and carbon dioxide and water will be generated.