Types of Chemical Reactions – Long Answer Questions

Medium Level (Application & Explanation)

Answer:

A combination reaction makes a single product from two or more reactants.
Example: CaO + H₂O → Ca(OH)₂ + heat. The beaker feels warm, so it is exothermic.
A decomposition reaction splits one compound into simpler substances.
Example: CaCO₃ → CaO + CO₂. This needs heat, so it is endothermic.
So, combination builds bonds and often releases energy.
Decomposition breaks bonds and usually absorbs energy.
Both change bonding and make new substances.

Answer:

When ferrous sulphate (FeSO₄) is heated, its green color fades.
It first loses water of crystallization and turns white to pale.
On strong heating, it decomposes to Fe₂O₃ (brown solid), SO₂, and SO₃ (gases).
You may notice a sharp smell due to sulphur oxides.
The reaction is a thermal decomposition because heat breaks the compound.
Word form: ferrous sulphate → iron(III) oxide + sulphur dioxide + sulphur trioxide.
This shows that a single reactant forms multiple products.

Answer:

In a displacement reaction, a more reactive metal replaces a less reactive metal from its compound.
Iron (Fe) is more reactive than copper (Cu).
When iron nails are put in CuSO₄ (blue), Fe displaces Cu from the solution.
The blue color fades, and reddish-brown copper deposits on the nails.
The solution slowly turns green due to FeSO₄ formation.
This proves Fe + CuSO₄ → FeSO₄ + Cu is a displacement reaction.
The key idea is the reactivity difference between metals.

Answer:

In a double displacement reaction, ions exchange places between two compounds.
When Pb(NO₃)₂(aq) is mixed with KI(aq), ions swap: Pb²⁺ meets I⁻.
A yellow/white precipitate of PbI₂(s) forms. KNO₃ stays in solution.
The equation: Pb(NO₃)₂ + 2KI → PbI₂(s) + 2KNO₃.
The solid formed is a precipitate, so this is also called a precipitation reaction.
Key signs: sudden solid formation and ion exchange in solution.
This proves the double displacement idea clearly.

Answer:

Combination: Many reactants give one product; often exothermic.
Decomposition: One reactant gives many products; needs heat/light; often endothermic.
Displacement: A more reactive element replaces a less reactive one in a compound.
Double displacement: Ions exchange between two compounds in solution.
Signs: temperature change, color change, gas release, or precipitate formation.
Think: build vs break, replace vs exchange.
Use observations to match the reaction type.

Answer:

(a) This is a combination reaction because two reactants form one product. The heat shows it is exothermic.
(b) This is a decomposition reaction because a single compound splits into simpler substances on heating.
(c) This is a displacement reaction because iron (more reactive) replaces copper from its compound.
(d) This is a double displacement reaction because ions exchange and form a precipitate (PbI₂).
Each reaction matches its pattern and observation.
Use the number of products/reactants, ion exchange, and reactivity to decide.

Answer:

Choose two metals: iron and copper. Prepare CuSO₄ solution in two test tubes.
Put a clean iron nail in tube A and a clean copper wire in tube B.
After 20–30 minutes, tube A: blue fades, copper deposits on iron, solution turns green (FeSO₄).
Tube B: no change because copper is less reactive and cannot displace iron.
This proves more reactive metal can displace a less reactive one.
It shows reactivity series in action and confirms displacement.
Keep safety: use gloves, avoid skin contact, and label tubes.

Answer:

A sudden solid formation is a precipitation in a double displacement reaction.
Likely reactants: Pb(NO₃)₂(aq) and KI(aq).
They exchange ions: Pb²⁺ from lead nitrate meets I⁻ from KI.
PbI₂(s) forms as a yellow precipitate. K⁺ and NO₃⁻ stay in solution as spectator ions.
The key is ion exchange and low solubility of PbI₂.
Thus, the reaction is double displacement with precipitation.
Observations match the given example in the chapter.

Answer:

Quicklime (CaO) reacts with water to form slaked lime (Ca(OH)₂).
The reaction is CaO + H₂O → Ca(OH)₂ + heat. It is exothermic and gets very hot.
Safety: add water slowly, use gloves and goggles, and keep distance from the hot container.
The beaker warms because energy is released when new bonds form.
Slaked lime is used to make mortar and plaster in building work.
The reaction shows a combination type with clear heat release.
Proper control prevents splashing and burns.

Answer:

For CaCO₃ → CaO + CO₂, pass gas through limewater (Ca(OH)₂); it turns milky due to CaCO₃.
For 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃, the gas has a sharp smell; bring moist litmus: it turns red (acidic).
You can also note choking odor typical of SO₂/SO₃.
Both reactions are thermal decomposition because heat breaks the compounds.
Record color change (green to brown solid for FeSO₄ heating) and gas tests.
These observations confirm the products and the reaction type.
Always heat gently and use a fume hood or good ventilation.