This MCQ module is based on: Displacement, Double Displacement and Redox Reactions
TOPIC 3 OF 50
Displacement, Double Displacement and Redox Reactions
🎓 Class 10
Science
CBSE
Theory
Ch 1 — Chemical Reactions and Equations
⏱ ~23 min
🌐 Language: [gtranslate]
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1.2.3 Displacement Reactions
In a displacement reaction, a more reactive element takes the place of a less reactive element in a compound. The more reactive element essentially "kicks out" the less reactive one.
Activity 1.9 — Iron Nails in Copper Sulphate Solution
L3 Apply
Predict first: Copper sulphate solution is blue. If you dip an iron nail into this solution and leave it for 20 minutes, what changes do you expect in the colour of the solution and the appearance of the nail?
- Take two iron nails and clean them with sandpaper.
- Take copper sulphate (CuSO4) solution in a test tube.
- Note the colour of the solution (blue) and the iron nails (grey).
- Dip the iron nails in the copper sulphate solution for about 20 minutes.
- Remove the nails and observe their surface. Also observe the colour of the solution.
Observations:
After 20 minutes: The blue colour of the CuSO4 solution has faded and turned greenish (due to FeSO4 forming). The iron nails now have a brownish deposit of copper metal on their surface.
Chemical equation:
After 20 minutes: The blue colour of the CuSO4 solution has faded and turned greenish (due to FeSO4 forming). The iron nails now have a brownish deposit of copper metal on their surface.
Chemical equation:
\(\text{Fe}(s) + \text{CuSO}_4(aq) \rightarrow \text{FeSO}_4(aq) + \text{Cu}(s)\)
Iron is more reactive than copper, so it displaces copper from copper sulphate. Iron goes into the solution (as FeSO4, which is green), while copper comes out and deposits on the nail.
Activity 1.10 — Other Displacement Reactions
L3 Apply
Predict first: If zinc is more reactive than copper, what will happen when zinc granules are placed in CuSO4 solution?
- Place zinc granules in CuSO4 solution and observe after some time.
- Similarly, place a lead strip in CuSO4 solution and observe.
Observations:
In both cases, the blue colour of CuSO4 fades and copper metal is deposited, because both zinc and lead are more reactive than copper.
Chemical equations:
In both cases, the blue colour of CuSO4 fades and copper metal is deposited, because both zinc and lead are more reactive than copper.
Chemical equations:
\(\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{ZnSO}_4(aq) + \text{Cu}(s)\)
\(\text{Pb}(s) + \text{CuSO}_4(aq) \rightarrow \text{PbSO}_4(aq) + \text{Cu}(s)\)
Reactivity order: Zn > Fe > Pb > Cu. A more reactive metal will displace a less reactive metal from its salt solution.
1.2.4 Double Displacement Reactions
In a double displacement reaction, two ionic compounds in solution exchange their ions. Think of it as two pairs of partners who swap and form new pairs. These reactions often result in the formation of a precipitate (an insoluble solid that separates from the solution).
Example: When sodium sulphate solution is mixed with barium chloride solution:
\(\text{Na}_2\text{SO}_4(aq) + \text{BaCl}_2(aq) \rightarrow \text{BaSO}_4(s)\downarrow + 2\text{NaCl}(aq)\)
A white precipitate of barium sulphate (BaSO4) is formed. This is an insoluble compound that separates out as a solid. The reaction where a precipitate forms is also called a precipitation reaction.
1.2.5 Oxidation and Reduction (Redox Reactions)
Activity 1.11 — Heating Copper Powder
L3 Apply
Predict first: Copper powder is reddish-brown. When heated in air, will it gain or lose something from the atmosphere? What colour change might occur?
- Take about 1 g of copper powder in a porcelain dish (china dish).
- Heat the dish over a burner for some time.
- Observe the change in the colour of the surface of the copper powder.
Observations:
The reddish-brown copper powder turns black on heating. This black substance is copper oxide (CuO). Copper has gained oxygen from the air -- it has been oxidised.
Chemical equation:
The reddish-brown copper powder turns black on heating. This black substance is copper oxide (CuO). Copper has gained oxygen from the air -- it has been oxidised.
Chemical equation:
\(2\text{Cu}(s) + \text{O}_2(g) \xrightarrow{\Delta} 2\text{CuO}(s)\)
If hydrogen gas is passed over this black CuO, the copper is regained (the oxygen is removed):\(\text{CuO}(s) + \text{H}_2(g) \xrightarrow{\Delta} \text{Cu}(s) + \text{H}_2\text{O}(l)\)
Here, CuO has lost oxygen -- it has been reduced. And H2 has gained oxygen -- it has been oxidised.
Defining Oxidation and Reduction
Oxidation: Gain of oxygen OR loss of hydrogen by a substance during a reaction.
Reduction: Loss of oxygen OR gain of hydrogen by a substance during a reaction.
Reduction: Loss of oxygen OR gain of hydrogen by a substance during a reaction.
In the reaction of CuO with H2:
\(\text{CuO}(s) + \text{H}_2(g) \xrightarrow{\Delta} \text{Cu}(s) + \text{H}_2\text{O}(l)\)
- CuO is reduced -- it loses oxygen to become Cu.
- H2 is oxidised -- it gains oxygen to become H2O.
Since both oxidation and reduction happen simultaneously in the same reaction, it is called a redox reaction (reduction-oxidation).
More Examples of Redox Reactions
\(\text{ZnO} + \text{C} \rightarrow \text{Zn} + \text{CO}\)
C is oxidised (gains O), ZnO is reduced (loses O)
C is oxidised (gains O), ZnO is reduced (loses O)
\(\text{MnO}_2 + 4\text{HCl} \rightarrow \text{MnCl}_2 + 2\text{H}_2\text{O} + \text{Cl}_2\)
HCl is oxidised (loses H), MnO2 is reduced (loses O)
HCl is oxidised (loses H), MnO2 is reduced (loses O)
Effects of Oxidation in Everyday Life
Corrosion
Corrosion is the gradual destruction of metals due to their reaction with substances in the environment such as oxygen, water, and acids. The most familiar example is the rusting of iron.
When iron is exposed to moist air for a prolonged period, it acquires a reddish-brown flaky coating called rust. Rust is hydrated iron(III) oxide: Fe2O3.xH2O. Unlike the oxide layer on aluminium (which is protective), rust is porous and flakes off, exposing more iron to corrosion.
Silver articles become black over time because silver reacts with hydrogen sulphide in air to form a layer of silver sulphide (Ag2S). Copper articles develop a green coating (patina) of basic copper carbonate [CuCO3.Cu(OH)2].
Prevention of corrosion: Painting, oiling/greasing, galvanising (coating with zinc), chrome-plating, and alloying (e.g., making stainless steel).
Rancidity
Rancidity is the oxidation of fats and oils in food when exposed to air, causing the food to develop a bad smell and taste. This is why food left uncovered for long periods becomes stale and smelly.
Prevention of rancidity:
- Adding antioxidants to food (like BHA, BHT, or natural ones like vitamin E)
- Vacuum packing (removing air)
- Flushing food packets with nitrogen gas (an inert atmosphere)
- Storing food in airtight containers
- Refrigeration (slows oxidation)
Interactive: Identify the Reaction Type L4 Analyse
For each reaction below, select the correct reaction type:
1. CaO + H₂O → Ca(OH)₂
Combination
Decomposition
Displacement
Double Displacement
2. 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
Decomposition
Combination
Redox
Displacement
3. Fe + CuSO₄ → FeSO₄ + Cu
Displacement
Double Displacement
Combination
Decomposition
4. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
Double Displacement
Displacement
Decomposition
Redox
5. CuO + H₂ → Cu + H₂O
Redox
Combination
Displacement
Decomposition
Competency-Based Questions
Anita left an iron gate unpainted in her garden. After the monsoon season, she noticed a reddish-brown flaky coating on the gate. Meanwhile, her grandmother's silver jewellery, kept in a cupboard for months, had turned black. Their neighbour mentioned that his copper roof had developed a greenish layer.
Q1. L1 Remember What is the chemical name and formula of rust?
Answer: B. Hydrated iron(III) oxide, Fe2O3.xH2O. Rust forms when iron reacts with oxygen and moisture (water) in the air over time.
Q2. L2 Understand Why did the silver jewellery turn black? Name the compound responsible and write the reaction. (2 marks)
Answer: Silver reacts with hydrogen sulphide (H2S) present in the air to form a thin layer of silver sulphide (Ag2S), which is black in colour. This is called tarnishing.
\(2\text{Ag}(s) + \text{H}_2\text{S}(g) \rightarrow \text{Ag}_2\text{S}(s) + \text{H}_2(g)\)
\(2\text{Ag}(s) + \text{H}_2\text{S}(g) \rightarrow \text{Ag}_2\text{S}(s) + \text{H}_2(g)\)
Q3. L4 Analyse In the reaction Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s), identify which substance is oxidised and which is reduced. Justify your answer. (3 marks)
Answer:
Iron (Fe) is oxidised -- it goes from Fe(0) to Fe(2+), losing electrons.
Copper ion (Cu2+) is reduced -- it goes from Cu(2+) to Cu(0), gaining electrons.
This is also a displacement reaction where a more reactive metal (Fe) displaces a less reactive metal (Cu) from its salt solution. It is simultaneously a redox reaction.
Iron (Fe) is oxidised -- it goes from Fe(0) to Fe(2+), losing electrons.
Copper ion (Cu2+) is reduced -- it goes from Cu(2+) to Cu(0), gaining electrons.
This is also a displacement reaction where a more reactive metal (Fe) displaces a less reactive metal (Cu) from its salt solution. It is simultaneously a redox reaction.
Q4. L3 Apply Food manufacturers often pack chips in bags flushed with nitrogen gas. Explain how this prevents rancidity. (2 marks)
Answer: Rancidity is caused by the oxidation of fats and oils in food when they come in contact with oxygen in air. By flushing chip packets with nitrogen gas (which is chemically inert and does not react with fats), the oxygen is removed from the packet. Without oxygen, oxidation cannot occur, so the chips stay fresh for a longer time.
Q5. L5 Evaluate List three different methods to prevent iron from rusting. For each method, briefly explain how it works. (3 marks)
Answer:
1. Painting/Oiling: A layer of paint or oil creates a barrier between iron and moist air, preventing oxygen and water from reaching the iron surface.
2. Galvanising: Coating iron with a layer of zinc. Even if the zinc layer is scratched, zinc (being more reactive) gets oxidised preferentially, protecting the iron beneath.
3. Alloying: Mixing iron with chromium and nickel to make stainless steel. The chromium forms a thin protective oxide layer (Cr2O3) that does not flake off, unlike rust, and prevents further corrosion.
1. Painting/Oiling: A layer of paint or oil creates a barrier between iron and moist air, preventing oxygen and water from reaching the iron surface.
2. Galvanising: Coating iron with a layer of zinc. Even if the zinc layer is scratched, zinc (being more reactive) gets oxidised preferentially, protecting the iron beneath.
3. Alloying: Mixing iron with chromium and nickel to make stainless steel. The chromium forms a thin protective oxide layer (Cr2O3) that does not flake off, unlike rust, and prevents further corrosion.
Assertion-Reason Questions
Assertion (A): When iron nails are dipped in copper sulphate solution, the blue colour of the solution fades.
Reason (R): Iron is more reactive than copper and displaces copper from the solution.
Answer: A. Both are true. Iron displaces copper from CuSO4 because iron is more reactive (R). As CuSO4 (blue) is consumed and FeSO4 (green) is formed, the blue colour fades (A). The Reason correctly explains the Assertion.
Assertion (A): Corrosion of iron can be prevented by galvanisation.
Reason (R): Galvanisation involves coating iron with a layer of copper.
Answer: C. The Assertion is true -- galvanisation does prevent rusting. But the Reason is false -- galvanisation involves coating iron with a layer of zinc, not copper. Zinc is more reactive than iron and sacrificially corrodes in its place.
Assertion (A): In the reaction CuO + H2 → Cu + H2O, hydrogen is the reducing agent.
Reason (R): A reducing agent is a substance that gets oxidised in a reaction and causes the reduction of another substance.
Answer: A. Both are true. Hydrogen is the reducing agent because it reduces CuO to Cu (by removing its oxygen). In doing so, hydrogen itself gets oxidised (gains oxygen to become H2O). The Reason correctly defines and explains why hydrogen is the reducing agent.
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