Explanation of the Chemical Reaction

When an iron nail is placed in a copper sulfate solution, a chemical displacement reaction occurs. During this reaction, iron displaces copper from the copper sulfate solution.

Displacement Reaction:

The iron nail reacts with the copper sulfate (CuSO₄) to form iron sulfate (FeSO₄) and free copper metal. This reaction is represented by the following equation:

Fe + CuSO₄ → FeSO₄ + Cu

As a result, the blue colour of the copper sulfate solution fades, and a green colour appears due to the formation of iron sulfate, which is green in colour.

Question 4: Example of a Double Displacement Reaction

Answer: Sodium carbonate reacts with calcium chloride to form calcium carbonate and sodium chloride. In this reaction, sodium carbonate and calcium chloride exchange ions to form two new compounds. Hence, it is a double displacement reaction.

Chemical Reaction:

Na2CO3 (aq) + CaCl2 (aq) → CaCO3 (s) + 2 NaCl (aq)

Question 5: Identification of Oxidation and Reduction in Reactions

(i) Oxidation and Reduction in the Reaction

Answer: In this reaction, sodium (Na) is oxidised as it gains oxygen, and oxygen (O₂) is reduced by gaining electrons.

Chemical Reaction:

2 Na (s) + O2 (g) → 2 Na2O (s)

(ii) Oxidation and Reduction in the Reaction

Answer: In this reaction, copper oxide (CuO) is reduced to copper (Cu), while hydrogen (H₂) gets oxidised to form water (H₂O).

Chemical Reaction:

CuO (s) + H2 (g) → Cu (s) + H2O (l)

Question 1: Which of the statements about the reaction below are incorrect?

2PbO(s) + C(s) → 2Pb(s) + CO₂(g)

Answer: (i) (a) and (b)

Statements:

• (a) Lead is getting reduced.
• (b) Carbon dioxide is getting oxidised.
• (c) Carbon is getting oxidised.
• (d) Lead oxide is getting reduced.

Select the correct option:

• (i) (a) and (b)
• (ii) (a) and (c)
• (iii) (a), (b) and (c)
• (iv) All

Question 2: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a

• (a) Combination reaction.
• (b) Double displacement reaction.
• (c) Decomposition reaction.
• (d) Displacement reaction.

Answer: (d) The given reaction is an example of a displacement reaction.

Question 3: What happens when dilute hydrochloric acid is added to iron filings?

Select the correct answer:

• (a) Hydrogen gas and iron chloride are produced.
• (b) Chlorine gas and iron hydroxide are produced.
• (c) No reaction takes place.
• (d) Iron salt and water are produced.

Answer: (a) Hydrogen gas and iron chloride are produced. The reaction is as follows:

Fe (s) + 2 HCl (aq) → FeCl2 (aq) + H2 (g)

Question 4: What is a balanced chemical equation? Why should chemical equations be balanced?

Answer: A reaction which has an equal number of atoms of all the elements on both sides of the chemical equation is called a balanced chemical equation.

The law of conservation of mass states that mass can neither be created nor destroyed. Hence, in a chemical reaction, the total mass of reactants should be equal to the total mass of the products. It means that the total number of atoms of each element should be equal on both sides of a chemical equation. Hence, it is for this reason that chemical equations should be balanced.

Question 5: Translate the following statements into chemical equations and then balance them.

(a) Hydrogen gas combines with nitrogen to form ammonia.

Answer:

N2 (g) + 3 H2 (g) → 2 NH3 (g)

(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.

Answer:

2 H2S (g) + 3 O2 (g) → 2 H2O (l) + 2 SO2 (g)

(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.

Answer:

BaCl2 (aq) + Al2(SO4)3 (aq) → 2 AlCl3 (aq) + BaSO4 (s)

(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer:

2 K (s) + 2 H2O (l) → 2 KOH (aq) + H2 (g)

Question 6: Balance the Following Chemical Equations

(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O

Balanced Equation:

2 HNO3 + Ca(OH)2 → Ca(NO3)2 + 2 H2O

(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Balanced Equation:

2 NaOH + H2SO4 → Na2SO4 + 2 H2O

(c) NaCl + AgNO₃ → AgCl + NaNO₃

Balanced Equation:

NaCl + AgNO3 → AgCl + NaNO3

(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Balanced Equation:

BaCl2 + H2SO4 → BaSO4 + 2 HCl

Question 7: Write the balanced chemical equations for the following reactions

(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Balanced Equation:

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)

(b) Zinc + Silver nitrate → Zinc nitrate + Silver

Balanced Equation:

Zn (s) + 2 AgNO3 (aq) → Zn(NO3)2 (aq) + 2 Ag (s)

(c) Aluminium + Copper chloride → Aluminium chloride + Copper

Balanced Equation:

2 Al (s) + 3 CuCl2 (aq) → 2 AlCl3 (aq) + 3 Cu (s)

(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Balanced Equation:

BaCl2 (aq) + K2SO4 (aq) → BaSO4 (s) + 2 KCl (aq)

Question 8: Write the balanced chemical equation for the following and identify the type of reaction in each case

(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)

Balanced Equation:

2 KBr (aq) + BaI2 (aq) → 2 KI (aq) + BaBr2 (s)

Type of Reaction: Double displacement (precipitation) reaction.

(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)

Balanced Equation:

ZnCO3 (s) → ZnO (s) + CO2 (g)

Type of Reaction: Decomposition reaction.

(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)

Balanced Equation:

H2 (g) + Cl2 (g) → 2 HCl (g)

Type of Reaction: Combination (synthesis) reaction.

(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Balanced Equation:

Mg (s) + 2 HCl (aq) → MgCl2 (aq) + H2 (g)

Type of Reaction: Single displacement (redox) reaction.

Question 9: What does one mean by exothermic and endothermic reactions? Give examples.

Answer:

Exothermic Reactions:
Chemical reactions that release energy in the form of heat, light, or sound are called exothermic reactions. In these reactions, the energy of the products is lower than the energy of the reactants, leading to the release of energy.
Example: A mixture of sodium (Na) and chlorine (Cl₂) to yield table salt (NaCl). This is a combination reaction that releases energy in the form of heat and light.
Chemical Equation:

2 Na (solid) + Cl2 (gas) → NaCl (solid)

• Sodium (Na) – Solid
• Chlorine (Cl₂) – Gas
• Sodium chloride (NaCl) – Solid

Endothermic Reactions:
Reactions that absorb energy or require energy in order to proceed are called endothermic reactions. In these reactions, the energy of the products is higher than the energy of the reactants, leading to the absorption of energy.
Example: In the process of photosynthesis, plants use the energy from sunlight as a catalyst to convert carbon dioxide (CO₂) and water (H₂O) into glucose (C₆H₁₂O₆). This is an example of an endothermic reaction.
Chemical Equation:

6 CO2 (gas) + 6 H2O (liquid) → C6H12O6 (aqueous) + 6 O2 (gas)

• Carbon dioxide (CO₂) – Gas
• Water (H₂O) – Liquid
• Glucose (C₆H₁₂O₆) – Aqueous
• Oxygen (O₂) – Gas

Question 10: Why is respiration considered an exothermic reaction? Explain.

Answer:

Energy is required to support life. Energy in our body is obtained from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy. The special name for this combustion reaction is respiration. Since energy is released in the whole process, it is an exothermic process.

C6H12O6 (glucose) + 6 O2 (oxygen) → 6 CO2 (carbon dioxide) + 6 H2O (water) + Energy

Substances Involved:

• Glucose (C₆H₁₂O₆)
• Oxygen (O₂)
• Carbon dioxide (CO₂)
• Water (H₂O)

Question 11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer:

Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Thus, they are the exact opposite of combination reactions in which two or more substances combine to give a new substance with the release of energy.

CaCO3 (calcium carbonate) → CaO (calcium oxide) + CO2 (carbon dioxide)

Substances Involved:

• Calcium carbonate (CaCO₃)
• Calcium oxide (CaO)
• Carbon dioxide (CO₂)

2 H2 (hydrogen) + O2 (oxygen) → 2 H2O (water)

Substances Involved:

• Hydrogen (H₂)
• Oxygen (O₂)
• Water (H₂O)

Question 12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light, or electricity.

Answer:

CaCO3 (calcium carbonate) → CaO (calcium oxide) + CO2 (carbon dioxide)

Substances Involved:

• Calcium carbonate (CaCO₃)
• Calcium oxide (CaO)
• Carbon dioxide (CO₂)

2 AgCl (silver chloride) → 2 Ag (silver) + Cl2 (chlorine)

Substances Involved:

• Silver chloride (AgCl)
• Silver (Ag)
• Chlorine (Cl₂)

2 H2O (water) → 2 H2 (hydrogen) + O2 (oxygen)

Substances Involved:

• Water (H₂O)
• Hydrogen (H₂)
• Oxygen (O₂)

Question 13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:

In a displacement reaction, a more reactive element replaces a less reactive element from a compound. Here, element A is more reactive than element B.

In a double displacement reaction, two atoms or a group of atoms switch places to form new compounds.

Displacement Reaction:

Zn (zinc) + CuSO4 (copper sulfate) → ZnSO4 (zinc sulfate) + Cu (copper)

Substances Involved:

• Zinc (Zn)
• Copper sulfate (CuSO₄)
• Zinc sulfate (ZnSO₄)
• Copper (Cu)

Double Displacement Reaction:

NaCl (sodium chloride) + AgNO3 (silver nitrate) → NaNO3 (sodium nitrate) + AgCl (silver chloride)

Substances Involved:

• Sodium chloride (NaCl)
• Silver nitrate (AgNO₃)
• Sodium nitrate (NaNO₃)
• Silver chloride (AgCl)

Question 14: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer:

In the refining of silver, copper metal displaces silver from its solution of silver nitrate. This is a typical displacement reaction, where a more reactive metal (copper) replaces a less reactive metal (silver) from its compound.

Displacement Reaction:

Cu (copper) + 2AgNO3 (silver nitrate) → Cu(NO3)2 (copper nitrate) + 2Ag (silver)

Substances Involved:

• Copper (Cu)
• Silver nitrate (AgNO₃)
• Copper nitrate (Cu(NO₃)₂)
• Silver (Ag)

Question 15: What do you mean by a precipitation reaction? Explain by giving examples.

Answer:

A reaction in which an insoluble solid (called precipitate) is formed is called a precipitation reaction.

For example:

CaCl2 (aq) + Na2CO3 (aq) → CaCO3 (s) + 2NaCl (aq)

In this reaction, calcium carbonate (CaCO₃) is obtained as a precipitate. Hence, it is a precipitation reaction.

Another example of a precipitation reaction is:

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

In this reaction, barium sulphate (BaSO₄) is obtained as a precipitate.

Question 16: Explain the following in terms of gain or loss of oxygen with two examples each.

(a) Oxidation

Oxidation is the gain of oxygen. For example:

2H2 (g) + O2 (g) → 2H2O (l)

In this equation, H₂ is oxidized to H₂O.

2Cu (s) + O2 (g) → 2CuO (s)

In this equation, Cu is oxidized to CuO.

(b) Reduction

Reduction is the loss of oxygen. For example:

CO2 (g) + H2 (g) → CO (g) + H2O (g)

In this equation, CO₂ is reduced to CO.

CuO (s) + H2 (g) → Cu (s) + H2O (g)

In this equation, CuO is reduced to Cu.

Question 17: A shiny brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.

Answer:

‘X’ is copper (Cu) and the black-coloured compound formed is copper oxide (CuO). The equation of the reaction involved on heating copper is given below:

2Cu (s) + O2 (g) → 2CuO (s)

Question 18: Why do we apply paint on iron articles?

Answer:

Iron articles are painted because it prevents them from rusting. When painted, the contact of iron articles with moisture and air is cut off. Hence, rusting is prevented as their presence is essential for rusting to take place.

Question 19: Oil and fat containing food items are flushed with nitrogen. Why?

Answer:

Nitrogen is an inert gas and does not easily react with these substances. On the other hand, oxygen reacts with food substances and makes them rancid. Thus, bags used in packing food items are flushed with nitrogen gas to remove oxygen inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat-containing food items is avoided.

Question 20: Explain the following terms with one example each.

(a) Corrosion

Corrosion is defined as a process where materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. For example:

4Fe (s) + 3O2 (g) + 6H2O (l) → 4Fe(OH)3 (s)

This is the formation of rust, which is hydrated iron oxide.

(b) Rancidity

The process of oxidation of fats and oils that can be easily noticed by the change in taste and smell is known as rancidity. For example:

The taste and smell of butter change when kept for a long time.

Rancidity can be avoided by:

• Storing food in airtight containers
• Storing food in refrigerators
• Adding antioxidants
• Storing food in an environment of nitrogen