Chemical Reactions and Equations

Intext Questions 1

Question 1

Why should a magnesium ribbon be cleaned before burning in air?

Answer

As magnesium is a very reactive metal hence it reacts with oxygen and a layer of magnesium oxide is formed on its surface, which prevents further reaction of magnesium with oxygen. Therefore, magnesium ribbon is cleaned before burning in air.

Question 2

Write the balanced equation for the following chemical reactions.

(i) Hydrogen + Chlorine ⟶ Hydrogen chloride

(ii) Barium chloride + Aluminium sulphate ⟶ Barium sulphate + Aluminium chloride

(iii) Sodium + Water ⟶ Sodium hydroxide + Hydrogen

Answer

(i) H₂ + Cl₂ ⟶ 2HCl

(ii) 3BaCl₂ + Al₂(SO₄)₃ ⟶ 3BaSO₄ + 2AlCl₃

(iii) 2Na + 2H₂O ⟶ 2NaOH + H₂ ↑

Question 3

Write a balanced chemical equation with state symbols for the following reactions.

(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer

(i) BaCl₂ (aq) + Na₂SO₄ (aq) ⟶ BaSO₄(s) + 2NaCl (aq)

(ii) NaOH (aq) + HCl (aq) ⟶ NaCl (aq) + H₂O (l)

Intext Questions 2

Question 1

A solution of a substance 'X' is used for whitewashing.

(i) Name the substance 'X' and write its formula.

(ii) Write the reaction of the substance 'X' named in (i) above with water.

Answer

(i) The substance 'X' is calcium oxide (Quick lime). Its chemical formula is CaO.

(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime).

CaO(s) + H₂O (l) ⟶ Ca(OH)₂ (aq)

Question 2

Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer

In Activity 1.7, electrolysis of water is carried out and the products obtained are hydrogen and oxygen. As water contains 2 parts of hydrogen element as compared to only 1 part of oxygen element, hence, the amount of hydrogen gas collected in one test tube is double of the amount of oxygen gas collected in the other test tube.

Intext Questions 3

Question 1

Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer

When an iron nail is dipped in a copper sulphate solution, iron being on top of copper in the metal reactivity series displaces copper from copper sulphate solution and forms iron sulphate, which is green in colour.

Fe (s) + CuSO₄ (aq) ⟶ FeSO₄ (aq) + Cu (s)

Hence, the colour of copper sulphate solution changes when an iron nail is dipped in it.

Question 2

Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer

Lead nitrate and sodium chloride react to form sodium nitrate and lead chloride.

Pb(NO₃)₂ + 2NaCl ⟶ 2NaNO₃ + PbCl₂ ↓

Question 3

Identify the substances that are oxidised and the substances that are reduced in the following reactions.

(i) 4Na (s) + O₂ (g) ⟶ 2Na₂O (s)

(ii) CuO (s) + H₂ (g) ⟶ Cu (s) + H₂O (l)

Answer

(i) 4Na (s) + O₂ (g) ⟶ 2Na₂O (s)

• Sodium (Na) is oxidised to Na₂O as it gains oxygen.
• Oxygen (O₂) is reduced to Na₂O as one atom of oxygen is removed from O₂.

(ii) CuO (s) + H₂ (g) ⟶ Cu (s) + H₂O (l)

• Copper oxide (CuO) is reduced to copper (Cu) as oxygen is lost.
• Hydrogen (H₂) is oxidised to water as it gains oxygen.

Exercises

Question 1

Which of the statements about the reaction below are incorrect?

2PbO (s) + C (s) ⟶ 2Pb (s) + CO₂ (g)

(a) Lead is getting reduced.

(b) Carbon dioxide is getting oxidised.

(c) Carbon is getting oxidised.

(d) Lead oxide is getting reduced

1. (a) and (b)
2. (a) and (c)
3. (a), (b) and (c)
4. all

Answer

(a) and (b)

Reason — Carbon is getting oxidised to carbon dioxide due to gain of oxygen and lead oxide is getting reduced to lead as oxygen is lost.

Question 2

Fe₂O₃ + 2Al ⟶ Al₂O₃ + 2Fe

The above reaction is an example of a

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

Answer

displacement reaction

Reason —

The above reaction is an example of a displacement reaction because Fe in Fe₂O₃ has been displaced by Al and the reaction is of type : YZ + X ⟶ XZ + Y

Question 3

What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.

1. Hydrogen gas and iron chloride are produced.
2. Chlorine gas and iron hydroxide are produced.
3. No reaction takes place.
4. Iron salt and water are produced.

Answer

Hydrogen gas and iron chloride are produced.

Reason — When dilute hydrochloric acid is added to iron fillings hydrogen gas and iron chloride are produced.

Fe(s) + 2HCl(aq) ⟶ FeCl₂(aq) + H₂ ↑

Question 4

What is a balanced chemical equation? Why should chemical equations be balanced?

Answer

A balanced equation is one in which the number of atoms of each element of the reactant is equal to the number of atoms of each element of the products.

An equation must be balanced in order to comply with the "Law of Conservation of Mass", which states that mass is neither created nor destroyed in the course of a chemical reaction. An unbalanced equation would imply that atoms have been created or destroyed.

Question 5

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

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

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

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

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

Answer

(i) 3H₂ (g) + N₂ (g) ⟶ 2NH₃ (g)

(ii) 2H₂S (g) + 3O₂ (g) ⟶ 2H₂O (l) + 2SO₂ (g)

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

(iv) 2K (s) + 2H₂O (l) ⟶ 2KOH (s) + H₂ (g)

Question 6

Balance the following chemical equations.

(a) HNO₃ + Ca(OH)₂ ⟶ Ca(NO₃)₂ + H₂O

(b) NaOH + H₂SO₄ ⟶ Na₂SO₄ + H₂O

(c) NaCl + AgNO₃ (aq) ⟶ AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ ⟶ BaSO₄ + HCl

Answer

(a) 2HNO₃ + Ca(OH)₂ ⟶ Ca(NO₃)₂ + 2H₂O

(b) 2NaOH + H₂SO₄ ⟶ Na₂SO₄ + 2H₂O

(c) NaCl + AgNO₃ ⟶ AgCl + NaNO₃

(d) BaCl₂ + H₂SO₄ ⟶ BaSO₄ + 2HCl

Question 7

Write the balanced chemical equations for the following reactions.

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

(b) Zinc + Silver nitrate ⟶ Zinc nitrate + Silver

(c) Aluminium + Copper chloride ⟶ Aluminium chloride + Copper

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

Answer

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

(b) Zn (s) + 2AgNO₃ (aq) ⟶ Zn(NO₃)₂ (aq) + 2Ag (s)

(c) 2Al (s) + 3CuCl₂ (aq) ⟶ 2AlCl₃ (aq) + 3Cu (s)

(d) BaCl₂ (aq) + K₂SO₄ (aq) ⟶ BaSO₄ (s) + 2KCl (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)

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

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

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

Answer

(a) 2KBr (aq) + BaI₂ (aq) ⟶ 2KI (aq) + BaBr₂ (s)
Type — Double displacement reaction

(b) ZnCO₃ (s) ⟶ ZnO (s) + CO₂ (g)
Type — Decomposition reaction

(c) H₂ (g) + Cl₂ (g) ⟶ 2HCl (g)
Type — Combination reaction

(d) Mg (s) + 2HCl (aq) ⟶ MgCl₂ (aq) + H₂ (g)
Type — Displacement reaction

Question 9

What does one mean by exothermic and endothermic reactions? Give examples?

Answer

Exothermic reactions — Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
For example, burning of natural gas gives out large amount of heat:
CH₄ (g) + 2O₂ (g) ⟶ CO₂ (g) + 2H₂O (g) + Heat

Endothermic reactions — Reactions in which energy is absorbed are known as endothermic reactions.
For example, when carbon is heated with sulphur at high temperature, liquid carbon disulphide is formed:
C + 2S $\xrightarrow{\text{Heat}}$ CS₂

Question 10

Why is respiration considered an exothermic reaction? Explain?

Answer

During digestion, food is broken down into simpler substances such as carbohydrates. These carbohydrates are broken down to form glucose. Glucose combines with oxygen in the cells and provides energy. The special name of this reaction is respiration.

As energy is released in this process, it is an exothermic process.

$\underset{\text{[Glucose]}}{\text{C}_6\text{H}_{12}\text{O}_6}\text{ (aq)} + 6\text{O}_2\text{ (aq)} \longrightarrow 6\text{CO}_2\text{ (aq)} + 6\text{H}_2\text{O}\text{ (l)} + \text{energy}$

Question 11

Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions?

Answer

In a decomposition reaction, a single reactant breaks down to give simpler products.

CaCO₃ $\xrightarrow{\Delta}$ CaO + CO₂

In a combination reaction, a single product is formed from two or more reactants.

CaO + H₂O ⟶ Ca(OH)₂

Hence, we can say that the decomposition reaction is opposite of combination reaction.

Question 12

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

Answer

1. CaCO₃ (s) $\xrightarrow{\text{Heat}}$ CaO (s) + CO₂ (g)

2. 2AgCl (s) $\xrightarrow{\text{sunlight}}$ 2Ag (s) + Cl₂ (g)

3. 2H₂O (l) $\xrightarrow{\text{electricity}}$ 2H₂ (g) + O₂ (g)

Question 13

What is the difference between displacement and double displacement reactions? Write equations for these reactions?

Answer

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

Cu (s) + 2AgNO₃ (aq) ⟶ Cu(NO₃)₂ (aq) + 2Ag (s)

Question 15

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

Answer

A chemical reaction in which two compounds in their aqueous state react to form an insoluble substance (precipitate) as one of the products is called a precipitation reaction.

Some examples of precipitation reactions are:

1. Take a solution of silver nitrate in a test tube and add dil. hydrochloric acid. A curdy white ppt. of AgCl is formed.
   AgNO₃ (aq) + HCl (l) ⟶ AgCl (s) ↓ + HNO₃ (g)

2. When sodium sulphate reacts with barium chloride, a white ppt. of barium sulphate appears.

Na₂SO₄ (aq) + BaCl₂ (aq) ⟶ BaSO₄ (s) ↓ + 2NaCl (aq)

Question 16

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

(a) Oxidation

(b) Reduction

Answer

(a) Oxidation is gain of oxygen by a substance.

For example,

1. Copper is oxidised to Copper oxide:
   2Cu + O₂ $\xrightarrow{\text{Heat}}$ 2CuO

2. Carbon is oxidised to Carbon dioxide
   C + O₂ ⟶ CO₂

(b) Reduction is loss of oxygen from a substance.

For example,

1. Copper (II) oxide is reduced to Copper
   CuO + H₂ $\xrightarrow{\text{Heat}}$ Cu + H₂O

2. Zinc (II) oxide is reduced to Zinc.
   ZnO + C ⟶ Zn + CO

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

Element 'X ' is copper. On heating in air it forms Copper (II) oxide, which is black in colour.

2Cu (s) + O₂ (g) $\xrightarrow{\text{Heat}}$ 2CuO (s)

Question 18

Why do we apply paint on iron articles?

Answer

Paint is applied on iron articles primarily to protect them from corrosion and rust formation by creating a barrier between the iron surface and moisture and oxygen in the air.

Question 19

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

Answer

Oil and fat containing food items are flushed with nitrogen to extend their shelf life by displacing oxygen, which can lead to oxidative rancidity and spoilage. Nitrogen helps create an oxygen-free environment that maintains the quality and freshness of the food items.

Question 20

Explain the following terms with one example each.

(a) Corrosion

(b) Rancidity

Answer

(a) Corrosion — When a metal is attacked by substances around it such as moisture, acids, etc, it is said to corrode and this process is called corrosion.
For example, iron articles are shiny when new, but get coated with a reddish brown powder when left for some time due to rusting.

(b) Rancidity — When taste and smell of food items containing fat and oil change due to oxidation, then this condition is called rancidity.
For example, when butter is left exposed to air for a long time, it becomes rancid, making it unsuitable for consumption and cooking.