Chapter 6 – Chemical Reactions

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Book Name	: Concise Chemistry Middle School
Subject	: Chemistry
Class	: 8
Publisher	: Selina Publisher PVT Ltd
Chapter Name	: Chemical Reactions

EXERCISE- I

Question 1

1. Define a chemical reaction
2. What happens during a chemical reaction?
3. What do you understand by a chemical bond?

Answer

1. A chemical reaction is a process that involves the transformation of one or more substances into different substances with different chemical and physical properties.
2. During a chemical reaction, the atoms of the reactants rearrange themselves to form new substances, and the bonds between the atoms are broken and reformed to create the products of the reaction.
3. A chemical bond is a force of attraction that holds two or more atoms together to form a molecule.

Question 2

Give one example each that illustrates the following characteristics of a chemical reaction:

1. evolution of a gas
2. change of colour
3. change in state

Answer

Evolution of gas: The reaction between hydrochloric acid (HCl) and magnesium (Mg) produces hydrogen gas (H₂). The balanced chemical equation for this reaction is:

2HCl + Mg → MgCl₂ + H₂

Change of colour: When sodium hydroxide (NaOH) is added to copper(II) sulfate (CuSO₄) solution, a precipitate of copper(II) hydroxide (Cu(OH)₂) forms, which has a characteristic blue colour. The balanced chemical equation for this reaction is:

NaOH + CuSO₄ → Cu(OH)₂ + Na₂SO₄

Change in the state: When calcium carbonate (CaCO₃) is heated, it undergoes thermal decomposition to form calcium oxide (CaO) and carbon dioxide gas (CO₂). The balanced chemical equation for this reaction is:

CaCO₃ (s) → CaO (s) + CO₂ (g)

Question 3

How do the following help in bringing about a chemical change? Explain each with a suitable example.

1. pressure
2. light
3. catalyst
4. heat.

Answer

(a) Pressure: Increasing the pressure on a reaction can cause the molecules to come closer together, which can increase the frequency of collisions between them. This can help to bring about a chemical change by increasing the rate of the reaction. For example, when carbon dioxide gas is dissolved in water under high pressure, it forms carbonic acid more readily:

CO₂ (g) + H₂O (l) ⇌ H₂CO₃ (aq)

(b) Light: Light can provide the energy needed to break the bonds between atoms or molecules, allowing a chemical reaction to occur.

2AgCl (s) + light → 2Ag (s) + Cl₂ (g)

(c) Catalyst: A catalyst is a substance that speeds up a chemical reaction without being consumed in the reaction itself. Catalysts work by lowering the activation energy of a reaction, which makes it easier for the reactants to come together and react.

2H₂O₂ (aq) + 2KI (aq) + MnO₂ (s) → 2H₂O (l) + 2KOH (aq) + O₂ (g) + MnO (s)

(d) Heat: Heat can provide the energy needed to break the bonds between atoms or molecules, allowing a chemical reaction to occur. This is because heat causes the molecules to move more quickly and collide with each other more frequently, which can increase the rate of the reaction. For example, when methane gas (CH₄) is burned in the presence of oxygen gas (O₂), heat is released and carbon dioxide gas (CO₂) and water vapour (H₂O) are produced:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g) + heat

Question 4

1. Define catalyst.
2. What are (i) positive catalysts and (ii) negative catalysts? Support your answer with one example for each of them.
3. Name three biochemical catalysts found in the human body.

Answer

1. A catalyst is a substance that speeds up a chemical reaction without being consumed in the reaction itself.
2. Positive catalysts increase the rate of a chemical reaction (e.g., iron in the Haber process).
   Negative catalysts decrease the rate of a chemical reaction (e.g., sodium cyanide in the reaction between hydrogen peroxide and potassium iodide).
3. Three biochemical catalysts found in the human body are enzymes, coenzymes, and ribozymes.

Question 5

What do you observe when

1. diluting sulphuric acid is added to granulated zinc?
2. a few pieces of iron are dropped in a blue solution of copper sulphate.
3. silver nitrate is added to a solution of sodium chloride?
4. ferrous sulphate solution is added to an aqueous solution of sodium hydroxide?
5. solid lead nitrate is heated?
6. when dilute sulphuric acid is added to barium chloride solution?

Answer

1. When diluting sulfuric acid is added to granulated zinc, hydrogen gas is produced and can be observed as bubbles.
2. When a few pieces of iron are dropped in a blue solution of copper sulfate, the iron displaces the copper from the solution, and a red-brown solid of copper is formed on the iron, and the blue color of the solution fades.
3. When silver nitrate is added to a solution of sodium chloride, a white precipitate of silver chloride is formed.
4. When ferrous sulfate solution is added to an aqueous solution of sodium hydroxide, a green precipitate of iron(II) hydroxide is formed.
5. When solid lead nitrate is heated, it decomposes to form a solid lead oxide, nitrogen dioxide gas, and oxygen gas.
6. When dilute sulfuric acid is added to the barium chloride solution, a white precipitate of barium sulfate is formed.

Question 6

Complete and balance the following chemical equations:

1. N₂ + O₂ →
2. H₂S + Cl₂ →
3. Na + H₂O →
4. NaCl + AgNO₃ →
5. Zn + H₂SO₄ →
6. FeSO₄ + NaOH →
7. Pb(NO₃)₂ →
8. BaCl₂ + H₂SO₄ →

Answer

1. N₂ + 2O₂ → 2NO
2. H₂S + Cl₂ → 2HCl + S
3. 2Na + 2H₂O → 2NaOH + H₂
4. NaCl + AgNO₃ → AgCl + NaNO₃
5. Zn + H2SO₄ → ZnSO₄ + H₂
6. FeSO₄ + 2NaOH → Fe(OH)₂ + Na₂SO₄
7. Pb(NO₃)₂ → PbO + 2NO₂ + O₂
8. BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

EXERCISE- II

Question 1

Fill in the blanks:

1. A reaction in which two or more substances combine to form a single substance is called a ___ reaction.
2. A ___ is a substance which changes the rate of a chemical reaction without undergoing a chemical change.
3. The formation of gas bubbles in a liquid during a reaction is called ___
4. The reaction between an acid and a base is called ___
5. Soluble bases are called ___
6. The chemical change involving iron and hydrochloric acid illustrates a ___ reaction.
7. In the type of reaction called ___ two compounds exchange their positive and negative radicals respectively.
8. A catalyst either ___ or ___ the rate of a chemical change but itself remains ___ at the end of the reaction.
9. The chemical reaction between hydrogen and chlorine is a ___ reaction.
10. When a piece of copper is added to silver nitrate solution, it turns ___ in colour.

Answer

1. synthesis
2. catalyst
3. effervescence
4. neutralization
5. alkalis
6. single displacement or substitution
7. increases, decreases, unchanged
8. combination or synthesis
9. exothermic
10. Silver

Question 2

Classify the following reactions as a combination, decomposition, displacement, precipitation and neutralization. Also, balance the equations.

1. CaCO₃ (s) → CaO (s) +CO₂ (g)
2. Zn (s) + H₂SO₄→ ZnSO₄ (s) + H₂ (g)
3. AgNO₃(aq) + NaCl (aq) → AgCl (s) + NaNO₃
4. NH₃ (g) + HCl (g) → NH₄Cl (s)
5. CuSO₄ (aq) + H₂S (g)→CuS (s) + H₂SO₄ (l)
6. Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
7. Ca (s) + O₂ (g) → CaO(s)
8. NaOH + HCl → NaCl + H₂O
9. KOH + H₂SO₄ → K₂SO₄ + H₂O

Answer

1. Decomposition: CaCO₃ (s) → CaO (s) + CO₂ (g)
2. Displacement: Zn (s) + H₂SO₄ → ZnSO₄ (s) + H₂ (g)
3. Precipitation: AgNO₃(aq) + NaCl (aq) → AgCl (s) + NaNO₃(aq)
4. Neutralization: NH₃ (g) + HCl (g) → NH₄Cl (s)
5. Precipitation: CuSO₄ (aq) + H₂S (g) → CuS (s) + H₂SO₄ (aq)
6. Displacement: Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
7. Combination: 2 Ca (s) + O₂ (g) → 2 CaO(s)
8. Neutralization: NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)
9. Neutralization: 2 KOH (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + 2 H₂O (l)

Question 3

Define:

1. precipitation
2. neutralization
3. catalyst

Answer

1. Precipitation is the process of the formation of an insoluble solid during a chemical reaction.
2. Neutralization is a chemical reaction between an acid and a base, which results in the formation of salt and water.
3. A catalyst is a substance that speeds up a chemical reaction without being consumed in the reaction itself.

Question 4

Explain the following types of chemical reactions giving two examples for each of them:

1. combination reaction
2. decomposition reaction
3. displacement reaction
4. double displacement reaction

Answer

(a) A reaction in which two or more reactants combine to form a single product is called a combination reaction.

Examples include:

• 2H₂ (g) + O₂ (g) → 2H₂O (l)
• Mg (s) + O₂ (g) → 2MgO (s)

(b) A reaction in which a single compound breaks down into two or more simpler substances is called a decomposition reaction.

Examples include:

• 2H₂O (l) → 2H₂ (g) + O₂ (g)
• 2KClO₃(s) → 2KCl(s) + 3O₂(g)

(c) A reaction in which an element or group in a compound is replaced by another element or group is called a displacement reaction.

Examples include:

• Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)
• Fe (s) + CuCl₂ (aq) → FeCl₂ (aq) + Cu (s)

(d) A reaction in which two compounds exchange ions to form two new compounds is called a double displacement reaction.

Examples include:

• Na₂SO₄ (aq) + BaCl₂ (aq) → 2NaCl (aq) + BaSO₄ (s)
• AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)

Question 5

Write the missing reactants and products and balance the equations :

1. NaOH + ___ → NaCl + ___
2. KClO₃ → ____ + ___
3. ____ + HCl → NaCl + H₂O + ___

Answer

1. NaOH + HCl → NaCl + H₂O
2. KClO₃ → KCl + O₂
3. Mg + HCl → MgCl₂ + H₂

Question 6

How will you obtain?

1. Magnesium oxide from magnesium.
2. Silver chloride from silver nitrate.
3. Nitrogen dioxide from lead nitrate.
4. Zinc chloride from zinc.
5. Ammonia from nitrogen.

Also, give balanced equations for the reactions.

Answer

(a) Magnesium oxide can be obtained by heating magnesium in the presence of air or oxygen.

2 Mg (s) + O₂ (g) → 2 MgO (s)

(b) Silver chloride can be obtained by mixing solutions of silver nitrate and sodium chloride.

AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq)

(c) Nitrogen dioxide can be obtained by heating lead nitrate.

2 Pb(NO₃)₂ (s) → 2 PbO (s) + 4 NO₂ (g) + O₂ (g)

(d) Zinc chloride can be obtained by reacting zinc with hydrochloric acid.

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

(e) Ammonia can be obtained by reacting nitrogen and hydrogen in the presence of a catalyst.

N₂(g) + 3 H₂(g) → 2 NH₃(g)

Question 7

What do you observe when

1. Iron nail is kept in copper sulphate solution for some time.
2. Phenolphthalein is added to the sodium hydroxide solution.
3. Blue litmus paper is dipped in dilute hydrochloric acid.
4. Lead nitrate is heated.
5. Magnesium ribbon is burnt in oxygen.
6. Ammonia is brought in contact with hydrogen chloride gas.

Answer

(a) When an iron nail is kept in copper sulfate solution, the iron nail will lose electrons, and copper ions will gain electrons to form a reddish-brown coating of copper on the iron nail.

(b) When phenolphthalein is added to sodium hydroxide solution, the solution turns pink, indicating the basic nature of sodium hydroxide.

(c) When blue litmus paper is dipped in dilute hydrochloric acid, the paper will turn red, indicating the acidic nature of hydrochloric acid.

(d) When lead nitrate is heated, it decomposes to form a lead oxide, nitrogen dioxide gas, and oxygen gas.

2Pb(NO₃)₂ (s) → 2PbO (s) + 4NO₂ (g) + O₂ (g)

(e) When magnesium ribbon is burnt in oxygen, it reacts with oxygen to form magnesium oxide, and bright light is observed.

2Mg (s) + O₂ (g) → 2MgO (s)

(f) When ammonia is brought in contact with hydrogen chloride gas, a dense white smoke of ammonium chloride is formed.

NH₃ (g) + HCl (g) → NH₄Cl (s)

Question 8

Give reason:

1. A person suffering from acidity is advised to take an antacid.
2. Acidic soil is treated with quick lime.
3. Wasp sting is treated with vinegar.

Answer

1. A person suffering from acidity is advised to take an antacid because antacids are basic in nature and neutralize the excess acid in the stomach, providing relief from acidity.
2. Acidic soil is treated with quicklime (CaO) because quicklime is alkaline in nature and can neutralize the excess acid present in the soil, making it more suitable for plant growth.
3. Wasp sting is treated with vinegar (acetic acid solution) because the venom of the wasp contains a basic solution. The acidic solution in vinegar can neutralize the basic solution in the venom, providing relief from pain and itching.

Question 9

What is meant by metal reactivity series? State its importance, (any two points).

Answer

The metal reactivity series is a list of metals arranged in order of their decreasing reactivity with other substances.

Importance of metal reactivity series

1. The reactivity series is important in predicting the products of reactions between metals and non-metals.
2. In determining the method of extraction for a particular metal.

Question 10

What are oxides? Give two examples of each of the following oxides.

1. Basic oxide
2. Acidic oxide
3. Amphoteric oxide
4. Neutral oxide

Answer

1. A basic oxide is an oxide that reacts with water to form a base. Two examples of basic oxides are sodium oxide (Na₂O) and calcium oxide (CaO).
2. An acidic oxide is an oxide that reacts with water to form an acid. Two examples of acidic oxides are sulfur dioxide (SO₂) and nitrogen dioxide (NO₂).
3. An amphoteric oxide is an oxide that can act as both an acid and a base. Two examples of amphoteric oxides are aluminium oxide (Al₂O₃) and zinc oxide (ZnO).
4. A neutral oxide is an oxide that does not react with water to form an acid or a base. Two examples of neutral oxides are carbon monoxide (CO) and nitrous oxide (N₂O).

Question 11

Define exothermic and endothermic reactions. Give two examples of each.

Answer

An exothermic reaction is a chemical reaction that releases energy in the form of heat, light, or sound. Two examples of exothermic reactions with equations are:

Combustion of methane:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (g) + heat

Neutralization reaction:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l) + heat

Endothermic reaction: An endothermic reaction is a chemical reaction that absorbs energy in the form of heat, light, or sound. Two examples of endothermic reactions with equations are:

Melting of ice:

H₂O(s) → H₂O(l) + heat

Photosynthesis:

6CO₂(g) + 6H₂O(l) + light energy → C₆H₁₂O₆ (aq) + 6O₂ (g)

Question 12

State the effect of:

1. an endothermic reaction
2. an exothermic reaction on the surroundings

Answer

1. Endothermic reaction: An endothermic reaction absorbs energy in the form of heat, light or sound from the surroundings. As a result, the surroundings feel cold or cooler after the reaction.
2. Exothermic reaction: An exothermic reaction releases energy in the form of heat, light or sounds into the surroundings. As a result, the surroundings feel hot or warmer after the reaction.

Question 13

What do you observe when

1. an acid is added to a basic solution?
2. ammonium chloride is dissolved in water.

Answer

When an acid is added to a basic solution, a neutralization reaction occurs, resulting in the formation of salt and water.

For example, when hydrochloric acid is added to sodium hydroxide solution, the following reaction occurs:

HCl + NaOH → NaCl + H₂O

When ammonium chloride (NH₄Cl) is dissolved in water, it undergoes dissociation to form ammonium ions (NH₄⁺) and chloride ions (Cl^–).

NH₄Cl (s) → NH₄⁺(aq) + Cl^– (aq)