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Sulfuric Acid : Chemical Properties, Uses and Structure

Sulfuric Acid Formula and Structure

Sulfuric acid reacts with PCl5 to form sulfuric chloride (SO2Cl2). The composition of sulfuric chloride is definite as one molecule of it is obtained by the combination of one molecule of SO2 and one molecule of chlorine. Sulfuryl chloride hydrolyzes to form sulfuric acid.

SO2 + Cl2 → SO2Cl2

Sulfuric Acid + 2PCl5  → SO2Cl2 + 2POCl3 + 2HCl

SO2Cl2 + 2H2O → Sulfuric Acid + 2HCl

From the above facts it is known that the molecular formula of sulfuric acid is H2SO4 and two hydroxyl (-OH) groups are present in it.

Sulfuric acid is a dibasic acid. Since isomerism is not found in mono-sulfonated carbon compounds, both hydroxyle groups are identical. On the basis of these two facts the structure of sulfuric acid is as follows –

Sulfuric Acid Structure

On this basis the sulfur atom in H2SO4 and SO42- ion is sp3 hybridised. In SO42- ion all the four S – O bonds are same and its shape is tetrahedral. Its binding formula and electron point formula are as follows –

binding formula
electron point formula

Sulfuric Acid Uses

The main use of sulfuric acid is in the manufacture of various chemical substances, fertilizers and explosives. It is a major reagent used in the laboratory. It is often used as an oxidising and dehydrating agent. It is also used in factories of paint, paper, clothes, medicine etc.

Sulfuric Acid Chemical Properties

On this basis the sulfur atom in H2SO4 and SO42- ion is sp3 hybridised. In SO42- ion all the four S – O bonds are same and its shape is tetrahedral. Its binding formula and electron point formula are as follows –

Effect of heat

On boiling it decomposes into sulfur trioxide and water.

H2SO4 ⇌ H2O + SO3

The process of decomposition occurs more rapidly on heating.

Action with Bases

Sulfuric acid is a dibasic acid. Therefore, it reacts with bases to form salts. Being dibasic, it forms two types of salts.

Example: By reacting with sodium hydroxide (NaOH), it forms sodium hydrogen sulfate(NaHSO₄) and sodium sulfate(Na2SO4) respectively.

NaOH + H2SO4 → NaHSO4 + H2O

NaOH + NaHSO4 → Na2SO4 + H2O

Oxidative Properties

It also acts as an oxidising agent. In the presence of substances to be oxidised, it provides nascent oxygen by the following reaction:

H2SO4 → H2O + SO2 + O

Oxidation of Carbon

It oxidises carbon to CO2.

[H2SO4 → H2O + SO2 + O] x 2

C + 2O → CO2

—————————————–

C + 2H2SO4 → CO2 + 2H2O + 2SO2

Oxidation of Sulfur

It oxidizes to sulfur dioxide.

[H2SO4 → H2O + SO2 + O] x 2

S + 2O → SO2

—————————————–

S + 2H2SO4 → 2H2O + 3SO2

Oxidation of Phosphorus

It oxidises phosphorus to phosphoric acid.

[H2SO4 → H2O + SO2 + O] x 10

P4 + 10O → 2P2O5

[P2O5 + 3H2O → 2H3PO4] x 2

—————————————–

P4 + 10H2SO4 → 4H2O + 10SO2 + 4H3PO4

Oxidation of HBr and HI

It oxidises HBr to bromine (Br2) and HI to iodine (I2).

2HBr + H2SO4 → 2H2O + SO2 + Br2

2HI + H2SO4 → 2H2O + SO2 + I2

Oxidation of bromides and iodides

It also oxidises bromides and iodides to bormine and iodine respectively.

example :

2KBr + H2SO4 → 2H2O + SO2 + Br2 + K2SO4

2KI + H2SO4 → 2H2O + SO2 + I2 + K2SO4

Oxidation of hydrogen sulfide :

It oxidises hydrogen sulfide (H2S) to sulfur (S).

2KBr + H2SO4 → 2H2O + SO2 + Br2 + K2SO4

2KI + H2SO4 → 2H2O + SO2 + I2 + K2SO4

In addition to the above, it also oxidizes many other substances in appropriate states. It oxidizes metals to their sulfates.

Reaction with Metals

In metals whose reduction potential is less than the reduction potential of hydrogen, hydrogen is displaced by the action of dilute sulfuric acid on those metals and sulfate is formed from metals.

Example: Zn, Fe, Cr, Mg, Mn and Al react with dilute sulfuric acid to form hydrogen.

Zn + H2SO4 → ZnSO4 + H2

2Fe + 3H2SO4 → Fe2(SO4)3 + 3H2

2Cr + 3H2SO4 → Cr(SO4)3 + 3H2

Mg + H2SO4 → MgSO4 + H2

Mn + H2SO4 → MnSO4 + H2

2Al + 3H2SO4 → Al2(SO4)3 + 3H2

For metals whose reduction potential is greater than the reduction potential of hydrogen, hydrogen is not displaced by the reaction of dilute sulfuric acid on those metals.

Most of these metals (Cu, Ag, Hg, etc.) on heating with concentrated sulfuric acid form sulfur di oxide gas and sulfate salts. In these reactions metals get oxidised to their cations.

example :

H2SO4 → H2O + SO2 + O

Cu + O → CuO

CuO + H2SO4 → CuSO4 + H2O

—————————————–

Cu + 2H2SO4 → 2H2O + CuSO4 + SO2

2Ag + 2H2SO4 → 2H2O + Ag2SO4 + SO2

Hg + 2H2SO4 → 2H2O + HgSO4 + SO2

Sulfuric acid does not react with gold and platinum.

Reaction with Salts

It reacts with salts to form their corresponding acid.

The following reactions of double decomposition take place in the forward direction as one or more of the products formed in this direction is separated as gaseous or precipitate.

FeS + H2SO4 → FeSO4 + H2S

Na2SO3 + H2SO4 → Na2SO4 + H2SO3

H2SO3 → SO2 + H2O

2NaCl + H2SO4 → Na2SO4 + 2HCl

BaCl2 + H2SO4 → BaSO4 + 2HCl

CaCO3 + H2SO4 → CaSO4 + H2CO3

H2CO3 → H2O + CO2

CuCO3 + H2SO4 → CuSO4 + H2CO3

H2CO3 → H2O + CO2

Pb(NO3)2 + H2SO4 → PbSO4 + 2HNO3

Dehydrating Effect

Concentrated sulfuric acid has dehydrating properties. It absorbs water. Being a dehydrating agent, it reacts with sugar (Sucrose C12H22O11) –

C12H22O11 + H2SO4 → 11H2O + H2SO4 + 12C

Due to the formation of carbon (C) in the above reaction, sugar is destroyed and its color becomes black.

Its dehydrating property is used in many reactions.

Example: In making ethyline from ethyl alcohol, its action is done with ethyl alcohol.

C2H5OH → C2H4 + H2O

Sulphonation

On heating with aromatic compounds it displaces one or more hydrogen atoms from their molecules by sulphonic acid group (-SO3H). In such reactions sulphonic acid is formed and these reactions are called Sulphonation.

Example:       

C6H6 + H2SO4 → C6H5SO3H + H2O

In some oxidation-reduction reactions, it acts as an acidic medium.

example :

Reaction of ferrous sulfate and KMnO4 in the presence of H2SO4

4KMnO4 + 8H2SO4 + 10FeSO4 → K2SO4 + 2MnSO4 + 5Fe2(SO4)3 + 8H2O

Reaction of SO2 and K2Cr2O7 in the presence of H2SO4

K2Cr2O7 + 3SO2 + H2SO4 → K2SO4 + Cr2(SO4)3 + H2O

The color of the solution turns green due to the formation of Cr2(SO4)3. Therefore, paper soaked with acidic K2Cr2O7 turns green when it comes in contact with sulfur dioxide gas.

Reaction of ferrous sulfate and K2Cr2O7 in the presence of H2SO4

K2Cr2O7 + 7H2SO4 + 6FeSO4 → K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O

Reaction of oxylic acid (H2C2O4) and KMnO4 in the presence of H2SO4

2KMnO4 + 3H2SO4 + 5 H2C2O4 → K2SO4 + 2MnSO4 + 8H2O + 10CO2

Reaction of oxylic acid (H2C2O4) and KMnO4 in the presence of H2SO4

Testing for Sulfate ion

On adding a solution of berium chloride or lead nitrate to a dilute solution of sulfuric acid or any sulfate salt, a white precipitate of berium sulfate or lead sulfate is obtained which does not dissolve in concentrated acids (HCl and HNO3).

BaCl2 + H2SO4 → BaSO4 + 2HCl

Pb(NO3)2 + H2SO4 → PbSO4 + 2HNO3

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