Concentrated Sulphuric Acid Can Turn Sugar Into a Rising Column of Black Carbon
Follow sulphur through the Contact Process to sulphuric acid, write the reactions of the dilute acid as a dibasic acid, see the concentrated acid act as an oxidising and dehydrating agent, understand why its non-volatile nature drives out other acids, and test for sulphate ions.
Why does sulphuric acid behave in so many different ways?
Pour a little concentrated sulphuric acid onto a heap of white sugar in a beaker. Within a minute the sugar turns yellow, then brown, then black, and a steaming column of spongy carbon rises out of the beaker. No other common laboratory acid does anything like it.
Sulphuric acid shows more distinct kinds of behaviour than the other acids in this chapter, and which one appears depends on whether it is dilute or concentrated:
- Dilute sulphuric acid behaves as a typical strong, dibasic acid — with metals, oxides, hydroxides and carbonates
- Hot concentrated sulphuric acid acts as an oxidising agent, attacking carbon and sulphur
- Concentrated sulphuric acid is a powerful dehydrating agent, pulling the elements of water out of sugar and out of blue copper sulphate crystals
- Sulphuric acid is non-volatile, so on heating it drives out more volatile acids such as hydrochloric and nitric acids from their salts
Its uses are correspondingly wide. It is the acid in vehicle batteries, a raw material for fertilisers such as ammonium sulphate and superphosphate, and a reagent in making detergents, dyes, paints and other chemicals.
This part covers:
- The Contact Process — the industrial manufacture, with conditions and reasons
- Dilute sulphuric acid as an acid, and why it is dibasic
- Concentrated sulphuric acid as an oxidising and dehydrating agent
- Its non-volatile nature, and the tests for the acid and the sulphate ion
The thread through the whole chapter. Sulphuric acid has already appeared as the reactant that makes hydrogen chloride and nitric acid, as the drying agent for hydrogen chloride, and in the absorption of sulphur trioxide. This part explains why it can do all of those jobs.
One safety rule sits above everything else. Concentrated sulphuric acid releases a great deal of heat when mixed with water and chars skin and paper. Always add the acid slowly to water, never water to the acid.
This page covers the ICSE Class 10 Chemistry study of sulphuric acid: the Contact Process, its acidic, oxidising and dehydrating properties, its non-volatile nature, and tests.
Sulphuric acid shows more distinct kinds of behaviour than the other acids in this chapter, and which one appears depends on whether it is dilute or concentrated:
- Dilute sulphuric acid behaves as a typical strong, dibasic acid — with metals, oxides, hydroxides and carbonates
- Hot concentrated sulphuric acid acts as an oxidising agent, attacking carbon and sulphur
- Concentrated sulphuric acid is a powerful dehydrating agent, pulling the elements of water out of sugar and out of blue copper sulphate crystals
- Sulphuric acid is non-volatile, so on heating it drives out more volatile acids such as hydrochloric and nitric acids from their salts
Its uses are correspondingly wide. It is the acid in vehicle batteries, a raw material for fertilisers such as ammonium sulphate and superphosphate, and a reagent in making detergents, dyes, paints and other chemicals.
This part covers:
- The Contact Process — the industrial manufacture, with conditions and reasons
- Dilute sulphuric acid as an acid, and why it is dibasic
- Concentrated sulphuric acid as an oxidising and dehydrating agent
- Its non-volatile nature, and the tests for the acid and the sulphate ion
The thread through the whole chapter. Sulphuric acid has already appeared as the reactant that makes hydrogen chloride and nitric acid, as the drying agent for hydrogen chloride, and in the absorption of sulphur trioxide. This part explains why it can do all of those jobs.
One safety rule sits above everything else. Concentrated sulphuric acid releases a great deal of heat when mixed with water and chars skin and paper. Always add the acid slowly to water, never water to the acid.
This page covers the ICSE Class 10 Chemistry study of sulphuric acid: the Contact Process, its acidic, oxidising and dehydrating properties, its non-volatile nature, and tests.
How is sulphuric acid manufactured by the Contact Process, and why are those conditions chosen?
Sulphur dioxide from burning sulphur is purified and oxidised to sulphur trioxide over vanadium pentoxide at about 450–500 °C and slightly above atmospheric pressure; the sulphur trioxide is absorbed in concentrated sulphuric acid to form oleum, which is diluted to sulphuric acid.
Step 1 — making sulphur dioxide. Sulphur is burnt in air, or iron pyrites is roasted:
Step 2 — purifying the gases. The mixture of sulphur dioxide and air is passed through dust chambers, a washing tower and a drying tower containing concentrated sulphuric acid, and impurities such as arsenic compounds are removed. Dust, moisture and arsenic poison the catalyst, so purification is essential.
Step 3 — catalytic oxidation in the contact tower.
Conditions:
- Catalyst: vanadium pentoxide,
- Temperature: about to
- Pressure: slightly above atmospheric, about to atmospheres
- Excess air, supplying extra oxygen
Step 4 — absorption. Sulphur trioxide is not dissolved directly in water, because the reaction is so violent that it produces a dense fog of acid droplets that does not condense easily. Instead it is absorbed in concentrated sulphuric acid, forming oleum, or pyrosulphuric acid:
Step 5 — dilution. Oleum is diluted with the right amount of water:
Reasons for the conditions:
- Temperature — the forward reaction is exothermic, so a lower temperature favours a higher yield of sulphur trioxide, but the reaction would be too slow. About 450–500 °C is the compromise, with the catalyst giving a good rate
- Pressure — three volumes of gas become two, so higher pressure favours sulphur trioxide. But the yield is already high at ordinary pressures, so only a slight excess pressure is used, avoiding the cost and hazard of very high pressure
- Excess oxygen — pushes the equilibrium towards sulphur trioxide and ensures the sulphur dioxide is used efficiently
- Catalyst — speeds up the attainment of equilibrium; it does not change the yield
The plant, as a flow of stages:
- Sulphur burner purification units preheater contact tower with vanadium pentoxide absorption tower with concentrated sulphuric acid dilution tank
Worked example — acid from sulphur. Each sulphur atom ends up in one molecule of sulphuric acid. What mass of sulphuric acid can be made from of sulphur? S , H , O .
Worked example — volumes in the contact tower. What volume of oxygen is needed to convert of sulphur dioxide completely to sulphur trioxide, under the same conditions?
An everyday connection. Many fertiliser plants produce sulphuric acid on site, because ammonium sulphate and superphosphate fertilisers both need it — linking the Contact Process to Haber's process in the same factory.
The boundary case. Absorbing sulphur trioxide in water looks simpler on paper, since . In practice the acid mist it creates escapes absorption, which is why the two-stage route through oleum is used.
Step 1 — making sulphur dioxide. Sulphur is burnt in air, or iron pyrites is roasted:
Step 2 — purifying the gases. The mixture of sulphur dioxide and air is passed through dust chambers, a washing tower and a drying tower containing concentrated sulphuric acid, and impurities such as arsenic compounds are removed. Dust, moisture and arsenic poison the catalyst, so purification is essential.
Step 3 — catalytic oxidation in the contact tower.
Conditions:
- Catalyst: vanadium pentoxide,
- Temperature: about to
- Pressure: slightly above atmospheric, about to atmospheres
- Excess air, supplying extra oxygen
Step 4 — absorption. Sulphur trioxide is not dissolved directly in water, because the reaction is so violent that it produces a dense fog of acid droplets that does not condense easily. Instead it is absorbed in concentrated sulphuric acid, forming oleum, or pyrosulphuric acid:
Step 5 — dilution. Oleum is diluted with the right amount of water:
Reasons for the conditions:
- Temperature — the forward reaction is exothermic, so a lower temperature favours a higher yield of sulphur trioxide, but the reaction would be too slow. About 450–500 °C is the compromise, with the catalyst giving a good rate
- Pressure — three volumes of gas become two, so higher pressure favours sulphur trioxide. But the yield is already high at ordinary pressures, so only a slight excess pressure is used, avoiding the cost and hazard of very high pressure
- Excess oxygen — pushes the equilibrium towards sulphur trioxide and ensures the sulphur dioxide is used efficiently
- Catalyst — speeds up the attainment of equilibrium; it does not change the yield
The plant, as a flow of stages:
- Sulphur burner purification units preheater contact tower with vanadium pentoxide absorption tower with concentrated sulphuric acid dilution tank
Worked example — acid from sulphur. Each sulphur atom ends up in one molecule of sulphuric acid. What mass of sulphuric acid can be made from of sulphur? S , H , O .
Worked example — volumes in the contact tower. What volume of oxygen is needed to convert of sulphur dioxide completely to sulphur trioxide, under the same conditions?
An everyday connection. Many fertiliser plants produce sulphuric acid on site, because ammonium sulphate and superphosphate fertilisers both need it — linking the Contact Process to Haber's process in the same factory.
The boundary case. Absorbing sulphur trioxide in water looks simpler on paper, since . In practice the acid mist it creates escapes absorption, which is why the two-stage route through oleum is used.
How does dilute sulphuric acid react as an acid, and why is it dibasic?
Dilute sulphuric acid gives two hydronium ions per molecule, so it is dibasic and forms both acid salts and normal salts, and it reacts with metals, oxides, hydroxides, carbonates, bicarbonates, sulphites, bisulphites and sulphides like any strong acid.
Why it is dibasic. Each molecule has two replaceable hydrogen atoms, which ionise in two steps:
So it forms two series of salts:
1. With an active metal — salt and hydrogen.
2. With a metal oxide — salt and water.
3. With a hydroxide — salt and water, as above with sodium hydroxide.
4. With a carbonate — salt, water and carbon dioxide.
5. With a hydrogen carbonate.
6. With a sulphite — salt, water and sulphur dioxide, on warming.
7. With a hydrogen sulphite.
8. With a sulphide — salt and hydrogen sulphide.
Worked check — balancing the hydrogen sulphite equation.
- Sodium:
- Hydrogen: on the left; on the right
- Sulphur: on the left; on the right
- Oxygen: on the left; on the right
Balanced.
Worked example — hydrogen from zinc. What volume of hydrogen at STP is produced when of zinc reacts completely with dilute sulphuric acid, and what mass of acid is used? Zn .
An everyday example. The acid in a lead-acid vehicle battery is dilute sulphuric acid. Spilt battery acid fizzes on marble or cement floors — the carbonate reaction above, with calcium carbonate.
Two boundary cases.
- Calcium carbonate and lead react only briefly, because insoluble calcium sulphate and lead sulphate coat the solid and stop the reaction
- Copper gives no hydrogen with dilute sulphuric acid, since it lies below hydrogen — though hot concentrated acid does attack it, as the next section shows
Why it is dibasic. Each molecule has two replaceable hydrogen atoms, which ionise in two steps:
So it forms two series of salts:
1. With an active metal — salt and hydrogen.
2. With a metal oxide — salt and water.
3. With a hydroxide — salt and water, as above with sodium hydroxide.
4. With a carbonate — salt, water and carbon dioxide.
5. With a hydrogen carbonate.
6. With a sulphite — salt, water and sulphur dioxide, on warming.
7. With a hydrogen sulphite.
8. With a sulphide — salt and hydrogen sulphide.
Worked check — balancing the hydrogen sulphite equation.
- Sodium:
- Hydrogen: on the left; on the right
- Sulphur: on the left; on the right
- Oxygen: on the left; on the right
Balanced.
Worked example — hydrogen from zinc. What volume of hydrogen at STP is produced when of zinc reacts completely with dilute sulphuric acid, and what mass of acid is used? Zn .
An everyday example. The acid in a lead-acid vehicle battery is dilute sulphuric acid. Spilt battery acid fizzes on marble or cement floors — the carbonate reaction above, with calcium carbonate.
Two boundary cases.
- Calcium carbonate and lead react only briefly, because insoluble calcium sulphate and lead sulphate coat the solid and stop the reaction
- Copper gives no hydrogen with dilute sulphuric acid, since it lies below hydrogen — though hot concentrated acid does attack it, as the next section shows
How does concentrated sulphuric acid act as an oxidising agent and a dehydrating agent?
Hot concentrated sulphuric acid gives up oxygen, oxidising carbon to carbon dioxide and sulphur to sulphur dioxide while being reduced itself; concentrated sulphuric acid also removes the elements of water from compounds, charring sugar to carbon and turning blue copper sulphate white.
1. As an oxidising agent. Hot concentrated sulphuric acid decomposes to give nascent oxygen:
Oxidation of carbon:
Oxidation of sulphur:
In both reactions, sulphuric acid is reduced to sulphur dioxide, recognised by its smell of burning sulphur and by turning acidified potassium dichromate green. Hot concentrated acid also attacks copper in the same way:
2. As a dehydrating agent. Concentrated sulphuric acid has a very strong attraction for water. **It removes hydrogen and oxygen in the ratio — the elements of water — from compounds.
With cane sugar:**
Observations: the sugar turns yellow, then brown, then black; a spongy mass of sugar charcoal rises from the beaker; steam is given off and the beaker becomes very hot.
With blue copper sulphate crystals:
Observation: the blue crystals turn white as their water of crystallisation is removed.
Drying agent versus dehydrating agent.
- A drying agent removes moisture — free water mixed with a substance — as when concentrated sulphuric acid dries hydrogen chloride gas
- A dehydrating agent removes the elements of water that are chemically combined in a compound, as with sugar
- Concentrated sulphuric acid is both, which is why it appears in both roles in this chapter
Worked check — balancing the carbon equation.
- Carbon:
- Hydrogen: on the left; on the right
- Sulphur:
- Oxygen: on the left; on the right
Balanced.
Worked example — carbon from sugar. What mass of carbon is left when of cane sugar is completely dehydrated? C , H , O .
Check: .
An everyday example. A drop of concentrated sulphuric acid on paper or cotton cloth leaves a black, charred hole. Paper and cotton are made of cellulose, a compound of carbon, hydrogen and oxygen, and the acid dehydrates it to carbon — which is also why the acid causes such severe burns on skin.
The boundary case. Dilute sulphuric acid shows none of these effects. Its water has already satisfied the acid's attraction for water, and it is too dilute to act as an oxidising agent — so the concentration decides the behaviour completely.
1. As an oxidising agent. Hot concentrated sulphuric acid decomposes to give nascent oxygen:
Oxidation of carbon:
Oxidation of sulphur:
In both reactions, sulphuric acid is reduced to sulphur dioxide, recognised by its smell of burning sulphur and by turning acidified potassium dichromate green. Hot concentrated acid also attacks copper in the same way:
2. As a dehydrating agent. Concentrated sulphuric acid has a very strong attraction for water. **It removes hydrogen and oxygen in the ratio — the elements of water — from compounds.
With cane sugar:**
Observations: the sugar turns yellow, then brown, then black; a spongy mass of sugar charcoal rises from the beaker; steam is given off and the beaker becomes very hot.
With blue copper sulphate crystals:
Observation: the blue crystals turn white as their water of crystallisation is removed.
Drying agent versus dehydrating agent.
- A drying agent removes moisture — free water mixed with a substance — as when concentrated sulphuric acid dries hydrogen chloride gas
- A dehydrating agent removes the elements of water that are chemically combined in a compound, as with sugar
- Concentrated sulphuric acid is both, which is why it appears in both roles in this chapter
Worked check — balancing the carbon equation.
- Carbon:
- Hydrogen: on the left; on the right
- Sulphur:
- Oxygen: on the left; on the right
Balanced.
Worked example — carbon from sugar. What mass of carbon is left when of cane sugar is completely dehydrated? C , H , O .
Check: .
An everyday example. A drop of concentrated sulphuric acid on paper or cotton cloth leaves a black, charred hole. Paper and cotton are made of cellulose, a compound of carbon, hydrogen and oxygen, and the acid dehydrates it to carbon — which is also why the acid causes such severe burns on skin.
The boundary case. Dilute sulphuric acid shows none of these effects. Its water has already satisfied the acid's attraction for water, and it is too dilute to act as an oxidising agent — so the concentration decides the behaviour completely.
Why is sulphuric acid called non-volatile, and how are sulphuric acid and sulphate ions tested?
Sulphuric acid has a very high boiling point, so when heated with chlorides or nitrates it stays behind and drives out the more volatile hydrochloric or nitric acid; sulphate ions are identified by a white precipitate with barium chloride that does not dissolve in hydrochloric acid.
Non-volatile nature. Sulphuric acid boils at about , far above hydrochloric acid and nitric acid. When it is heated with a salt of a more volatile acid, the volatile acid escapes as vapour, and the reaction is driven forward.
With chlorides — hydrogen chloride is driven out:
With nitrates — nitric acid is driven out:
These are exactly the laboratory preparations of hydrogen chloride and nitric acid from earlier in the chapter.
Tests for sulphuric acid:
- With barium chloride solution — a white precipitate of barium sulphate, insoluble in dilute hydrochloric or nitric acid:
- With lead nitrate solution — a white precipitate of lead sulphate, insoluble in acids:
- Concentrated acid chars sugar, and the dilute acid gives hydrogen with zinc
Test for the sulphate radical in any solution:
- Add dilute hydrochloric acid, then barium chloride solution
- A white precipitate that does not dissolve confirms sulphate:
Why hydrochloric acid is added first. Sulphites and carbonates also give white precipitates with barium chloride, but barium sulphite and barium carbonate dissolve in hydrochloric acid. Only barium sulphate stays undissolved, so the acid makes the test specific.
Worked example — mass of precipitate. What mass of barium sulphate forms when excess barium chloride is added to a solution containing of sulphuric acid? Ba .
Worked check — balancing the sodium sulphate test. Sodium ; sulphur ; oxygen ; barium ; chlorine . Balanced.
An everyday example. A bottle of concentrated sulphuric acid left open gradually increases in volume as it absorbs moisture from the air — it is hygroscopic. Its non-volatility and its thirst for water together explain why it is used in desiccators to keep laboratory samples dry.
The boundary case. Non-volatile does not mean unreactive. Sulphuric acid drives out hydrogen chloride because it stays behind, not because it is a stronger acid in water — hydrochloric acid is also a strong acid. The reaction succeeds because one product escapes as a gas.
Non-volatile nature. Sulphuric acid boils at about , far above hydrochloric acid and nitric acid. When it is heated with a salt of a more volatile acid, the volatile acid escapes as vapour, and the reaction is driven forward.
With chlorides — hydrogen chloride is driven out:
With nitrates — nitric acid is driven out:
These are exactly the laboratory preparations of hydrogen chloride and nitric acid from earlier in the chapter.
Tests for sulphuric acid:
- With barium chloride solution — a white precipitate of barium sulphate, insoluble in dilute hydrochloric or nitric acid:
- With lead nitrate solution — a white precipitate of lead sulphate, insoluble in acids:
- Concentrated acid chars sugar, and the dilute acid gives hydrogen with zinc
Test for the sulphate radical in any solution:
- Add dilute hydrochloric acid, then barium chloride solution
- A white precipitate that does not dissolve confirms sulphate:
Why hydrochloric acid is added first. Sulphites and carbonates also give white precipitates with barium chloride, but barium sulphite and barium carbonate dissolve in hydrochloric acid. Only barium sulphate stays undissolved, so the acid makes the test specific.
Worked example — mass of precipitate. What mass of barium sulphate forms when excess barium chloride is added to a solution containing of sulphuric acid? Ba .
Worked check — balancing the sodium sulphate test. Sodium ; sulphur ; oxygen ; barium ; chlorine . Balanced.
An everyday example. A bottle of concentrated sulphuric acid left open gradually increases in volume as it absorbs moisture from the air — it is hygroscopic. Its non-volatility and its thirst for water together explain why it is used in desiccators to keep laboratory samples dry.
The boundary case. Non-volatile does not mean unreactive. Sulphuric acid drives out hydrogen chloride because it stays behind, not because it is a stronger acid in water — hydrochloric acid is also a strong acid. The reaction succeeds because one product escapes as a gas.
Exam tip
What earns full marks on sulphuric acid questions?
For the Contact Process, give every step's equation and each condition with its reason; for properties, state whether the acid is dilute or concentrated before writing the equation.
- Write all five equations of the Contact Process, from burning sulphur to diluting oleum
- Name vanadium pentoxide as the catalyst, with the temperature and pressure
- Give reasons for the temperature compromise, the modest pressure and the excess air
- Explain why sulphur trioxide is not absorbed in water — the acid mist
- Mention purification and why arsenic and dust must be removed
- Show dibasic behaviour with both the acid salt and the normal salt
- Say hot and concentrated for the oxidation of carbon and sulphur, and name sulphur dioxide as the reduced product
- Describe the sugar observations in order: yellow, brown, black, spongy mass, steam, heat
- Distinguish dehydrating from drying
- For the sulphate test, add dilute hydrochloric acid before barium chloride, and state the precipitate is insoluble
The misconception to name. The catalyst in the Contact Process does not increase the amount of sulphur trioxide at equilibrium. It only makes the equilibrium arrive faster at the chosen temperature. Writing that vanadium pentoxide increases the yield loses the mark.
A second trap. Writing that dilute sulphuric acid chars sugar or oxidises carbon. Those are properties of the concentrated acid only, and the word concentrated must appear in the answer.
- Write all five equations of the Contact Process, from burning sulphur to diluting oleum
- Name vanadium pentoxide as the catalyst, with the temperature and pressure
- Give reasons for the temperature compromise, the modest pressure and the excess air
- Explain why sulphur trioxide is not absorbed in water — the acid mist
- Mention purification and why arsenic and dust must be removed
- Show dibasic behaviour with both the acid salt and the normal salt
- Say hot and concentrated for the oxidation of carbon and sulphur, and name sulphur dioxide as the reduced product
- Describe the sugar observations in order: yellow, brown, black, spongy mass, steam, heat
- Distinguish dehydrating from drying
- For the sulphate test, add dilute hydrochloric acid before barium chloride, and state the precipitate is insoluble
The misconception to name. The catalyst in the Contact Process does not increase the amount of sulphur trioxide at equilibrium. It only makes the equilibrium arrive faster at the chosen temperature. Writing that vanadium pentoxide increases the yield loses the mark.
A second trap. Writing that dilute sulphuric acid chars sugar or oxidises carbon. Those are properties of the concentrated acid only, and the word concentrated must appear in the answer.
Did you know
How does the sulphuric acid in a vehicle battery store electricity?
Under the bonnet of most cars, buses and many motorcycles sits a lead-acid battery — a box of lead plates standing in dilute sulphuric acid. It is recharged every time the engine runs and supplies the burst of current that starts the engine the next time. The acid is not just a filling; it is one of the reactants.
Each cell has two kinds of plate:
- One made of spongy lead,
- One coated with lead dioxide,
When the battery supplies current, both plates react with the sulphuric acid, and both turn into lead sulphate. The overall change during discharge is:
Checking the balance: lead ; sulphur ; oxygen on the left and on the right; hydrogen . Balanced.
Notice what happens to the acid. Sulphuric acid is used up and water is produced, so the acid becomes more dilute as the battery runs down. When the battery is recharged, a current is forced through it the other way, the reaction reverses, lead and lead dioxide re-form, and the acid becomes stronger again.
That change gives mechanics a simple way to check a battery. Sulphuric acid is denser than water, so the density of the liquid in each cell falls as the battery discharges. A small float instrument called a hydrometer measures that density and shows at a glance how charged the cell is.
This is electrolysis and the chemistry of sulphuric acid meeting in one device. The recharging step is an electrolytic process driven by the car's generator; the discharging step is the reverse, producing electricity from a chemical reaction — and in both, sulphuric acid supplies the ions and takes part in the change.
Each cell has two kinds of plate:
- One made of spongy lead,
- One coated with lead dioxide,
When the battery supplies current, both plates react with the sulphuric acid, and both turn into lead sulphate. The overall change during discharge is:
Checking the balance: lead ; sulphur ; oxygen on the left and on the right; hydrogen . Balanced.
Notice what happens to the acid. Sulphuric acid is used up and water is produced, so the acid becomes more dilute as the battery runs down. When the battery is recharged, a current is forced through it the other way, the reaction reverses, lead and lead dioxide re-form, and the acid becomes stronger again.
That change gives mechanics a simple way to check a battery. Sulphuric acid is denser than water, so the density of the liquid in each cell falls as the battery discharges. A small float instrument called a hydrometer measures that density and shows at a glance how charged the cell is.
This is electrolysis and the chemistry of sulphuric acid meeting in one device. The recharging step is an electrolytic process driven by the car's generator; the discharging step is the reverse, producing electricity from a chemical reaction — and in both, sulphuric acid supplies the ions and takes part in the change.
Exam relevance
How does sulphuric acid chemistry carry into JEE and NEET?
This is foundation work for Class 12 The p-Block Elements, Class 11 Equilibrium, Class 12 Electrochemistry and Class 11 Redox Reactions, all examined in JEE Main and NEET Chemistry.
Where the Contact Process leads. Class 12 The p-Block Elements describes the manufacture of sulphuric acid by the Contact Process among the compounds of group 16, with the vanadium pentoxide catalyst, the oxidation of sulphur dioxide and absorption as oleum. Questions on the catalyst, the reason for oleum and the conditions appear in both exams.
Where the conditions lead. The oxidation is, with Haber's process, a standard example of Le Chatelier's principle in Class 11 Equilibrium. **Predicting the effect of pressure, temperature and excess oxygen, and writing for this reaction, are recurring question types.
Where the properties lead. The same p-block chapter treats sulphuric acid as an oxidising agent, a dehydrating agent and a low-volatility acid that prepares more volatile acids from their salts. The equations with carbon, sulphur and sugar on this page are used there, and assertion-reason questions often test the dehydrating action.
Where the battery leads. Class 12 Electrochemistry describes the lead storage battery, with its discharge and charging reactions at each electrode. The overall equation in this lesson is the starting point for those half-reactions.
Where the oxidation leads. In Class 11 Redox Reactions**, sulphur in sulphuric acid has oxidation number and in sulphur dioxide . Identifying the acid as the oxidising agent in its reaction with carbon uses exactly that change.
Where the sulphate test leads. JEE Main practical chemistry includes the chemical principles of detecting sulphate among anions, using barium chloride in acid solution — the test described here.
Question types to expect. At this level: the Contact Process with conditions and reasons, acidic reactions, oxidising and dehydrating reactions, and tests. In competitive papers: Le Chatelier predictions, oxidation-number changes, lead storage battery reactions and anion detection.
The single trap that costs marks. Saying the catalyst shifts the equilibrium towards sulphur trioxide. It changes the rate only, a point both exams test in statement-based questions.
A second trap. Treating dilute and concentrated sulphuric acid as having the same properties. Oxidising and dehydrating action belong only to the concentrated acid, and questions frequently hinge on that single word.
Board versus competitive emphasis. The ICSE paper marks full process descriptions, balanced equations and observations; a competitive paper marks an equilibrium prediction, an oxidation state or a product. The transferable habit is asking whether the acid is behaving as an acid, an oxidising agent or a dehydrating agent — because the answer decides the product.
Where the Contact Process leads. Class 12 The p-Block Elements describes the manufacture of sulphuric acid by the Contact Process among the compounds of group 16, with the vanadium pentoxide catalyst, the oxidation of sulphur dioxide and absorption as oleum. Questions on the catalyst, the reason for oleum and the conditions appear in both exams.
Where the conditions lead. The oxidation is, with Haber's process, a standard example of Le Chatelier's principle in Class 11 Equilibrium. **Predicting the effect of pressure, temperature and excess oxygen, and writing for this reaction, are recurring question types.
Where the properties lead. The same p-block chapter treats sulphuric acid as an oxidising agent, a dehydrating agent and a low-volatility acid that prepares more volatile acids from their salts. The equations with carbon, sulphur and sugar on this page are used there, and assertion-reason questions often test the dehydrating action.
Where the battery leads. Class 12 Electrochemistry describes the lead storage battery, with its discharge and charging reactions at each electrode. The overall equation in this lesson is the starting point for those half-reactions.
Where the oxidation leads. In Class 11 Redox Reactions**, sulphur in sulphuric acid has oxidation number and in sulphur dioxide . Identifying the acid as the oxidising agent in its reaction with carbon uses exactly that change.
Where the sulphate test leads. JEE Main practical chemistry includes the chemical principles of detecting sulphate among anions, using barium chloride in acid solution — the test described here.
Question types to expect. At this level: the Contact Process with conditions and reasons, acidic reactions, oxidising and dehydrating reactions, and tests. In competitive papers: Le Chatelier predictions, oxidation-number changes, lead storage battery reactions and anion detection.
The single trap that costs marks. Saying the catalyst shifts the equilibrium towards sulphur trioxide. It changes the rate only, a point both exams test in statement-based questions.
A second trap. Treating dilute and concentrated sulphuric acid as having the same properties. Oxidising and dehydrating action belong only to the concentrated acid, and questions frequently hinge on that single word.
Board versus competitive emphasis. The ICSE paper marks full process descriptions, balanced equations and observations; a competitive paper marks an equilibrium prediction, an oxidation state or a product. The transferable habit is asking whether the acid is behaving as an acid, an oxidising agent or a dehydrating agent — because the answer decides the product.
Key takeaways
What must you be able to do from this part?
One industrial process, eight acid reactions, three roles of the concentrated acid and two tests.
- Contact Process: ; purification; ; ;
- Conditions: vanadium pentoxide, about –, about – atmospheres, excess air, purified gases
- Reasons: exothermic reaction needs a compromise temperature; fewer gas molecules favour slight pressure; catalyst speeds equilibrium only
- Sulphur trioxide is absorbed in concentrated acid, because water forms an acid mist
- ** of sulphur** gives of sulphuric acid
- Dibasic: two replaceable hydrogens, forming and
- Dilute acid: metals give hydrogen; oxides and hydroxides give salt and water; carbonates and bicarbonates give CO2; sulphites and bisulphites give SO2; sulphides give H2S
- ** of zinc** gives of hydrogen using of acid
- Oxidising agent (hot, concentrated): ;
- Dehydrating agent (concentrated): sugar gives black spongy carbon — of sugar leaves of carbon; blue copper sulphate turns white
- Drying removes moisture; dehydrating removes combined elements of water
- Non-volatile: drives out HCl from chlorides and HNO3 from nitrates
- Tests: white BaSO4 with barium chloride and white PbSO4 with lead nitrate, both insoluble in acid; chars sugar; hydrogen with zinc
- Sulphate test: dilute HCl, then barium chloride — white precipitate insoluble in HCl
The sharpest self-test is one acid in three bottles. For dilute, hot concentrated and cold concentrated sulphuric acid, write one reaction each that only that form can do — then explain in one line what the concentration changes.
- Contact Process: ; purification; ; ;
- Conditions: vanadium pentoxide, about –, about – atmospheres, excess air, purified gases
- Reasons: exothermic reaction needs a compromise temperature; fewer gas molecules favour slight pressure; catalyst speeds equilibrium only
- Sulphur trioxide is absorbed in concentrated acid, because water forms an acid mist
- ** of sulphur** gives of sulphuric acid
- Dibasic: two replaceable hydrogens, forming and
- Dilute acid: metals give hydrogen; oxides and hydroxides give salt and water; carbonates and bicarbonates give CO2; sulphites and bisulphites give SO2; sulphides give H2S
- ** of zinc** gives of hydrogen using of acid
- Oxidising agent (hot, concentrated): ;
- Dehydrating agent (concentrated): sugar gives black spongy carbon — of sugar leaves of carbon; blue copper sulphate turns white
- Drying removes moisture; dehydrating removes combined elements of water
- Non-volatile: drives out HCl from chlorides and HNO3 from nitrates
- Tests: white BaSO4 with barium chloride and white PbSO4 with lead nitrate, both insoluble in acid; chars sugar; hydrogen with zinc
- Sulphate test: dilute HCl, then barium chloride — white precipitate insoluble in HCl
The sharpest self-test is one acid in three bottles. For dilute, hot concentrated and cold concentrated sulphuric acid, write one reaction each that only that form can do — then explain in one line what the concentration changes.