Sulphuric Acid (H₂SO₄) is one of the most important industrial chemicals, often called the ‘King of Chemicals’ because of its wide range of uses.
1. Manufacture: Contact Process
- It is manufactured industrially by the Contact Process. The main steps are:
- Production of Sulphur Dioxide: Done by burning sulphur or iron pyrites in air.
- Purification: The gases are thoroughly cleaned to remove impurities that could poison the catalyst.
- Conversion to Sulphur Trioxide: Sulphur dioxide is reacted with excess air (oxygen) over a Vanadium Pentoxide (V₂O₅) catalyst at around 450°C to form Sulphur Trioxide (SO₃).
- Formation of Oleum: Sulphur Trioxide is not directly dissolved in water as it creates a corrosive mist. Instead, it is absorbed in 98% sulphuric acid to form Oleum (H₂S₂O₇).
- Dilution: Oleum is then carefully diluted with water to get sulphuric acid of the desired concentration.
2. Physical Properties
- It is a dense, oily liquid (hence the old name ‘Oil of Vitriol’).
- It is colourless and odorless.
- It is highly corrosive and mixes with water, releasing a great deal of heat.
3. Chemical Properties & Tests
- The chapter explains its behavior as a:
- Strong Acid: Turns blue litmus red, reacts with metals, bases, and carbonates.
- Dehydrating Agent: It has a strong affinity for water and can remove water from other substances. For example, it chars sugar and turns blue copper sulphate crystals white.
- Oxidizing Agent: It can oxidize non-metals like carbon and metals like copper.
- Test for Acid: Sulphuric acid reacts with barium chloride solution to form a white precipitate of barium sulphate, which is insoluble in all acids. This is a key identification test.
4. Uses of Sulphuric Acid
- Its major uses include making:
- Fertilizers (like ammonium sulphate, superphosphate).
- Other chemicals (like hydrochloric acid, nitric acid).
- Paints, pigments, and explosives.
- Soaps, detergents, and synthetic fibers.
- As a drying agent and in car batteries.
1) Comment, sulphuric acid is referred to as:
(a) King of chemicals
(b) Oil of vitriol
Ans: (a) King of chemicals – This is because it is the most widely used chemical in industry for making countless other chemicals, fertilizers, detergents, and more. Its central role earns it the “king” title.
(b) Oil of vitriol – This is its historical name, given by early alchemists who prepared it by heating green vitriol (ferrous sulphate) crystals, which produced an oily-looking, highly corrosive liquid.
2) Sulphuric acid is manufactured by contact process
(a) Give two balanced equations to obtain SO2 in this process.
(b) Give the conditions for the oxidation of SO2
Ans: (a) Two balanced equations to obtain SO2 is:
(i) 4FeS + 11O —>2FeO + 8SO
(ii) S + O2—> SO2
(b) Conditions for the oxidation of SO₂:
Catalyst: Vanadium pentoxide (V₂O₅)
Temperature: Around 450 °C
Pressure: A little above atmospheric pressure (1-2 atm)
3) Why is water not added to concentrated H2SO4 in order to dilute it?
Ans: Adding water to concentrated sulfuric acid is dangerous because it releases a large amount of heat instantly. This can cause the water to boil violently, leading to splattering of the hot, corrosive acid.To dilute it safely, the concentrated acid is slowly added to water with constant stirring. This method allows the heat to be safely absorbed and dissipated by the larger volume of water, preventing dangerous splashes.
4) Why must the impurity of arsenic oxide be removed before passing the mixture of SO4 and air through the catalytic chamber?
Ans: The impurity of arsenic oxide must be removed because it acts as a “catalyst poison.”In the Contact Process for making sulfuric acid, the conversion of sulfur dioxide (SO₂) to sulfur trioxide (SO₃) uses a vanadium pentoxide (V₂O₅) catalyst.
Arsenic oxide settles on the surface of this catalyst and blocks the active sites where the reaction should occur. This drastically reduces the efficiency of the catalyst, slowing down the reaction and lowering the overall yield of sulfuric acid.
Therefore, removing it beforehand is essential to keep the catalyst working effectively.
5) Give two balanced reactions of each type to show the following properties of sulphuric acid:
(a) Acidic nature.
(b) Oxidising agent,
(c) Hygroscopic nature,
(d) Non-volatile nature
Ans: (a) Acidic nature:
(i) Dilute H2SO4 reacts with basic oxides to form sulphate and water.
2 NaOH + H2SO4 —>Na2SO4 + 2H2O
(ii) CuO + H2SO4 —>CuSO4 + H2O
(iii) It reacts with carbonate to produce CO2.
Na2CO3 + H2SO4 —>Na2SO4 + H2O + CO2
(b) Oxidising agent: H2SO4 —>H2O +SO2 + [O]
Nascent oxygen oxidizes non-metals, metals and inorganic compounds.
For example,
Carbon to carbon dioxide C+H2SO4—> CO2 +H2O +2SO2
Sulphur to sulphur dioxide S +H2SO4 —> 3SO2 +2H2O
(c) Hygroscopic nature: -It has great affinity for water. It readily absorbs moisture from atmospheric air.
HCOOH 2 —> HSO CO + H2O
C6H12O6 —> .HSO 6C + 6H2O
(d) Non-volatile nature: It has a high boiling point (356oC) so it is considered to be non-volatile. Therefore, it is used for preparing volatile acids like hydrochloric acid, nitric acid from their salts by double decomposition reaction.
NaCl + H2SO4—> NaHSO4 + HCl
KCl + H2SO4 —> KHSO4 + HCl
6) Give a chemical test to distinguish between:
(a) dilute sulphuric acid and dilute hydrochloric acid
(b) dilute sulphuric acid and conc. Sulphuric acid
Ans: a) Distinguish between dilute sulphuric acid and dilute hydrochloric acid
- Test: Add a few drops of barium chloride solution or lead nitrate solution to separate samples of each acid.
- Observation for Dilute Sulphuric Acid: A white precipitate of barium sulphate or lead sulphate is formed.
- Observation for Dilute Hydrochloric Acid: No precipitate is formed.
- The sulphate ions (SO₄²⁻) from H₂SO₄ react with Ba²⁺ or Pb²⁺ ions to form an insoluble precipitate. Chloride ions (Cl⁻) from HCl do not form a precipitate with these ions.
(b) Distinguish between dilute sulphuric acid and conc. sulphuric acid
- Test 1: The Copper Test
- Add a small piece of copper metal to separate samples and warm gently.
- Observation for Conc. Sulphuric Acid: A blue solution is formed and a colourless, pungent gas (SO₂) is evolved.
- Observation for Dilute Sulphuric Acid: No reaction occurs. Copper does not react with dilute acids.
- Test 2: The Sugar Test
- Add a teaspoon of sugar to separate samples.
- Observation for Conc. Sulphuric Acid: Sugar turns into a black, porous mass of carbon due to its strong dehydrating property.
- Observation for Dilute Sulphuric Acid: Sugar simply dissolves to form a sweet, colourless solution. No charring occurs.
7) Name the products formed when hot and concentrated sulphuric acid reacts with the following:
(a) Sulphur
(b) NaOH,
(c) Sugar
(d) Carbon
(e) Copper.
Ans: (a) Reaction with Sulphur
When concentrated sulphuric acid reacts with sulphur, it results in the formation of sulphur dioxide gas. This is an example of an oxidation reaction where the acid oxidizes sulphur.
(b) Reaction with Sodium Hydroxide (NaOH)
Concentrated sulphuric acid reacts with sodium hydroxide in a neutralization reaction. This chemical process produces sodium sulphate, which is a salt, and water.
(c) Reaction with Sugar
Concentrated sulphuric acid has a very strong affinity for water. When it is added to sugar (sucrose), it acts as a powerful dehydrating agent. It removes hydrogen and oxygen from the sugar molecules in the ratio of water (H₂O), leaving behind a black, charred mass of porous carbon.
(d) Reaction with Carbon
When concentrated sulphuric acid reacts with carbon, it gets reduced. In this redox reaction, the acid is reduced to sulphur dioxide, while the carbon is oxidized to carbon dioxide.
(e) Reaction with Copper
Concentrated sulphuric acid reacts with heated copper metal. In this reaction, copper is oxidized to form copper sulphate, and the acid is reduced to sulphur dioxide gas. Water is also produced as a by-product.
8) Why is: (a) Concentrated sulphuric acid kept in air tight bottles?
(b) H2SO4 not a drying agent for H2S?
(c) Sulphuric acid used in the preparation of HCI and HNO3? Give equations in both cases.
Ans: (a) Concentrated sulphuric acid is kept in air tight bottles because it is hygroscopic. It readily absorbs moisture from the air, which dilutes the acid and reduces its strength and effectiveness.
(b) H₂SO₄ is not a drying agent for H₂S because it oxidizes H₂S gas. Concentrated sulphuric acid is a strong oxidizing agent and reacts with hydrogen sulfide instead of just drying it.
Equation:
3H₂SO₄ (conc.) + H₂S → 4SO₂ + 4H₂O
(c) Sulphuric acid is used in the preparation of HCl and HNO₃ because it is a non-volatile acid. It can displace more volatile acids from their salts.
For HCl preparation (from NaCl):
NaCl + H₂SO₄ (conc.) → NaHSO₄ + HCl
For HNO₃ preparation (from KNO₃):
KNO₃ + H₂SO₄ (conc.) → KHSO₄ + HNO₃
9) What property of conc. H2SO4 is made use of in each of the following cases? Give an equation for the reaction on each case:
(a) in the production of HCI gas when it reacts with a chloride,
(b) in the preparation of CO and HCOOH,
Ans: (a) Due to its reducing property. i.e, it is a non-volatile acid.
NaCl + H2SO4—> NaHSO4 + HCl (Conc.)
(b) It is a dehydrating agent.
HCOOH 2—>.HSO CO + H2O
(c) Magnesium is present above hydrogen in the reactivity series so sulphuric acid is able to liberate hydrogen gas by reacting with magnesium strip.
Mg + H2SO4—> MgSO4 + H2
(d) Due to its oxidizing character
Cu + H2SO4→ CuSO4 + 2H2O + SO2
(e) Due to its oxidizing property Hydrogen sulphide gas is passed through concentrated sulphuric acid to liberate sulphur dioxide and sulphur is formed. H2S + H2SO4 S—> + 2H2O + SO2
10) What is the name given to the salts of:
(a) sulphurous acid
(b) sulphuric acid?
Ans: (a) Salts of sulphurous acid
The salts of sulphurous acid (H₂SO₃) are called sulphites.The naming follows a standard pattern in chemistry. When sulphurous acid reacts with a base, it forms a salt and water. In this reaction, the negative ion that is produced is the sulphite ion, which has the chemical formula SO₃²⁻. Therefore, any salt containing this ion is a sulphite.
Example: Sodium sulphite (Na₂SO₃) is a common salt formed when sodium hydroxide reacts with sulphurous acid.
(b) Salts of sulphuric acid
The salts of sulphuric acid (H₂SO₄) are called sulphates.
Similarly, when sulphuric acid reacts with a base, it produces the sulphate ion, SO₄²⁻. Salts that contain this polyatomic ion are known collectively as sulphates.
Example: Copper sulphate (CuSO₄) is a well-known blue, crystalline salt formed from sulphuric acid.
11) Give reasons for the following:
(a) Sulphuric acid forms two types of salts with NaOH,
(b) Red brown vapours are produced when concentrated sulphuric acid is added to hydrogen bromide.
(c) A piece of wood becomes black when concentrated sulphuric acid is poured on it, (d) Brisk effervescence is seen when oil of vitriol is added to sodium carbonate.
Ans: (a) Sulphuric acid forms two types of salts with NaOH.
- This happens because sulphuric acid (H₂SO₄) is a dibasic acid, meaning it has two replaceable hydrogen atoms (H⁺ ions) per molecule.
- Step-wise Neutralization: When it reacts with a base like sodium hydroxide (NaOH), the neutralization can occur in two distinct steps.
- Formation of Acid Salt (Sodium Hydrogen Sulphate): In the first step, only one hydrogen ion is replaced. This results in the formation of an acid salt.
Reaction: H₂SO₄ + NaOH → NaHSO₄ + H₂O
- Formation of Normal Salt (Sodium Sulphate): If more NaOH is available, the second hydrogen ion is also replaced, forming a normal salt.
Reaction: NaHSO₄ + NaOH → Na₂SO₄ + H₂O
Therefore, depending on the amount of NaOH used, we can get either the acid salt (NaHSO₄) or the normal salt (Na₂SO₄).
(b) Red brown vapours are produced when concentrated sulphuric acid is added to hydrogen bromide.
- This occurs because concentrated sulphuric acid acts as a strong oxidizing agent and oxidizes the hydrogen bromide (HBr).
- The Reaction: Hydrogen bromide is a reducing agent. When concentrated H₂SO₄ is added to it, it oxidizes the bromide ion (Br⁻) to bromine (Br₂).
Chemical Change: 2HBr + H₂SO₄ → Br₂ + SO₂ + 2H₂O
- Origin of Vapours: The bromine (Br₂) produced is a volatile liquid at room temperature, but its vapours have a distinctive red-brown colour. The sulphur dioxide (SO₂) gas produced is colourless, so the observed red-brown fumes are entirely due to the evolution of bromine vapour.
(c) A piece of wood becomes black when concentrated sulphuric acid is poured on it.
This is a demonstration of the dehydrating property of concentrated sulphuric acid.
- Composition of Wood: Wood is primarily composed of cellulose, a carbohydrate (a polymer of glucose, C₆H₁₂O₆). Carbohydrates have a general formula that can be thought of as containing water molecules (i.e., they are hydrates of carbon).
- Powerful Dehydration: Concentrated sulphuric acid has a very strong affinity for water. It violently rips out hydrogen and oxygen atoms from the cellulose molecule in the ratio of 2:1 (as water, H₂O).
- Resulting Residue: After the removal of water, the remaining material is almost pure carbon. This black, spongy mass of carbon is what we see as the charred or blackened wood.
(d) Brisk effervescence is seen when oil of vitriol is added to sodium carbonate.
“Oil of Vitriol” is an old name for concentrated sulphuric acid.
- Acid-Carbonate Reaction: Sulphuric acid is a strong acid, and sodium carbonate (Na₂CO₃) is a carbonate salt. Acids react with carbonates to produce a salt, water, and carbon dioxide (CO₂) gas
- The Chemical Reaction:
Na₂CO₃ + H₂SO₄ → Na₂SO₄ + H₂O + CO₂ (g)
- Cause of Effervescence: The carbon dioxide gas is produced very rapidly in this reaction. As the gas bubbles out vigorously from the solution, it causes the observed brisk effervescence.
12) Copy and complete the following table:
| Column 1 Substance reacted with acid | Column 2 Dilute or concentrated acid | Column 3 Gas |
| hydrogen | ||
| Carbon dioxide | ||
| Only chloride |
Ans:
| Column 1 Substance reacted with acid | Column 2 Dilute or concentrated acid | Column 3 Gas |
| Substance reacted with acid | Dilute or concentrated sulphuric acid | Gas |
| Zinc | Dilute sulphuric acid | hydrogen |
| Calcium carbonate C | Concentrated sulphuric acid | Carbon dioxide |
| Bleaching power CaOCl2 | Dilute sulphuric acid | Only chloride |
Question 1(2004): Name a gas that can be oxidized to sulphur.
Ans: A gas that can be oxidized to sulphur is hydrogen sulphide (H₂S).
Here is a simple explanation of the process:
Hydrogen sulphide is a flammable gas with a characteristic rotten egg smell. When it undergoes combustion (a form of oxidation) in a limited supply of air or oxygen, it can be partially oxidized to produce elemental sulphur and water vapour.
The chemical reaction can be represented as:
2H₂S(g) + O₂(g) → 2S(s) + 2H₂O(g)
In this reaction, the hydrogen sulphide gas (H₂S) is oxidized, and the oxygen gas (O₂) acts as the oxidizing agent. The sulphur (S) is deposited as a solid.
This principle is actually used in industrial processes to remove toxic H₂S from natural gas and refinery streams, recovering valuable sulphur in the process.
Question 2(2004): Give the odour of the gas evolved and name the gas produced Ans: when sodium sulphide is added to solution of HCI in water.
When sodium sulphide (Na₂S) is added to a solution of hydrochloric acid (HCl) in water, a brisk reaction occurs, producing a gas with a very distinctive and unpleasant smell.
Odour of the gas: The gas has a powerful and characteristic smell of rotten eggs.
Name of the gas: The gas produced is hydrogen sulphide (H₂S).
Explanation in simple terms:
The sodium sulphide reacts with the hydrochloric acid in a typical acid-carbonate/sulphide type of reaction. The sulphide ions (S²⁻) from the sodium sulphide combine with the hydrogen ions (H⁺) from the acid to form hydrogen sulphide gas. This is the same gas that is often responsible for the smell of decaying organic matter in swamps or bad breath.
Question 3(2004):
(a) Name the catalyst which helps in the conversion of sulphur dioxide to sulphur trioxide in step C.
(b) In the contact process for the manufacture of sulphuric acid, sulphur trioxide is not converted to sulphuric acid by reacting it with water. Instead a two-steps procedure is used. Write the equations for the two steps involved in D.
(c) What type of substance will liberate sulphur dioxide from sodium sulphite in step E?
(d) Write the equation for the reaction by which sulphure dioxide is converted to sodium sulphite in step F.
Ans: (a) Catalyst for SO₂ to SO₃ oxidation
Vanadium pentoxide (V₂O₅).
(b) Reason for two-step absorption
It prevents the formation of a fine, corrosive sulphuric acid mist that is difficult to condense and handle.
(c) Acid for SO₂ liberation from sodium sulphite
A dilute acid, like dilute hydrochloric acid (HCl) or dilute sulphuric acid (H₂SO₄).
(d) Reaction with a base
With sodium hydroxide: SO₂ + 2NaOH → Na₂SO₃ + H₂O
With sodium carbonate: SO₂ + Na₂CO₃ → Na₂SO₃ + CO₂
Question 1(2006): (a) Name the process used for the large-scale manufacture of sulphuric acid.
(b) Which property of sulphuric acid accounts for its use as a dehydrating agent?
(c) Concentrated sulphuric acid is both an oxidizing agent and a non-volatile acid. Write one equation each to illustrate the above mentioned properties of sulphuric acid.
Ans: (a) Contact Process
(b) Its strong affinity for water (it reacts exothermically with water).
(c)As an oxidizing agent: It oxidizes carbon to carbon dioxide.
C + 2H₂SO₄ (conc.) → CO₂ + 2SO₂ + 2H₂O
As a non-volatile acid: It displaces volatile acids from their salts.
2NaCl + H₂SO₄ (conc.) → Na₂SO₄ + 2HCl
Question 1(2007): Some properties of sulphuric acid are listed below. Choose the property A, B, C or D which is responsible for the reactions (i) to (v) some properties may be repeated.
A. Acid
B. Dehydrating agent
C. Non-volatile acid
D. Oxidizing agent
(i) C12H22O11 + NH2SO4 —>12C + 11H2O + nH2SO4
(ii) S 2HSO —> 3SO 2HO
(iii) NaCI+ H2SO4—> NaHSO4+ HCI
(iv) CuO+ H2SO4—> CuSO4+ H2O
(v) Na2CO3+ H2SO4—> Na2SO4+ H2O+ CO2
Ans:
(i) B
(ii) D
(iii) C
(iv) A
(v) A
Question 2(2007): (a) Name the acid formed when sulphur dioxide dissolves in water (b) Name the gas released when sodium carbonate is added to a solution of sulphur dioxide.
Ans: (a) Sulphurous acid
(b) Carbon dioxide
Question 1(2008): (a) What is the property of concentrated sulphuric acid which allows it to be used in the preparation of hydrogen chloride and nitric acid?
(b) What property of concentrated sulphuric acid is in action when sugar turns black in its presence?
Ans: (a) Concentrated sulphuric acid is used because it is a non-volatile acid and a strong dehydrating agent. Its low volatility allows it to displace more volatile acids like HCl and HNO₃ from their salts.
(b) The property in action is its strong dehydrating nature. It removes water (H and O in the ratio of 2:1) from sugar (sucrose), leaving behind a black, spongy mass of carbon.
