Important Questions (2 Marks): Chemical Reactions & Equations - Class 9 MCQ

Reaction of magnesium with nitrogen is a combination reaction as these two reactants (Mg and N₂) combine to form a single product as magnesium nitride (Mg₃N₂). The equation can be written as: 3Mg(s) + N₂(g) → Mg₃N₂(s).

Copper metal is more reactive than silver nitrate to displace silver and form copper nitrate.

2AgNO₃ + Cu → Cu (NO₃)₂ + 2Ag

Metals present on top of the other metal in the activity series can displace the metal at the bottom from its salt solution. So zinc is present at the top of the iron and can displace iron from its sulphate solution.

 

 

A metathesis reaction is also called a double displacement reaction is a chemical process involving the exchange of ions of cations and anions between reacting species resulting in the creation of two new chemical species. These are many types like precipitation reaction, neutralization reaction etc.

Observation:
- Rahul took zinc granules in a test tube.
- He added dilute HCl (Hydrochloric acid) to the test tube.
- He observed a change in the color of the zinc granules.

The color of the zinc granules changed to white after the addition of dilute HCl. The reaction between zinc and hydrochloric acid can be explained as follows:
1. Reaction between zinc and hydrochloric acid:
- Zinc (Zn) reacts with hydrochloric acid (HCl) to form zinc chloride (ZnCl2) and hydrogen gas (H2).
- The chemical equation for the reaction is: Zn + 2HCl → ZnCl2 + H2
2. Formation of zinc chloride:
- Zinc chloride is a white solid compound.
- When zinc granules react with hydrochloric acid, zinc chloride is formed.
- The white color of zinc chloride imparts a white color to the zinc granules.
Therefore, the color of the zinc granules changed to white after the addition of dilute HCl.

BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq)

White ppt

Explanation:
When iron nails are kept in copper sulphate solution for two hours, a chemical reaction takes place between the iron and copper sulphate. This reaction results in a change in the color of the solution.
Here is a detailed explanation of the process:
1. Initial state:
- The copper sulphate solution is initially blue in color.
2. Chemical reaction:
- The iron nails are made of iron, which is more reactive than copper.
- In the presence of water and oxygen, iron undergoes oxidation, losing electrons and forming Fe2+ ions.
- Copper ions (Cu2+) from the copper sulphate solution accept these electrons and are reduced to metallic copper.
- As a result, the iron nails corrode and dissolve in the solution, while copper is deposited onto the nails.
3. Color change:
- As copper is deposited onto the iron nails, the solution gradually loses its blue color.
- The color changes from blue to light green due to the presence of copper ions in the solution.
4. Conclusion:
- After two hours, the color of the solution changes from blue to light green.
Therefore, the correct answer is C: from blue to light green.

SO₂ and SO₃ gases are evolved which cause choking and suffocation.

BaCl₂ (aq) + Na₂SO₄ (aq) → BaSO₄ (s) + 2NaCl (aq)

White ppt of BaSO₄ settles at the bottom.

The reaction is exothermic and heat is produced with hissing sound.

BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl

A lot of heat is produced in the reaction, which may even cause the water to boil. Hence it is a combination reaction as well as exothermic reaction.

On heating, ferrous sulphate crystals lose water and anhydrous ferrous sulphate (FeSO₄) is formed. So their colour changes from light green to white. On further heating, anhydrous ferrous sulphate decomposes to form ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃).

Hydrogen is prepared in the lab by the action of acids on metals dilute. Sulphuric acid is added to granulated zinc. Zinc sulphate or zinc chloride is formed in solution and the hydrogen that is evolved is collected over water in a trough.

To determine which solution is colored, we need to analyze the chemical compounds given.
A: ZnSO₄
- Zinc sulfate is a white crystalline solid and is colorless when dissolved in water. It does not produce a colored solution.
B: FeSO₄
- Iron(II) sulfate is a pale green or pale blue crystalline solid. When dissolved in water, it forms a light green solution. Therefore, this solution is colored.
C: Al₂(SO₄)₃
- Aluminum sulfate is a white crystalline solid and is colorless when dissolved in water. It does not produce a colored solution.
D: Na₂SO₄
- Sodium sulfate is a white crystalline solid and is colorless when dissolved in water. It does not produce a colored solution.
Conclusion:
The solution that is colored is option B: FeSO₄.

Answer:

The aqueous solution of which of the following is colorless?

• A: FeSO₄


• B: ZnSO₄


• C: Al₂(SO₄)₃


• D: Both (b) and (c)

Answer: The correct answer is option D, Both (b) and (c).

Explanation:

Let's analyze the options to determine the color of their aqueous solutions:

• A: FeSO₄ - The aqueous solution of iron(II) sulfate, FeSO₄, is pale green in color.


• B: ZnSO₄ - The aqueous solution of zinc sulfate, ZnSO₄, is colorless.


• C: Al₂(SO₄)₃ - The aqueous solution of aluminum sulfate, Al₂(SO₄)₃, is colorless.

Therefore, both options B and C, ZnSO₄ and Al₂(SO₄)₃, have colorless aqueous solutions.

Thus, the correct answer is option D, Both (b) and (c).

Explanation:

Observation:

• Zn rod became thinner


• Zn rod became thicker due to iron deposition


• Zn rod remains as it was


• Zn rod has holes

Reasoning:

• Zn rod became thinner: This is not possible because Zn is more reactive than Fe. In FeSO₄ solution, Zn will undergo oxidation and lose electrons, resulting in the formation of Zn²⁺ ions. These ions will then react with SO₄^2- ions in the solution to form ZnSO₄. This reaction will lead to the gradual dissolution of the Zn rod, making it thinner.


• Zn rod became thicker due to iron deposition: This is also not possible because FeSO₄ solution contains Fe²⁺ ions, not Fe metal. Therefore, there is no source of iron to deposit on the Zn rod.


• Zn rod remains as it was: This is not possible because Zn will react with FeSO₄ solution to form ZnSO₄ and dissolve gradually, resulting in a change in the appearance of the Zn rod.


• Zn rod has holes: This is the correct observation. As Zn undergoes oxidation and dissolves in the FeSO₄ solution, it forms Zn²⁺ ions and leaves behind empty spaces or holes in the rod.

Therefore, the correct answer is option D - Zn rod has holes.

Observations:
(i) Pale green solution becomes colourless.
- This observation is incorrect because when zinc granules are added to an iron sulphate solution, the resulting solution should turn pale green, not colorless.
(ii) Black deposit seen on zinc granules.
- This observation is correct. When zinc reacts with iron sulphate, a black deposit is formed on the surface of the zinc granules.
(iii) Red deposit seen on zinc granules.
- This observation is incorrect. There should not be a red deposit formed on the zinc granules when they react with iron sulphate.
(iv) Colourless solution becomes pale green.
- This observation is correct. The addition of zinc granules to an iron sulphate solution would cause the colorless solution to turn pale green.
Conclusion:
Based on the given observations, the incorrect observation is (iii) Red deposit seen on zinc granules.
Answer: B: (iii) and (iv)

Observation:
The correct observation for the change in color of the solution made by Sapna after adding a strip of aluminum can be described as follows:
Explanation:
When a strip of aluminum is added to a solution of FeSO₄, a redox reaction takes place between the aluminum and the iron ions present in the solution. This reaction leads to the formation of a new compound and a change in the color of the solution. Based on this, the correct observation can be determined as follows:
- A: Pale green colored solution turns colorless: This option is incorrect because the addition of aluminum does not change the color of the solution to colorless.
- B: Colorless solution turned pale green: This option is incorrect because the initial color of the solution is not colorless.
- C: Pale green colored solution remained pale green: This option is incorrect because the addition of aluminum causes a change in the color of the solution.
- D: Colorless solution turned blue: This option is incorrect because the addition of aluminum does not change the color of the solution to blue.
Therefore, the correct observation is option A: Pale green colored solution turns colorless.

In the chemical reaction Fe₂O₃ + 2Al → Al₂O₃ + 2Fe

The above reaction is known as displacement reaction iron oxide reacts with aluminium to form aluminium oxide and iron metal.