Types of Chemical Reactions – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Differentiate between combination and decomposition reactions using the given activities and explain the associated energy changes.
Answer:
A combination reaction occurs when two or more reactants combine to form a single product. In the activity with calcium oxide (CaO) and water (H₂O), they combine to form calcium hydroxide, Ca(OH)₂, releasing heat. This shows that many combination reactions are exothermic because new bonds form and energy is released.
A decomposition reaction is the breakdown of a single compound into simpler substances. When ferrous sulphate (FeSO₄) is heated, it decomposes to iron(III) oxide (Fe₂O₃) and gases SO₂ and SO₃, while calcium carbonate (CaCO₃) decomposes into calcium oxide (CaO) and carbon dioxide (CO₂). These reactions generally require heat, so they are often endothermic because energy is absorbed to break chemical bonds.
In short, combination = joining + exothermic (often), whereas decomposition = splitting + endothermic (often), with clear examples from the experiments.
Q2. Describe the CaO and water activity in detail. What do the observations reveal about the nature of the reaction?
Answer:
In this activity, a small amount of calcium oxide (CaO) is placed in a beaker, and water (H₂O) is added gradually. Upon touching the beaker, it feels warm, indicating heat release. The product formed is calcium hydroxide, Ca(OH)₂ (aq), also called slaked lime.
The chemical equation is: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
The rise in temperature shows the reaction is exothermic. The formation of only one product confirms it is a combination reaction. The reaction also illustrates how bond formation in the product releases energy.
This experiment emphasizes safe handling: add water slowly to control heat release. The outcome explains a key idea—when substances combine to form a single product with heat release, the reaction is a combination and exothermic.
Q3. When ferrous sulphate crystals are heated, what changes do you see and what products are formed? Explain with reasons and equation.
Answer:
On heating ferrous sulphate (FeSO₄) in a boiling tube, its green color fades and it gradually turns brown due to the formation of iron(III) oxide (Fe₂O₃). A characteristic smell is noticed because sulphur dioxide (SO₂) and sulphur trioxide (SO₃) gases are released.
The balanced chemical equation is:
2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
This is a decomposition reaction because one compound breaks into simpler substances. It requires heating, showing it is generally endothermic since bond breaking needs energy.
The visual change (green to brown), the evolution of gases, and the need for heat are clear signs of decomposition. The experiment helps you connect observations (color change, odor) with products formed, reinforcing the concept of thermal decomposition.
Q4. Explain the displacement reaction between iron nails and copper sulphate solution. What observations support your conclusion?
Answer:
When clean iron nails are placed in copper sulphate (CuSO₄) solution, after some time the blue color fades, and a reddish-brown deposit of copper appears on the iron nails. This indicates that iron has displaced copper from the solution.
The reaction can be represented as: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s).
This is a displacement reaction, where a more reactive element replaces a less reactive element from its compound. Here, iron replaces copper in copper sulphate, forming iron(II) sulphate (which is pale green in solution) and copper metal.
Essential steps include cleaning the nails to remove oxide layers and immersing them adequately. The loss of blue color and deposition of copper are strong evidence that a displacement reaction has occurred.
Q5. Describe the double displacement reaction between lead(II) nitrate and potassium iodide. What indicates that a new compound has formed?
Answer:
When solutions of lead(II) nitrate, Pb(NO₃)₂, and potassium iodide, KI, are mixed, a white precipitate of lead iodide (PbI₂) forms, while potassium nitrate (KNO₃) remains in solution.
The chemical equation is:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
This is a double displacement reaction because ions are exchanged between two compounds: Pb²⁺ pairs with I⁻, and K⁺ pairs with NO₃⁻. The solid precipitate (PbI₂) shows that a new, insoluble product is formed, driving the reaction forward.
The sudden formation of a solid from clear solutions is a key indicator of precipitation as part of double displacement. The solution that remains contains KNO₃, confirming that both ions have exchanged partners to make two new compounds.
High Complexity (Analytical & Scenario-Based)
Q6. Compare energy changes in combination and decomposition reactions using the given examples. Why do these differences occur?
Answer:
In the combination reaction of CaO + H₂O → Ca(OH)₂, the beaker becomes warm, showing heat is released. This is exothermic because new bonds form in Ca(OH)₂, and bond formation typically releases energy.
In decomposition, such as 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃ and CaCO₃ → CaO + CO₂, heating is necessary to make the reactions happen. These are generally endothermic, as breaking bonds in a compound needs energy input.
The difference arises from the energy profile of the reactions: combination makes stronger, stable bonds, releasing energy, while decomposition must overcome bond energies to split a compound.
Observations (temperature rise vs. need for heating) connect the macroscopic signs to the microscopic bond changes, helping you remember: combination → exothermic; decomposition → endothermic (often).
Q7. A lab mixes iron nails with CuSO₄ solution and also mixes Pb(NO₃)₂ with KI. Analyze how the type of reaction and visible changes help identify the process in each case.
Answer:
In the iron + CuSO₄ setup, the blue color fades and copper deposits on the iron nails. This indicates a displacement reaction, where iron takes the place of copper in the solution to form FeSO₄ and copper metal. The change in color and metal deposition are the key signs.
In the Pb(NO₃)₂ + KI mixture, a white precipitate of PbI₂ forms immediately, while KNO₃ stays dissolved. This is a double displacement reaction, where ions exchange partners and a new insoluble solid forms. The sudden precipitate is the main indicator.
Thus, look for: single element replacement + color/deposit for displacement, and ion exchange + precipitate for double displacement. The observations directly match the reaction type.
Q8. Design a procedure to confirm that a double displacement reaction has occurred when mixing two salt solutions. What evidence would you collect and how would you interpret it?
Answer:
Steps:
Mix measured volumes of two aqueous salt solutions (e.g., Pb(NO₃)₂ and KI) in a clean test tube.
Observe for immediate turbidity or solid formation indicating a precipitate.
Allow the mixture to stand, then filter it to separate the solid from the filtrate.
Evidence to collect:
Presence of a visible precipitate (e.g., PbI₂(s)) shows formation of a new, insoluble product.
The filtrate remains clear, consistent with a soluble salt (e.g., KNO₃) remaining in solution.
Record color and texture of the precipitate and note that two new compounds have formed by ion exchange.
Interpretation:
Formation of an insoluble solid from two soluble reactants is strong evidence for a double displacement (precipitation) reaction.
The change cannot be explained by simple dilution; it reflects exchange of ions and creation of a new compound.
Q9. In the iron–copper sulphate experiment, imagine no visible change occurs after 20 minutes. Analyze possible reasons and propose corrective steps.
Answer:
Possible reasons:
The iron nails are not clean and have an oxide layer, preventing contact with the solution.
The CuSO₄ solution is too dilute, so the color change is not noticeable.
Insufficient time or low temperature slows the reaction.
The surface area is small (only one nail in contact) compared to the successful setup with two nails on a thread.
Corrective steps:
Clean the iron nails by rubbing with sandpaper to expose fresh metal.
Use a fresh, adequately concentrated CuSO₄ solution with a clear blue color at the start.
Increase contact time and gently agitate the test tube for better interaction.
Increase surface area by using more nails or thinner pieces.
With these adjustments, the expected fading of blue color and copper deposition should be observed, confirming a displacement reaction.
Q10. Cement production uses the decomposition of CaCO₃. Explain how this process illustrates the nature of decomposition reactions and their energy needs.
Answer:
In cement manufacture, calcium carbonate (CaCO₃) is heated to form calcium oxide (CaO) and carbon dioxide (CO₂):
CaCO₃(s) → CaO(s) + CO₂(g)
This is a decomposition reaction because one compound splits into two simpler products. The process requires continuous heating, showing it is endothermic—energy is absorbed to break bonds in CaCO₃.
The reaction demonstrates key features of decomposition:
