Exemplar Questions and Solutions

Important CBSE Questions - 2 Mark Questions

1) What do you understand by precipitation reaction? Explain with suitable examples.

Answer: A precipitation reaction is a type of chemical reaction in which two soluble substances in a solution combine to form an insoluble solid called a precipitate. This reaction occurs when the positive and negative ions in the reactants switch partners. For example:

Example: When silver nitrate (AgNO3) is mixed with sodium chloride (NaCl) in a solution, silver chloride (AgCl) precipitates out as a solid.

2) What is the relationship between oxidation and oxidising agent in a redox reaction? Write an example of a redox reaction showing the relationship of the same.

Answer: In a redox (reduction-oxidation) reaction, oxidation is the loss of electrons, while reduction is the gain of electrons. An oxidizing agent is a substance that causes oxidation by accepting electrons from another substance. The relationship between oxidation and an oxidizing agent can be explained by the following example:

Example: In the reaction between magnesium (Mg) and oxygen (O2) to form magnesium oxide (MgO), magnesium undergoes oxidation by losing electrons to oxygen, which acts as an oxidizing agent.

3) Give any two applications of displacement reactions.

Answer: Displacement reactions are chemical reactions in which a more reactive element displaces another less reactive element from a compound. Two common applications of displacement reactions are:

• Electroplating: Displacement reactions are used in electroplating processes where a metal coating is applied to another metal object, such as chromium plating on car parts for protection against corrosion.
• Extraction of metals: Displacement reactions are also crucial in the extraction of metals from their ores, such as the extraction of iron from its ore using carbon in a blast furnace.

4) Give any two industrial applications of combination reactions.

Answer: Two important industrial applications of combination reactions are-

• Synthesis of ammonia (NH3): The Haber process is a combination reaction used to produce ammonia from nitrogen and hydrogen gases. Ammonia is a vital component in the production of fertilizers.
• Manufacture of sulfuric acid (H2SO4): The Contact process is a combination reaction used to produce sulfuric acid by combining sulfur dioxide and oxygen in the presence of a catalyst.

5) Is corrosion an oxidation reaction?

Answer: Yes, corrosion is an oxidation reaction. Corrosion is a process in which metals undergo oxidation when exposed to elements such as oxygen, water, and chemicals. The metal loses electrons and forms metal oxides or salts, leading to the deterioration of the metal surface.

6) An Aluminium can is used to store ferrous sulphate solution. It is observed that in a few days, holes started to appear on the can. Explain the observation and write the chemical equation to support your answer:

Answer: This observation can be explained by the phenomenon of corrosion. When aluminium comes in contact with ferrous sulphate solution, a reaction occurs where the aluminium metal reacts with the sulphate ions in the solution. This reaction leads to the formation of aluminium sulphate, along with the release of hydrogen gas. The production of hydrogen gas creates pressure inside the can, leading to the formation of holes due to the build-up of gas.

The chemical equation for this reaction can be represented as:

7) Give any three examples of double displacement reaction in our everyday life:

Answer:

• Precipitation Reaction: When two solutions containing different ions are mixed, and an insoluble product (precipitate) is formed, it is a double displacement reaction. For example, the reaction between silver nitrate and sodium chloride to form silver chloride precipitate.
• Neutralization Reaction: The reaction between an acid and a base to form salt and water is also a type of double displacement reaction. For instance, the reaction between hydrochloric acid and sodium hydroxide to form sodium chloride and water.
• Formation of Soap: The saponification reaction of fats with sodium hydroxide is another example of a double displacement reaction. This reaction produces soap and glycerol from fats and alkali.

8) Why cannot a chemical change be normally reversed?

Answer: Chemical changes involve the breaking and formation of chemical bonds, resulting in the formation of new substances with different properties. These changes are usually irreversible because once the bonds are broken and new compounds are formed, it is challenging to recreate the original reactants from the products. Unlike physical changes, where the substance retains its original properties, chemical changes result in the formation of entirely new substances.

9) Explain the term Corrosion with examples:

Answer: Corrosion is the gradual degradation or destruction of materials, usually metals, due to chemical reactions with their environment. One common example of corrosion is the rusting of iron, where iron reacts with oxygen and water in the presence of moisture to form iron oxide (rust). Another example is the tarnishing of silver, where silver reacts with sulphur compounds in the air to form silver sulphide.

10) Why do marble statues corrode when kept in the open for a long time?

Answer: Marble statues corrode when exposed to the elements due to the process of chemical weathering. The calcium carbonate present in marble reacts with acidic pollutants in the air, such as sulfur dioxide and nitrogen oxides, to form calcium sulfate and carbon dioxide. This reaction weakens the structure of the marble, leading to its corrosion and deterioration over time.

11) Why are decomposition reactions called the opposite of combination reactions? Explain with equations of these reactions:

Answer: Decomposition reactions involve the breakdown of a compound into its constituent elements or simpler compounds. This process is the opposite of combination reactions, where two or more substances combine to form a single compound. For example, the decomposition of hydrogen peroxide into water and oxygen can be represented by the equation:

12) What is the rule for a displacement reaction?

Answer: A displacement reaction occurs when a more reactive element displaces a less reactive element from a compound. The general rule for displacement reactions is that a more reactive element will always displace a less reactive element from its compound. For example, when zinc is added to a solution of copper sulfate, it displaces copper and forms zinc sulfate.

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