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 –
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 –
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
