Class 8 SELINA Solutions Chemistry Chapter 7: Hydrogen
Hydrogen Exercise Ex.
Solution A 1(i)
Correct option: (c) Copper
Solution:
Copper does not react with acid or water to produce hydrogen gas.
Copper is less reactive than hydrogen, so it cannot displace hydrogen from acid. Copper is below hydrogen in the electrochemical series, which means it cannot liberate hydrogen from dilute hydrochloric acid.
Solution A 1(ii)
Correct option: (b) MgO
Solution:
Magnesium oxide (MgO) is the compound formed with hydrogen gas when magnesium reacts with steam. The balanced equation for this reaction is Mg(s) + H2O (steam) → MgO(s) + H2(g).
Solution A 1(iii)
Correct option: (b) O2
Solution:
At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode in order to complete the circuit.
Solution A 1(iv)
Correct option: (a) hydrogen chloride
Solution:
Equal volumes of hydrogen and chlorine are exposed to diffused sunlight to prepare colourless hydrogen chloride gas.
Solution A 1(v)
Correct option: (c) calcium
Solution:
The very active metals like sodium, potassium, and calcium liberate hydrogen gas from cold water.
Solution A 1(vi)
Correct option: (a) top
Solution:
In the metal reactivity series, the more reactive metals are at the top.
Solution A 1(vii)
Correct option: (d) all of the above
Solution:
Hydrogen is responsible for producing heat and light, hydrogenated oil and fertilisers.
Solution A 1(viii)
Correct option: (a) combustible.
Solution:
Hydrogen is combustible but not a supporter of combustion.
Solution A 1(ix)
Correct option: (b) carbon monoxide and hydrogen
Solution:
Water gas is a mixture of carbon monoxide and hydrogen.
Solution A 2
(a) Hydrogen is lighter than air.
(b) Hydrogen is sparingly soluble in water.
(c) Hydrogen burns with an oxyhydrogen pale blue flame, and a pop sound is heard.
(d) A metal sodium hydrogen in the reactivity series gives hydrogen with water.
(e) Hydrogen reacts with metal oxides to form metal and water.
(f) Oxidation is the removal of or addition of oxygen.
(g) In redox reactions, oxidation and reduction occur simultaneously.
Solution A 3
(a) True.
(b) False: Adding hydrogen to a substance is called reduction.
(c) False: Nitric acid is a strong oxidising agent (even when dilute), and it oxidises hydrogen gas into water, defeating the purpose. Dilute hydrochloric acid and dilute sulphuric acid do not oxidise hydrogen into water; hence, they are preferred for the preparation of hydrogen gas.
(d) True.
(e) True.
(f) False: Hydrogen is a very combustible element in the presence of oxygen, but it does not support combustion.
(g) False: Hydrogen gas is not easily liquefiable.
Solution A 4
|
| Column A |
| Answers |
| (a) | A metal which reacts with cold water to form hydrogen. | (iv) | Sodium |
| (b) | A gas which is inflammable and a non-supporter of combustion. | (v) | Hydrogen |
| (c) | A process in which Vanaspati ghee is prepared from vegetable oils. | (ii) | Hydrogenation |
| (d) | The removal of hydrogen or addition of oxygen. | (iii) | Oxidation |
| (e) | The addition of hydrogen or removal of oxygen. | (i) | Reduction |
Solution A 5
(a) Oxidation
(b) Bosch process
(c) Electrodes
(d) Ions
(e) Reducing agent
Solution A 6
(a) Sodium, Potassium
(b) Iron
(c) Copper oxide
(d) Saltwater, such as sodium chloride solution
(e) Platinum
Solution A 7
(a) H2 + Cl2 ⟶ 2HCl
(b) H2 + S ⟶ H2S
(c) Zn + 2HCl ⟶ ZnCl2 + H2
(d) CuO + H2 ⟶ Cu + H2O
(e) Fe + H2O ⟶Fe3O4 + H2
(f) K + H2O ⟶ KOH + H2
Solution B 1
(a) Catalytic hydrogenation: Catalytic hydrogenation is a process by which hydrogen gas is passed through vegetable oils in the presence of a catalyst like Ni, Pt or Pd to convert the oils into Vanaspati ghee.
(b) Oxidation: Oxidation is defined as a chemical process that involves addition of oxygen to a substance or removal of hydrogen from a substance.
(c) Reduction: Reduction is a chemical process that involves addition of hydrogen to a substance or removal of oxygen from a substance.
(d) Redox reaction: Redox reactions are those in which reduction and oxidation both take place simultaneously, i.e. one substance is reduced while the other gets oxidised.
(e) Electrolysis: Electrolysis is a process in which an electric current is passed through an aqueous solution or a molten state of a compound (also called an electrolyte) to bring about a chemical change.
(f) Electrolyte: Electrolytes are compounds in aqueous or molten states that conduct electric current through them because they contain free mobile ions.
Solution B 2
Hydrogen has one electron in its first (and only) electron shell.
Hydrogen atoms are combined to make a diatomic molecule that completes its shell.
It exists in the diatomic form in nature, so two hydrogen atoms make one molecule, H2.
Diatomic hydrogen has a total of 2 electrons and 2 protons, which cancel out in charge, hence leaving the diatomic form of hydrogen neutral.
Therefore, hydrogen gas is neutral.
Solution B 3
(a) Because of its high heat of combustion, hydrogen is used as a fuel; Coal gas, water gas (CO + H₂), and liquid hydrogen are some significant hydrogen-based fuels. Liquid hydrogen is used as a rocket fuel. Hydrogen can become the mass fuel of the future, replacing the hydrocarbons. In addition, hydrogen is a pollution-free fuel.
(b) Hydrogen is lighter than air but is not collected by the downward displacement of air since a mixture of hydrogen and air can lead to an explosion.
(c) Hydrogen burns with a "pop" sound because it is highly reactive in nature or highly flammable, whereas Carbon dioxide, Nitrogen, and Oxygen are not flammable. However, we need oxygen for the combustion process. The reaction of hydrogen with oxygen is very fast.
(d) Hydrogen is not used in the meteorological balloon as it is highly inflammable in nature. It is lighter than air, but hydrogen is avoided as it is highly combustible. Hence, hydrogen is replaced by helium in the meteorological balloon.
(e) Nitric acid, even in its diluted form, is not used in the preparation of hydrogen from metals because metals, in reaction with nitric acid, cannot replace the hydrogen atom.
Solution B 4
(a)
(i) Reduced: PbO
(ii) Oxidised: H2
(b) The reactant ‘PbO’ acts as an oxidising agent by donating oxygen.
Solution C 1
(a) Bosch process: Hydrogen is prepared on a large scale by this process.
(b) Equations for Bosch process:
(i) Formation of water gas
(c)
Removal of carbon dioxide from gaseous mixture: The products are hydrogen, carbon dioxide and some unreacted carbon monoxide. Hydrogen is separated from carbon dioxide by passing the mixture through water under pressure, in which carbon dioxide gets dissolved, leaving behind hydrogen. Carbon dioxide can also be separated by passing through a caustic potash (KOH) solution.
Removal of carbon monoxide: To separate carbon monoxide, the gaseous mixture is passed through ammoniacal cuprous chloride, in which carbon monoxide dissolves, leaving behind hydrogen. Thus, hydrogen gas is obtained.
Solution C 2
The substance that supplies hydrogen or removes oxygen is called a reducing agent.
Examples: Hydrogen, hydrogen sulphide, carbon, carbon monoxide, etc. The reducing agent itself gets oxidised during the reaction.
Redox reactions: Redox reactions are those in which reduction and oxidation take place simultaneously, i.e., one substance is reduced while the other gets oxidised. Most reactions are redox reactions. However, double displacement reactions are not redox reactions.
In the above reaction, copper oxide gets reduced to copper while hydrogen is oxidised into water. Thus, reduction and oxidation take place simultaneously. (Here, copper oxide acts as an oxidising agent).
Solution C 3
Uses of Hydrogen:
1. For cutting and welding metals: Hydrogen and oxygen, when burnt together, give a flame known as oxy. hydrogen flame. It has a temperature of about 2800°C-3000°C, which is sufficient to melt the metals. The flame is therefore used for cutting and welding of metals.
2. As a fuel: Because of its high heat of combustion, hydrogen is used. Coal gas, water gas (CO + H₂) and liquid hydrogen are significant hydrogen-based fuels. Liquid hydrogen is used as a rocket fuel. Hydrogen can become the mass fuel of the future, replacing the hydrocarbons. In addition, hydrogen is a pollution-free fuel.
3. For hydrogenation of vegetable oil:
Hydrogen is used in the preparation of vanaspati ghee from liquid vegetable fats like groundnut oil, coconut oil, etc. This process is called catalytic hydrogenation of oils.
4. For the manufacture of chemicals:
Hydrogen gas is used extensively in the manufacture of ammonia gas, which, in turn, is used to produce fertilisers and nitric acid. The gas is also used in the manufacture of hydrochloric acid, methanol, urea, etc.
Solution C 4
(a) Cathode and anode:
| Cathode | Anode |
| 1. The electrode connected to the negative terminal of the battery is called the cathode. | 1. The electrode connected to the positive terminal of the battery is called the anode. |
| 2. Current leaves the electrolyte through the cathode. | 2. Current enters the electrolyte through the anode. |
| 3. Cations migrate and discharge at the cathode. | 3. Anions migrate and discharge at the anode. |
(b) Cation and anion:
| Cation | Anion |
| 1. Cations are positively charged atoms or group of atoms. | 1. Anions are negatively charged atoms or group of atoms. |
| 2. During electrolysis, cations migrate towards the cathode. | 2. During electrolysis, anions migrate towards the anode. |
| 3. Examples: Na+, Ba2+, Ca2+, Al3+, Pb2+, NH4+, etc. | 3. Examples: Cl⁻, OH⁻, Br⁻, SO42-, HCO3⁻, CO32-, etc. |
Solution C 5
(a) The chemicals required to prepare hydrogen gas in the laboratory are granulated zinc and dilute hydrochloric acid.
(b) Balanced chemical equation for the reaction:
Zn + 2HCl ⟶ ZnCl2 + H2 (g)
Zinc chloride
(c) A neat and well-labelled diagram for the laboratory preparation of hydrogen is as follows:
(d) Hydrogen gas is collected by the downward displacement of water.
(e) One chemical test for hydrogen:
1. Ordinary hydrogen burns in the air with a characteristic pop sound.
2. Pure hydrogen burns in air or oxygen with a pale blue flame, and water is formed upon condensation, which can be tested upon condensation, which can be tested by sprinkling a few drops of it on white anhydrous copper sulphate, which turns into blue hydrated copper sulphate.
Solution C 6
(a) A = Hydrogen, B = Oxygen, D = Acidulated water, E = cathode made up of platinum, F = anode made up of platinum.
(b) The addition of acid to water makes it a good conductor of electricity.
(c) The ratio of hydrogen and oxygen thus collected is 2:1 by volume.
Disclaimer: In question 6(a), the ‘C’ label is not mentioned in the image.