Sulphuric Acid

A molecule of Sulphuric Acid, H2SO4, consists of two atoms of hydrogen, one atom of sulphur and four atoms of oxygen

Sulphuric acid is a colourless viscous corrosive oily liquid, which has

Sulphuric acid is the strong acid produced by dissolving sulphur trioxide in water.

	SO3	+	H2O	==>	H2SO4

The Strength of Acids is determined by the degree to which they are ionised in aqueous solution.
For example, Sulphuric Acid, H2SO4, which is a strong acid is fully dissociated, and all the displaceable hydrogen in the acid is present in solution as Hydrogen Ion, H(+).

	H2SO4  	==>   H(+)   +   SO4	
		     100% as H(+)	
In contrast, the weak acids ethanoic acid, CH3COOH, is partially ionised in solution, and only approximately 5% of the displaceable Hydrogen in the acid is present in solution as hydrogen ion, H(+).
	CH3COOH   ==>   H(+)   +   CH3COO(-)	
			5% as H(+)

Properties of Sulphuric Acid

A Dibasic Acid has two acidic hydrogen atoms in its molecules which can be ionised. Sulphuric Acid, H2SO4, is a dibasic acid, because it contains two hydrogens atoms which ionise in aqueous solution to become Hydrogen Ions, H(+).

	H2SO4    ==>    2 H(+)    +    SO4(2 -)

Sulphuric acid is an important industrial chemical and it has many uses as a strong oxidising agent and a powerful dehydrating agent.

Commercially available sulphuric acid is as a 96-98% solution of the acid in water.

It is a powerful protonating agent.

It is also a powerful dehydrating agent and is used to remove a molecule of Water, HO2, from many organic compounds.

The Dehydration Reactions of Alcohols results in their converted into an alkene, and involves the elimination of a molecule of water. Dehydration requires the presence of an acid and the application of heat.

Preparation of Sulphuric Acid

Combustion of Sulphur

When a small amount of Sulphur, S, is kindled on a deflagrating spoon, it burns with a bright blue flame when introduced into a gas jar containing Oxygen, O2. A gas, Sulphur Dioxide, SO2, is the main product of the combustion. However, a little Sulphur Trioxide, SO3, is also formed, which makes the gas slightly cloudy.

		S   +   O2   ==>	SO2	
           			   Sulphur Dioxide	

		2S   +  3O2   ==>       2SO3
				   Sulphur Trioxide	

When shaken with water, the products of combustion dissolve, forming an acidic solution which turns litmus red.

		SO2   +   H2O   ==>	H2SO3	
		Sulphur			Sulphurous	
		Dioxide			Acid	

		SO2   +   H2O   ==>   H2SO4	
		Sulphur			Sulphuric	
		Trioxide		Acid	

Manufacture of Sulphuric Acid

Sulphuric acid was manufactured by the lead-chamber process until the mid-1930s, but this process has now been replaced by the contact process, involving the catalytic oxidation of sulphur dioxide.

Properties of Sulphuric Acid

The Contact Process is used for manufacturing sulphuric acid and fuming sulphuric acid from sulphur dioxide, which is made by burning sulphur or by roasting sulphide ores and oxygen (in the form of air) which combine to form sulphur trioxide in the presence of a catalyst. The reaction is exothermic and the conditions are controlled to keep the temperature at 450 degC. The catalyst used is valadium oxide (V2O5). The sulphur trioxide is dissolved in sulphuric acid to form fuming sulphuric acid, this is called oleum.

	S(s)	+	O2(g)	==>	SO2(g)

	2SO2(g)	+	O2(g)	==>	2SO3(g)	

	SO3(g)	+	H2SO4(l)==>	H2S2O7(l)	

This Oleum, H2S2O7, is then diluted with Water, H2O, to produce concentrated Sulphuric Acid, H2SO4.

	H2S2O7(l)+	H2O (l)	==>	2 H2SO4(l) 	
	Oleum				Sulphuric	

Reactions of Sulphuric acid

Electrolysis of a Solution of dilute Sulphuric Acid

The Electrolysis of an Aqueous Solution of dilute Sulphuric Acid is often carried out in a Hofmann Voltammeter, an apparatus in which the gases evolved at the anode and cathode can be collected in separate graduated tubes. When the solution is electrolyzed hydrogen is produced at the cathode and oxygen at the anode. These gases can be shown to be present in a 2 to 1 ratio and result from the electrolysis of water under acidic conditions.

Sulphuric acid is a strong electrolyte is fully dissociated in aqueous solution.

		H2SO4   ==>   2 H(+)   +   SO4(2 -)	

Water is a weak electrolyte and is only slightly dissociated

		H2O   ==>   H(+)   +   OH(-)	

During electrolysis, the Hydrogen Ions, H(+), migrates towards the cathode, and are discharged there (i.e. they gain an electron and are converted to hydrogen gas).

		2 H(+)   +   2 e(-)   ==>   H2-	

At the anode the concentration of Hydroxyl Ions, HO(-),is too low to maintain a reaction and the Sulphate Ions, SO4(2 -) are not oxidized but remain on in solution at the end. Water molecules must be the species reacting at the anode.

		2 H2O   ==>   O2   +   4 H(+)   +   4 e(-)	

The overall reaction is

Cathode Reaction :

		 2 H(+)   +   2e(-)   ==>   H2	
		 4 H(+)   +   4e(-)   ==>   2H2	

Anode Reaction :

		 2 H2O   ==>   O2  +  4 H(+)  +  4 e(-)	

Overall Cell Reaction:

	4 H(+)   +   2 H2O   ==>   2 H2   +   O2   +  4 H(+)	

For every Hydrogen Ions, H(+), discharged at the anode, another hydrogen ion is formed at the cathode. The net result is that the concentration of the Sulphuric Acid, H2SO4, remains constant and this electrolysis consists of the decomposition of water with the overall reaction

		2H2O   ==>   2H2-   +   O2-

Ferrous Sulphate, Fe(II)SO4, is the salt formed when Iron, Fe, is dissolved in Sulphuric Acid, H2SO4.

Hydrogen Chloride, HCl, may be prepared in the laboratory by heating Concentrated Sulphuric Acid, H2SO4, with Sodium Chloride, NaCl.

	NaCl   +    H2SO4   ==>   NaHSO4   +   HCl	

Many Metallic Chlorides liberate Chlorine, Cl2, when treated with Sulphuric Acid, H2SO4, and Manganese Dioxide, MnO2).

Many Metallic Chlorides liberate Hydrogen Chloride gas, HCl, when warmed with concentrated Sulphuric Acid, H2SO4.

Sulphur Trioxide, SO3, is prepared by heating concentrated Sulphuric Acid, H2SO4, with a large excess of Phosphorus Pentoxide, P2O5.

		H2SO4   +   P2O5   ==>   SO3   +   2 HPO3	

Sulphur Dioxide, SO2, is usually made in the laboratory by heating concentrated Sulphuric Acid, H2SO4, with Copper turnings, Cu.

	Cu   +   2 H2SO4   ==>   CuSO4   +   SO2   +   2 H2O

Hydrogen Fluoride, HF, can be prepared in the laboratory by heating Concentrated Sulphuric Acid, H2SO4, with Calcium Fluoride, CaF2.

		H2SO4   +   CaF2   ==>   2 HF   +   CaSO4	

Hydrogen Iodide, HI, can be prepared by direct combination of the elements using a platinum catalyst. In the laboratory it is prepared by heating Concentrated Sulphuric Acid, H2SO4, with Sodium Iodide, NaI.

		H2SO4   +   2 NaI   ==>   2 HI   +   Na2SO4

Methanol, CH3OH, does not undergo dehydration reactions. Instead, in reaction with Sulphuric Acid, H2SO4, the ester, Dimethyl Sulphate, (CH3)2SO4, is formed.

	2CH3OH		==>	(CH3)2SO4  +	H2O	
	Methanol		Dimethyl	Water	

Sulphuric Acid, H2SO4, absorbs Ethylene, C2H4, at room temperature to form Ethyl Hydrogen Sulphate, C2H5.HSO4, with much evolution of heat.


	              C2H4   +   H2SO4		==>	C2H5.HSO4	

If this is treated with Water, H2O and warmed, Ethanol, C2H5OH, is formed.

                C2H5.HSO4   +   H2O     	==>	C2H5OH + H2SO4   	

Uses of Sulphuric Acid

The Daniell Cell, which is a primary voltaic cell having a positive electrode of Copper, Cu, and a negative electrode of Zinc Amalgam, Zn (in alloy with Hg), was invented by the British chemist John Daniel in 1836AD.

The Zinc Amalgam electrode is placed in an electrolyte of dilute Sulphuric Acid solution, H2SO4, or Zinc Sulphate solution, ZnSO4, in a porous earthenware pot. This porous pot stands in a solution of Copper Sulphate, CuSO4, in which the Copper electrode is immersed.

The Zinc electrode, Zn, acts as a source of electrons, which flow through an external wire which connects the two electrodes, while the Zinc Ions, Zn(2 +), from the electrode go into solution.

Cathode reaction : Zn    ==>   Zn(++)    +   2 e(-)	

On reaching the Copper Electrode, these electrons combine with Copper Ions, Cu(2 +), in solution and the discharged copper ions are deposited on the copper electrode as Copper metal, Cu.

Anode reaction : Cu(++)    +   2 e(-)    ==>    Cu	

An equation for the overall chemical process is obtained by adding together the two half-cell reactions in such a way that the electrons "cancel out".

		Zn   +   Cu(++)   ==>   Zn(++)   +   Cu	

While the reaction takes place ions move through the porous pot, but when it is not in use the cell should be dismantled to prevent the diffusion of one electrolyte into the other.

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