Chapter 7 – Hydrogen

Selina Concise Chemistry 2023 solutions for class 8 Chemistry. Chapter 7 – Hydrogen is provided here with simple step-by-step explanations. These solutions for Hydrogen are extremely popular among class 10 students. The Hydrogen solution is handy for quickly completing your homework and preparing for exams.

Question 1

Fill in the blanks :

1. Hydrogen is ____ than air.
2. Hydrogen is ____ soluble in water.
3. Hydrogen burns with a ____ flame and ____ sound is heard.
4. A metal ____ hydrogen in the reactivity series gives hydrogen with ____
5. Hydrogen reacts with metal oxides to form ____ and ____
6. Oxidation is the removal of ____ or the addition of ____
7. In redox reactions oxidation and reduction occur ____

Answer

1. lighter
2. poorly
3. pale, popping
4. above, acid
5. metal, water
6. hydrogen, oxygen
7. simultaneously

Question 2

Indicate which of the following statements are true and which are false:

1. The hydrogen molecule is monovalent.
2. The removal of hydrogen from a substance is called reduction.
3. Nitric acid can not be used to prepare hydrogen by its action on active metals.
4. The reaction between hydrogen and nitrogen to form ammonia is reversible.
5. Zinc can liberate hydrogen from water, acid and alkali solutions.
6. Hydrogen is combustible as well as a supporter of combustion.
7. Hydrogen gas is easily liquefiable.

Answer

1. True
2. False
3. True
4. True
5. True
6. True
7. False

Question 3

Complete and balance the following equations:

1. H₂ + ____ →  2HCl
2. H₂ + S → ____
3. Zn + ____ → ZnCl₂ + H₂
4. CuO + ____ → Cu + ____
5. Fe + H₂O → ____ + ____
6. K+ H₂O → ____ + ____

Answer

1. H₂ + Cl₂ →  2HCl
2. H₂ + S → H2S
3. Zn + 2HCl → ZnCl₂ + H₂
4. CuO + H₂ → Cu + H₂O
5. Fe + H₂O → Fe₃O₄ + H₂
6. K + H₂O → KOH + H₂

Question 4

Give reasons for the following:

1. Hydrogen can be used as fuel.
2. Though hydrogen is lighter than air it cannot be collected by the downward displacement of air.
3. A pop sound is produced when hydrogen gas is burnt.
4. Helium has replaced hydrogen in weather observation balloons.
5. Nitric acid is not used for the preparation of hydrogen gas.

Answer

1. Hydrogen is a clean fuel that produces only water vapour as a byproduct when burned.
2. Hydrogen is highly diffusive and rapidly mixes with air, making it difficult to collect.
3. The rapid combustion of hydrogen in the presence of oxygen produces a shock wave that creates a popping sound.
4. Helium is non-flammable and safer to use than hydrogen in weather balloons.
5. Nitric acid is a strong oxidizing agent that reacts with and oxidizes hydrogen gas.

Question 5

Name the following:

1. Two metals give hydrogen in cold water.
2. A metal which liberates hydrogen only when steam is passed over red hot metal.
3. The process in which oxygen is added or hydrogen is removed.
4. A metallic oxide can be reduced into metal by hydrogen.

Answer

1. Sodium and Lithium give hydrogen in cold water.
2. Iron liberates hydrogen only when steam is passed over red hot metal.
3. Oxidation
4. Iron oxide (Fe₂O₃) can be reduced into metal by hydrogen.

Question 6

1. Name the chemicals required to prepare hydrogen gas in the laboratory.
2. Give a balanced chemical equation for the reaction.
3. Draw a neat and well-labelled diagram for the laboratory preparation of hydrogen.
4. How is hydrogen gas collected? Why?

Answer

(a) Zinc and HCl

(b) Zn + 2HCl → ZnCl₂ + H₂

(c) Laboratory preparation of hydrogen

(d) Hydrogen gas is collected over water using the method of upward displacement of water to prevent contamination of the gas by air and to dissolve any impurities present in the gas.

Question 7

How would you show that hydrogen :

1. is a non-supporter of combustion?
2. is lighter than air ?

Answer

1. Hold a hydrogen gas-filled jar with its mouth downwards. Place a lighted candle and cover it with the jar. The candle gas gets extinguished but the gas burns with the pop sound. This shows that hydrogen is combustible but is a non-supporter of combustion.
2. To show that hydrogen is lighter than air, we can fill a balloon with hydrogen gas and release it. The balloon will rise rapidly and float upwards, demonstrating that hydrogen is lighter than air and tends to rise and disperse quickly.

Question 8

Hydrogen is a good reducing agent. What do you understand by the above statement? Explain with the help of copper oxide as an example.

Answer

Hydrogen is a good reducing agent because it readily gives up electrons to other substances, causing them to be reduced.

For example, when hydrogen gas is passed over copper oxide at high temperatures, a reduction reaction occurs, and the copper oxide is converted to copper metal and water.

CuO + H₂ → Cu + H₂O

This reaction demonstrates the reducing power of hydrogen gas.

Question 9

1. Name a process by which hydrogen gas is manufactured.
2. Give equations for the reactions.
3. How is hydrogen separated from carbon dioxide and carbon monoxide?

Answer

(a) Bosch Process

(b) Equations for the reactions

C + H₂O {\overset{1000^oC}{\longrightarrow}} (CO + H₂) – Heat

(CO + H₂) + H₂O {\overset{Fe_2O_3, 450^oC}{\longrightarrow}} CO₂ + 2H₂ + Heat

(c) Carbon dioxide can be separated by passing it through caustic potash (KOH) Solution.

CO₂ + 2KOH → K₂CO₃ + H₂O

To separate carbon monoxide, the gaseous mixture is passed through ammoniacal cuprous chloride.

CuCl + CO + 2H₂O → CuCl.CO.2H₂O

Question 11

State four uses of hydrogen.

Answer

The four uses of hydrogen are

1. Hydrogen is used for cutting and welding metals.
2. Hydrogen is used as a fuel.
3. Hydrogen is used for the hydrogenation of vegetable oil.
4. Hydrogen is used for the manufacture of chemicals like ammonia.

Question 12

Define :

1. catalytic hydrogenation
2. oxidation
3. reduction
4. redox reaction

Answer

1. Hydrogen is used in the preparation of solid vanaspati ghee from liquid vegetable fats like groundnut oil, coconut oil, etc. This process is called the catalytic hydrogenation of oils.
2. Oxidation is defined as a chemical process that involves the addition of oxygen or the removal of hydrogen.
3. Reduction is defined as a chemical process that involves the addition of hydrogen or the removal of oxygen.
4. A redox reaction, also known as an oxidation-reduction reaction, is a chemical reaction that involves both oxidation and reduction processes.

Question 13

Multiple Choice Questions

(a) Equal volumes of hydrogen and chlorine are exposed to diffused sunlight to prepare

1. hydrogen chloride
2. water
3. sodium hydroxide
4. hydrochloric acid

(b) The metal which reacts with cold water to produce hydrogen is

1. magnesium
2. aluminium
3. calcium
4. iron

(c) In the metal reactivity series the more reactive metals are at

1. top
2. bottom
3. middle
4. none

(d) Hydrogen is responsible for producing

1. heat and light
2. hydrogenated oil
3. fertilizers
4. all of the above

(e) Hydrogen is

1. combustible
2. non-combustible
3. supporter of combustion
4. neither supporter nor combustible

(f) Water gas is a mixture of of combustion.

1. carbon monoxide and oxygen
2. carbon monoxide and hydrogen
3. hydrogen and oxygen
4. hydrogen and nitrogen.

Answer

(a) Hydrogen chloride

(b) Calcium

(c) Top

(d) All of the above

(e) Combustible

(f) Carbon monoxide and hydrogen