Formic Acid Formula: Ever wondered why do we have an itching or burning sensation after an ant bite? It happens due to the entry of formic acid into the body. The IUPAC name of Formic acid is methanoic acid. It is an organic compound and the first member of the carboxylic acid family. It is found in red ants, honey flies, wasps and grass scorpions. Let’s explore more about this chemical compound.

Structure of Formic Acid

The formula, hybridization, molecular geometry  of Formic Acid are explained below

Formula of Formic Acid

The IUPAC name of formic acid is Methanoic acid. It is the simplest carboxylic acid with the condensed formula \({\rm{C}}{{\rm{H}}_2}{{\rm{O}}_2}\) The chemical formula of formic acid is \({\rm{HCOOH}}\).

Molar Mass of Formic Acid

The molar mass of Formic acid, \({\rm{HCOOH}}\) is:

\(\left( {{\rm{The}}\,{\rm{atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{carbon}}} \right) + 2\left( {{\rm{The}}\,{\rm{atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen}}} \right)2 + \left( {{\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{Oxygen}}} \right) = 12.01{\mkern 1mu} {\rm{u}} + 2\left( {1.007{\mkern 1mu} {\rm{u}}} \right) + 2\left( {15.999\,{\rm{u}}} \right)\)

\( = 46.02\;{\rm{g}}\;{\rm{mo}}{{\rm{l}}^{ – 1}}\)

Hence, one mole of Formic acid weighs \(46.02\,{\rm{amu}}.\)

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Hybridisation of Formic Acid

Formic acid is the simplest carboxylic acid. The general structural formula of the carboxylic acid is \({\rm{RCOOH}}\), where \({\rm{R}}\) stands for any side group comprising \({\rm{H}}\) or an alkyl group or another \( – {\rm{C}}\) bonded to a certain chain.

Hybridisation of the carboxylic carbon atom \( = \) Number of atoms attached \( + \) Lone pairs

\(= 3 + 0 = 3\)

The carbon atom of the formic acid is \({\rm{s}}{{\rm{p}}^2}\) hybridised \(\left( {1\;{\rm{s}} + 2{\rm{P}} = 3{{{\mathop{\rm sp}\nolimits} }^2}} \right).\) The atomic orbitals of the carbon atom undergo intermixing to form \({3{{{\mathop{\rm sp}\nolimits} }^2}}\) hybridised orbitals.

Hence, the carbon atom of the formic acid forms three sigma bonds. The carbonyl group has a basic trigonal structure with bond angles of \(120\) degrees due to the creation of three sigma bonds. Only two out of three \({\rm{p}}\) orbitals participate in hybridisation; hence, one \({\rm{p}}\) orbital is unhybridised.

This \({\rm{p}}\) orbital forms a pi bond with the unhybridised \({\rm{p}}\) orbital of the carbonyl oxygen atom and is directed above and below the plane of the paper. The oxygen atom bonded to hydrogen and the carbon atom is \({\rm{s}}{{\rm{p}}^3}\) hybridised.

There are two lone pairs on the oxygen atom of the carbonyl group and that of the hydroxyl group. One of the hydroxyl oxygen’s lone pair electrons can conjugate with the carbonyl group’s pi system. As a result, the carboxyl group becomes planar and can be represented using the resonance structure below.

Molecular Geometry of Formic Acid

The VSEPR (Valence Shell Electron Pair Repulsion) theory can determine the molecular geometry or shape.

In formic acid, the carbon atom is the central atom, and it has three bond pairs without any lone pair of electrons. When predicting the structure of a molecule, a double bond is treated as one bond pair.

As a result, the following table may easily predict the structure of formic acid.

General formula	Number of bond pairs	Molecular shape/geometry
\({\rm{AX}}\)	\(1\)	Linear
\({\rm{A}}{{\rm{X}}_2}\)	\(2\)	Linear
\({\rm{A}}{{\rm{X}}_3}\)	\(3\)	Trigonal planar
\({\rm{A}}{{\rm{X}}_4}\)	\(4\)	Tetrahedral
\({\rm{A}}{{\rm{X}}_5}\)	\(5\)	Trigonal bipyramidal
\({\rm{A}}{{\rm{X}}_6}\)	\(6\)	Octahedral

Hence, formic acid is of \({\rm{A}}{{\rm{X}}_3}\) type and will have a trigonal planar geometry around the carbon atom. A tetrahedral geometry around oxygen atom as it has two lone pairs and two bond pairs.

Bond Angle of Formic Acid

The \({\rm{H}} – {\rm{C}} = {\rm{O}}\) and \({\rm{O}} = {\rm{C}} – {\rm{O}}\) bond angle is found to be \({120^{\rm{o}}}\). This bond angle also accounts for the trigonal planar geometry of formic acid around the carbon atom. It minimises bond pair-bond pair repulsions around the carbon atom. However, the actual bond angle is slightly different from \({120^{\rm{o}}}\) due to larger repulsion between the double bond pair and the single bond pair. Hence, the \({\rm{H}} – {\rm{C}} = {\rm{O}}\) and \({\rm{O}} = {\rm{C}} – {\rm{O}}\) bond angles are greater than \({120^{\rm{o}}}\).

Similarly, the bond angle around the oxygen atom, i.e., \({\rm{C}} – {\rm{O}} – {\rm{H}}\), should be \({109.5^{\rm{o}}}\), being a tetrahedral geometry. But, the \({\rm{C}} – {\rm{O}} – {\rm{H}}\) bond angle is found to be \({106^{\rm{o}}}\). This happens to minimise the repulsion between two lone pairs present on the oxygen atom.

The hybridisation of the carbon and oxygen atom in the formic acid can be determined by the valence bond theory (VBT) and steric number.

Dipole Moment of Formic Acid

In formic acid, the electronegativity difference for the \({\rm{C}} – {\rm{H}}\) bond is \(2.5 – 2.2 = 0.3\), whereas for \({\rm{C}} – {\rm{O}}\) and \({\rm{O}} – {\rm{H}}\) bond is \(1.0\) and \(1.3\), respectively.

Considering the above values, the \({\rm{C}} – {\rm{H}}\) bond is slightly polar, and the \({\rm{O}} – {\rm{H}}\) and \({\rm{C}} – {\rm{O}}\) bonds are polar covalent bonds. Due to the electronegativity difference, a dipole develops in the \({\rm{C}} – {\rm{O}}\) bond with a partial positive charge on carbon and a partial negative charge on the oxygen atom. Similarly, the \({\rm{O}} – {\rm{H}}\) bond also develops a dipole that contributes to the polar nature of the formic acid.

Hence, formic acid is a polar molecule and readily dissolves  in water and most of the polar solvents.

Lewis Structure of Formic Acid

The Lewis structures or electron dot structures are the two-dimensional diagrams representing the valence electrons between atoms of the molecule. It represents bonding as well as nonbonding electrons (lone pairs) present in the outermost shell of an atom.

In formic acid, there are two hydrogen atoms, two oxygen atoms, and one carbon atom. Hence, the total number of valence electrons in formic acid is: \({\rm{4C + }}\left( {{\rm{1*2H}}} \right){\rm{  +  }}\left( {{\rm{6*2O}}} \right) = 18\) electrons.

The carbon atom is the least electronegative atom, acts as the central atom. The \(18\) valence electrons will be distributed over \({\rm{C}},{\rm{ H}}\) and \({\rm{O}}\) with Carbon as the central atom. The distribution occurs so that the \({\rm{C}}\) and \({\rm{O}}\) atoms satisfy the octet rule and hydrogen satisfies its duplet. The remaining electrons are present as lone pairs.

The two connecting electrons form one connection and the four connecting electrons become a two-fold connection.

The Lewis formic acid structure can therefore be drawn:

Skeletal Structure of Formic Acid

Formic acid, also known as  methanoic acid consists of a single carboxylic acid group \(( – {\rm{COOH}})\) attached to a hydrogen atom. The \({\rm{C}} – {\rm{H}}\) and \({\rm{C}} – {\rm{O}} – {\rm{H}}\) are single bonds whereas \( – {\rm{CO}}\) is a double bond. There are \(4\) sigma bonds and one pi bond.

Preparation of Formic acid

1. In the earlier days, formic acid was separated from the distillation of ant bodies, and it is now made industrially from methanol.
2. Laboratory Method

a. From Glycerol And Oxalic Acid:

Formic acid is obtained by heating a Glycerol and oxalic acid mixture to \({100^{\rm{o}}} – 110\,^\circ {\rm{C}}\) temperature.

About \(40\) grams of crystal Oxalic Acid is heated slowly with \(50{\rm{ ml}}\) Anhydrous glycerol in a distillation flask at \({100^{\rm{o}}} – 110\,^\circ {\rm{C}}.\)

The receptor distils an aqueous solution of formic acid. The Glycerol remains in the distillation bottle after the reaction has ended.

Formic acid is again obtainable in the distillation flask by heating and adding oxalic acid. In the following three steps the reaction is completed.

Step 1:  Glycerol and oxalic acid combine to form glycerol mono oxalate.

Step 2: Glycerol mono oxalate converts to additive glycerol monoformate with the release of one molecule of carbon dioxide at \(100\,^\circ {\rm{C}}\) temperature.

Step 3: Glycerol Monoformate reacts with water obtained in step \(1\) to form Formic acid and Glycerol.

b. Anhydrous Formic Acid From An Aqueous Solution Of Formic Acid

Some amount of water is present in formic acid obtained by the laboratory method. As formic acid has a boiling point of \(100\,^\circ {\rm{C}},\) it cannot be dehydrated by the efficient distillation method.

Therefore, to make it anhydrous, the aqueous solution of oxalic acid is heated with lead carbonate. On cooling this filtered fluid, the lead becomes crystal of Lead Formate, which are separated and dried.
\(2{\rm{HCOOH}} + {\rm{PbC}}{{\rm{O}}_3} \to {({\rm{HCOOH}})_2}\;{\rm{Pb}} + {{\rm{H}}_2}{\rm{O}} + {\rm{C}}{{\rm{O}}_2}\)

To retrieve formic acid from Lead Formate, the crystals are heated by steam and hydrogen Sulfide is added to it. On doing this, formic acid and lead sulfide are formed.
\({({\rm{HCOOH}})_2}{\rm{Pb}} + {{\rm{H}}_2}{\rm{S}} \to {\rm{HCOOH}} + {\rm{PbS}}\)

The formic acid obtained in this way is pure and anhydrous.

Industrial Method

a. From Carbon Mono Oxide:

Heating a mixture of carbon monoxide and sodium hydroxide at \({150^{\rm{o}}}{\rm{C}}\) forms sodium formate. Adding sulphuric acid to sodium formate gives formic acid.
\({\rm{CO}} + {\rm{NaOH}} \to {\rm{HCOONa}}\)
\({\rm{HCOONa}} + {{\rm{H}}_2}{\rm{S}}{{\rm{O}}_4} \to {\rm{HCOOH}} + {\rm{NaHS}}{{\rm{O}}_4}\)

b. Methyl Alcohol:

By oxidation of methyl alcohol results in formaldehyde which on further oxidation forms formic acid.

c. Hydrogen Cyanide And Methyl Formate:

Water decomposition of hydrogen cyanide and methyl formate results in formic acid.
\({\rm{HCN}} + 2{{\rm{H}}_2}{\rm{O}} \to {\rm{HCOOH}} + {\rm{N}}{{\rm{H}}_3}\)
\({\rm{HCOOC}}{{\rm{H}}_3} + {{\rm{H}}_2}{\rm{O}} \to {\rm{HCOOH}} + {{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}}\)

Physical Properties of Formic Acid

1. The molar mass of formic acid is \(46.03{\rm{ g}}/{\rm{mol}}\).
2. Formic acid has boiling point which is nearly equal to the water molecule, i.e., \(100.8{\,^{\rm{o}}}{\rm{C}}.\)
3. Formic acid is a colourless, hygroscopic liquid with a pungent and penetrating odor.
4. It is highly soluble in water and polar solvents.
5. It exists as a hydrogen-bonded dimer in the vapour phase as well as in hydrocarbons.
6. It forms blisters on the skin and is corrosive.

Chemical Properties of Formic Acid

Formic acid consists of a \( – {\rm{COOH}}\) group attached to an \({\rm{H}}\) atom. It comprises of  dual functional group, i.e., an aldehyde group and a carboxylic group. Due to this reason, formic acid is found to be more functional than other members of this group.

The stability of the formic acid can be attributed to the resonance structure of the formate ion.

Some of the major chemical reactions of formic acid are listed below.

a. Reducing Properties: It is a good reducing agent due to the presence of an aldehydic group in it.

\( \Rightarrow \) Silver Mirror Test: When formic acid is boiled with Tollen’s reagent, a silver mirror is deposited.

Other carboxylic acids do not give this test.

\( \Rightarrow \) Fehling’s Solution: When formic acid is warmed with Fehling’s solution, a brick red ppt. of cuprous oxide is obtained.

Other carboxylic acids do not give this test.

\( \Rightarrow \) Action with acidified \({\rm{KMn}}{{\rm{O}}_4}\) 

When formic acid is treated with acidified \({\rm{KMn}}{{\rm{O}}_4}\) solution, the pink colour of \({\rm{KMn}}{{\rm{O}}_4}\) is discharged.

Other carboxylic acids do not give this test.

b. Acidic Properties: Formic acid is the most acidic compared to its category of acids and exhibits all the common properties of acids. It forms salts with alkalis and esters with alcohols.

\({\rm{HCOOH}} + {\rm{NaOH}} \to {\rm{HCOONa}} + {{\rm{H}}_2}{\rm{O}}\)

\({\rm{HCOOH}} + {{\rm{C}}_2}{{\rm{H}}_5}{\rm{OH}} \to {\rm{HCOO}}{{\rm{C}}_2}{{\rm{H}}_5} + {{\rm{H}}_2}{\rm{O}}\)

It also forms salts by reacting with metals like sodium, zinc etc. with the release of hydrogen gas.

Example:

\(2{\rm{HCOOH}} + 2{\rm{Na}} \to 2{\rm{HCOONa}} + {{\rm{H}}_2}\)

c. Decomposition – Formic acid on being heated at \(160{\,^{\rm{o}}}{\rm{C}}\) temperature decomposes into \({\rm{C}}{{\rm{O}}_2}\) and \({{\rm{H}}_2}\).

\({\rm{HCOOH}} \to {\rm{C}}{{\rm{O}}_2} + {{\rm{H}}_2}\)

d. Dehydration: Formic acid on being heated with concentrated sulphuric acid forms carbon mono oxide and water.

e. Reaction of phosphorus pentachloride/thionyl chloride: Formic acid with phosphorus pentachloride forms formyl chloride \(({\rm{HCOCl}})\), decomposes into carbon mono oxide and hydrochloric acid.

\({\rm{HCOOH}} + {\rm{PC}}{{\rm{l}}_5} \to {\rm{HCOCl}} + {\rm{POC}}{{\rm{l}}_3} + {\rm{HCl}}\)

\({\rm{HCOCl}} \to {\rm{CO}} + {\rm{HCl}}\)

f. Effect of heat on salts:

1. Sodium oxalate and hydrogen are formed by heating sodium or potassium formate to \(360{\,^{\rm{o}}}{\rm{C}}.\)

2. Hydrogen gas is released when sodium or potassium formate is heated with soda lime.
\({\rm{HCOOH}} + {\rm{NaOH}} \to {\rm{N}}{{\rm{a}}_2}{\rm{C}}{{\rm{O}}_3} + {{\rm{H}}_2}\)

3. Formamide is formed on heating ammonium formate.
\({\rm{HCOON}}{{\rm{H}}_4} \to {\rm{HCON}}{{\rm{H}}_2} + {{\rm{H}}_2}{\rm{O}}\)

4. Formaldehyde is formed by dry distillation of calcium formate.
\({({\rm{HCOO}})_2}{\rm{Ca}} \to {\rm{HCHO}} + {\rm{CaC}}{{\rm{O}}_3}\)

5. Dry distillation of calcium formate with calcium acetate produces acetaldehyde.
\({\left( {{\rm{C}}{{\rm{H}}_3}{\rm{COO}}} \right)_2}{\rm{Ca}} + {({\rm{HCOO}})_2}{\rm{Ca}} \to 2{\rm{C}}{{\rm{H}}_3}{\rm{CHO}} + 2{\rm{CaC}}{{\rm{O}}_3}\)

Formic Acid Tests

1. The following general tests are carried out by all carboxylic acids.
2. Carboxylic acids turn blue litmus paper to red.
3. Carbon dioxide is released with effervescence on adding  saturated solution of \({\rm{NaHC}}{{\rm{O}}_3}\) to carboxylic acids.
4. Silver mirror is achieved in formic acid by heating ammonium silver nitrate.
5. When formic acid is heated with Fehling’s solution, a red precipitate of cuprous oxide is formed. A white precipitate is formed due to the formation of mercurous chloride when formic acid is heated with mercuric chloride solution.
6. It is neutralised by ammonium hydroxide, and red colour is obtained by adding a few drops of ferric chloride solution.
7. On heating this acid with concentrated sulfuric acid, carbon monoxide gas is released, which burns with flame.

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Uses of Formic Acid

1. Formic acid is used as a preservative and antibacterial agent in livestock feed.
2. It is used to process organic latex (sap) into raw rubber.
3. It is used in the textile industry and for the tanning of leather.
4. Some formate esters are artificial flavourings or perfumes.
5. It is the active ingredient for household limescale remover.
6. In the laboratory, formic acid is also used as a source for carbon monoxide, which is set by the addition of sulfuric acid.
7. Fuel cells that use modified formic acid are promising.

Summary

Formic acid is a unique compound. This is because it consists of an aldehyde group along with a carboxylic group. Hence, it is important to know its structure. In this article, we learned the chemical bonding in the formic acid by drawing its Lewis structure, its molecular geometry, and hybridisation. We also learnt its polar nature and the special tests shown by it.

FAQs on Formic Acid Formula

Q.1. What is the chemical formula for formic acid?
Ans: The chemical formula of formic acid is \({\rm{HCOOH}}\). It consists of a carboxyl group along with an aldehyde group.

Q.2. What is the use of formic acid?
Ans: Formic acid is used as an antibacterial agent and as a preservative in livestock feed. It is an active ingredient in household limescale remover.

Q.3. Is formic acid a strong acid?
Ans: The reaction for formic acid in an aqueous solution is given below.
\({\rm{HCCOH}} \to {\rm{HCO}}{{\rm{O}}^ – } + {{\rm{H}}^ + }\)
Formic acid does not release all its hydrogen ions into the solution; hence it is termed a weak acid.

Q.4. What is the common name for HCOOH?
Ans: The common name of \({\rm{HCOOH}}\) is formic acid. As it contains only one carbon atom, its IUPAC name is Methanoic acid.

Q.5. What is the pH of formic acid?
Ans: The \({\rm{pH}}\) of a \(0.1{\rm{M}}\) formic acid solution is \(2.38\).

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