Category: Chemistry

Laboratory Preparation of (Anhydrous) Formic Acid

Laboratory Preparation of (Anhydrous) Formic Acid

Introduction to Formic Acid

Formic acid is a weak organic acid. Its molecular formula is HCOOH. According to IUPAC, it is termed as methanoic acid. Laboratory preparation of formic acid involves the reaction of oxalic acid crystals with glycerol.

Principle

Principle for laboratory preperation of formic acid
  • Formation of glycerol monoformate: Firstly, glycerol is treated with oxalic acid crystals at about 120oC or 393 K to form glycerol monoxalate. Glycerol monoxalate decomposes itself to give glycerol monoformate.
  • Hydrolysis of glycerol monoformate: The glycerol monoxalate thus formed reacts with fresh water [obtained from oxalic acid crystals, (COOH)2.2H2O] to produce formic acid.

In this reaction, glycerol is recovered back which can be used over again and again. Hence glycerol acts as catalyst.

Procedure

The mixture of about 40 g oxalic acid crystal and 50 mL anhydrous glycerol is taken in a distillation flask which is placed over a sand bath. The flask is provided with the thermometer and the delivery tube is surrounded with a condenser. Upon heating the mixture at about 120oC, the reaction initiates with the evolution of CO2. When the evolution of CO2 stops, then the flask is cooled and fresh oxalic acid crystals are again added in the flask. The flask is again heated upto 120oC to form aqueous formic acid. Finally it is collected in reciever.

Laboratory Preparation of Formic Acid ( Methanoic Acid )

Preparation of anhydrous formic acid

Formic acid is soluble in water and forms aqueous formic acid during its preparation. Hence we must dehydrate it to get anhydrous formic acid in lab as follows: 

The aqueous formic acid is first neutralized by lead carbonate (PbCO3) or litharge(PbO) to get lead formate. The solution of lead formate thus formed is filtered and concentrated to get lead formate crystals.

The crystals of lead formate are filled in the inner tube of condenser. One end of the condenser is fitted with hydrogen sulphide gas generator and the other end is fitted with a buchner flask guarded with a calcium chloride tube as shown in the above figure.

Conversion of aqueous formic acid into anhydrous formic acid
Conversion of aqueous formic acid into anhydrous formic acid

H2S gas is passed through he condenser while steam is passed through its outer jacket. The anhydrous formic acid collects into the buchner flask.

Reaction for conversion of lead formate into formic acid

Note

  • We cannot subject aqueous formic acid to fractional distillation to remove water because boiling point of formic acid is nearly equal to that of water. (Boiling point of water : 100oC; Boiling point of formic acid : 100.5oC)
  • Dehydrating agents like Conc. H2SO4, KOH, P2O5 etc cannot be used as formic acid reacts with them.

General properties of formic acid

  • Molecular mass: 42.03 g/mol
  • State: Liquid due to intermolecular H-bonding
  • Odor: Sharp pungent smell
  • Solubility: Soluble in water and most of the polar organic solvents
  • Melting point: 281 K
  • Boiling point: 374 K
  • Molecular shape: Planar
  • pH: Acidity (3.77)

Introduction and Properties of Ionic Compounds

Introduction and Properties of Ionic Compounds

Introduction to Ionic Compounds

Those compounds which are formed by combination of ions are called ionic compounds. They are also called electrovalent compounds. Electrostatic force of attraction between positively charged and negatively charged ions hold them together in the crystal lattice. The bond between oppositely charged ions in these compounds is called ionic bond or electrovalent bond.

Formation of Ionic Compounds

Before going through the properties of ionic compounds, lets look about their formation. Ionic compounds are formed by the combination of metal and non-metal. Metal atom loses electron whereas non metal atom gains electrons, and thereby both attain octet. The metal atom becomes positively charged after losing electron and non-metal atom becomes negatively charged after gaining electron. These charged species are called ions.

Being oppositely charged species, metal and non-metal ions attract each other with strong electrostatic force of attraction. This bond which is formed by the complete transfer of one or more electrons from one atom to other is called ionic bond and the compounds thus formed are called electrovalent or ionic compounds. Examples of ionic compounds are: NaCl, KCl, NaOH, CaCl2, HCl etc.

Let us see the formation of an ionic compound, NaCl as an example.
Its formation takes place in following steps:

  • Sodium atom loses the outermost electron and becomes positively charged ion with octet.
    Na → Na+ + e
  • Chlorine gains that electron, and becomes negatively charged with octet.
    Cl + e → Cl
  • Sodium and chloride ions are held together by electrostatic forces of attraction.
    Na+ + Cl → Na+Cl or NaCl
Note: Electrovalency of an element is the number of electrons lost or gained by an atom of the element while forming an ionic bond.
In above example, elecrtovalency of sodium and chlorine is 1

Formation of Ionic Compound
Formation of NaCl

Properties of Ionic Compounds

  • High melting and boiling point:

    Ionic compounds have high melting and boiling points. Since the constituents of ionic compounds are held together by strong electrostatic force of attraction, they require large amount of energy to dissociate.
  • Formation of crystal lattice:

    Ions in ionic compounds are arranged in regular three dimensional pattern called crystal lattice. Ions are oriented in fixed direction and placed in fixed place unlike amorphous solid.
  • Hard and Brittle:

    Since force of attraction between ions is very high ionic compounds are hard in nature. Also, due to the formation of crystal lattice, these are generally brittle in nature and break into pieces on strong hitting.
  • High enthalpy of fusion and vaporization:

    Enthalpy of fusion is the amount of energy required to change one mole of solid into its liquid form at its melting point. Similarly, enthalpy of vaporization is the amount of energy required to change one mole of liquid into its vapour form at its boiling point. Due to the strong electrostatic force acting on ions, they require high energy to dissociate and have high enthalpy of fusion and vaporization.
  • Electrical conductivity:

    Although ionic solids consists of ions, they do not conduct electricity due to immobility of ions. But in solution form, ions change into electrolytes and conduct electricity. However, electrolytic solution have low conductivity than that of metals.
  • Solubility:

    Ionic solids are generally soluble in polar solvent like water and are insoluble in non polar solvent like benzene.