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NCERT Class 11 Chemistry Chapter 9 Hydrogen
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Hydrogen
Chapter: 9
| Part – II |
1. Justify the position of hydrogen in the periodic table on the basis of its electronic configuration.
Ans: Hydrogen is the first element of the periodic table. Its electronic configuration is [1s1]. Due to the presence of only one electron in its 1s shell, hydrogen exhibits a dual behaviour, i.e., it resembles both alkali metals and halogens.
Hydrogen, with the electronic configuration 1s1, shares similarities with alkali metals and halogens. Like alkali metals, it has one valence electron and can lose it to form a unipositive ion. It also resembles halogens by requiring one electron to complete its octet and forming covalent compounds, including diatomic molecules. However, hydrogen differs as it lacks metallic characteristics, possesses a high ionisation enthalpy, and is less reactive than halogens. Due to these distinctions, hydrogen cannot be grouped with alkali metals or halogens and is best placed separately in the periodic table.
2. Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?
Ans: Three naturally existing isotopes of hydrogen are protium, deuterium, and tritium.
3. Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?
Ans: The ionisation energy of hydrogen is quite high, amounting to 1312 kJ/mol. Hence, it is very hard to remove its only electron. As a result, its tendency to exist in the monatomic form is rather low. Instead, hydrogen forms a covalent bond with another hydrogen atom and exists as a diatomic (H2) molecule.
4. How can the production of dihydrogen obtained from ‘Coal gasification’ be increased?
Ans: Dihydrogen production in coal gasification can be increased by reacting CO in syngas with steam using iron chromate catalysts.
With the removal of CO2 the reaction shifts in the forward direction and thus, the production of dihydrogen will be increased.
5. Describe the bulk preparation of dihydrogen by electrolytic method. What is the role of an electrolyte in this process?
Ans: Dihydrogen is produced through electrolysis of acidified or alkaline water using platinum electrodes, typically employing 15-20% sulfuric acid (H₂SO₄) or sodium hydroxide (NaOH) as the electrolyte for efficient conduction.
Reduction of water occurs at the cathode as: 2H2O + 2 e– →2H2 + 2OH–
At the anode, oxidation of OH– ions takes place as: 2OH- →H2O + 1/2O2 + 2e–
∴ Net reaction can be represented as: H2O(l) ⟶ H2(g) + 1/2O2(g)
The electrical conductivity of pure water is very low owing to the absence of ions in it. Therefore, electrolysis of pure water also takes place at a low rate. If an electrolyte such as an acid or a base is added to the process, the rate of electrolysis increases. Adding an electrolyte provides ions necessary for conducting electricity, enabling the electrolysis process to occur efficiently.
6. Complete the following reaction:
(i)
Ans:
(ii)
Ans:
(iii)
Ans:
(iv)
Ans:
7. Discuss the consequences of high enthalpy of H-H bond, in terms of chemical reactivity of dihydrogen.
Ans: The ionisation enthalpy of H–H bond is very high (1312 kJ mol–1). This indicates that hydrogen has a low tendency to form H+ ions. Its ionisation enthalpy value is comparable to that of halogens. Hence, it forms diatomic molecules (H2), hydrides with elements, and a large number of covalent bonds. Due to its high ionisation enthalpy, hydrogen lacks metallic properties such as lustre and ductility, unlike typical metals.
8. What do you understand by.
(i) Electron-deficient.
Ans: A molecule in which the valence subshell of p electrons of one of the atoms appears to have more than eight electrons. Examples of electron-rich compounds include phosphorus pentachloride (PCl₅).
(ii) Electron-precise.
Ans: Those compounds in which exact number of electrons are present in the central atom or the central atom contains a complete octet are called precise hydrides e.g., CH4, SiH4, GeH4 etc. are precise hydrides.
(iii) Electron-rich compounds of hydrogen? Provide justification with suitable examples.
Ans: Compounds where the central atom has one or more lone pairs of electrons are known as electron-rich hydrides, such as ammonia (NH3) and water (H2).
9. What characteristics do you expect from an electron-deficient hydride with respect to its structure and chemical reaction?
Ans: Electron-deficient hydrides typically have a trigonal planar or linear structure, as they lack sufficient electrons for full octets. Chemically, they tend to act as Lewis acids, readily accepting electron pairs during reactions. Their reactivity is characterised by the formation of bonds with electron-rich species, often leading to complex formation.
10. Do you expect the carbon hydride of type Cn H2n + 2 to act as ‘Lewis’ acid or base? Justify your answer.
Ans: Carbon hydrides of the type CnH2n + 2 are classified as electron-precise hydrides. Because they have atoms with an exact number of electrons to form covalent bonds. Thus, they do not behave as Lewis acid or base. Since they have no tendency to gain or lose electrons.
11. What do you understand by the term ‘non-stoichiometric hydrides’? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer.
Ans: Non-stoichiometric hydrides are compounds formed by hydrogen and metals, where the ratio of metal to hydrogen is not fixed. This means that the composition of non-stoichiometric hydrides can vary, and they do not follow the law of constant composition.
Alkali metals do not form non-stoichiometric hydrides as these metals readily lose their valence electron and behave as strong reducing agents. Thus, they form ionic or stoichiometric hydrides.
12. How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.
Ans: Metallic hydrides are useful for hydrogen storage because they can absorb hydrogen and store it without the need for compression. In metallic hydrides, hydrogen is adsorbed as H-atoms. Due to the adsorption of H atoms the metal Lattice expands and become unstable. Thus, when metallic hydride is heated, it decomposes to form hydrogen and finely divided metal. The hydrogen evolved can be used as fuel.
13. How does the atomic hydrogen or oxy-hydrogen torch function for cutting and welding purposes ? Explain.
Ans: When hydrogen is burnt in oxygen the reaction is highly exothermic, it produces a very high temperature nearly 4000°C which is used for cutting and welding purposes.
14. Among NH3 H2O and HE, which would you expect to have the highest magnitude of hydrogen bonding and why?
Ans: Atomic hydrogen is produced by the dissociation of dihydrogen using an electric arc. This releases a huge amount of energy (435.88 kJ mol–1). This energy can be used to generate a temperature of 4000 K, which is ideal for welding and cutting metals. Hence, atomic hydrogen or oxy-hydrogen torches are used for these purposes. For this reason, atomic hydrogen is permitted to recombine on the surface during welding to achieve the desired temperature.
15. Saline hydrides are known to react with water violently producing fire. Can C02, a well known fire extinguisher, be used in this case? Explain.
Ans: No, CO2 cannot be used to extinguish fires caused by saline hydrides reacting with water because CO2 is reduced in the reaction, producing more hydrogen gas.
Saline hydrides like NaH and CaH₂ react vigorously with water, producing metal hydroxides and hydrogen gas. This reaction releases significant energy, which can ignite the hydrogen.
16. Arrange the following:
(i) CaH2, BeH2 and TiH2 in order of increasing electrical conductance.
Ans: BeH2 < TiH2 < CaH2
(ii) LiH, NaH and CsH in order of increasing ionic character.
Ans: LiH < NaH < CsH
(iii) H-H, D—D and F—F in order of increasing bond dissociation enthalpy.
Ans: F—F < H—H < D—D
(iv) NaH, MgH2 and H2O in order of increasing reducing property.
Ans: H2O < MgH2 < NaH
17. Compare the structures of H2O and H2O2
Ans: In gaseous phase, the water molecule has a bent form with a bond angle of 104.5°. The O–H bond length is 95.7 pm. The structure can be shown as:
Hydrogen peroxide has a non-planar structure both in gas and solid phase. The dihedral angle in gas and solid phase is 111.5° and 90.2° respectively.
18. What do you understand about the term ‘auto-protolysis’ of water? What is its significance?
Ans: Auto-protolysis is a chemical reaction that occurs when a proton is transferred between two identical molecules. This reaction is also known as self-ionisation of water, and is a type of molecular autoionization.
This reaction shows that water can act as both an acid and a base. Water can react with both acids and bases.
19. Consider the reaction of water with F2 and suggest, in terms of oxidation and reduction, which species are oxidised/reduced ?
Ans: The reaction between fluorine and water can be represented as:
2F2(ag) + 2H20(l) → O2(g) + 4H + (aq) + 4F–(aq)
Fluorine is reduced from zero to (- 1) oxidation state. A decrease in oxidation state indicates the reduction of fluorine.
Water is oxidised when its oxidation state increases from -2 to 0. This change signifies that water undergoes oxidation.
20. Complete the following chemical reactions.
(i) PbS(s) + H2O2 (aq) ————->
(ii) MnO4– (aq) + H2O2 (aq) ————->
(iii) CaO(s) + H2O(g) ————->
(iv) AlCl3(g) + H2O(l)————->
(v) Ca3N2(S) + H2O(l) ————->
Classify the above into (a) hydrolysis, (b) redox and (c) hydration reactions.
Ans: (i) PbS(s) + 4H2O2(aq) ————-> PbSO4(s) + 4H2O(l)
(ii) 2MnO4 – (aq) +H2O2(aq) + 6H + (aq) ————-> 2Mn (aq) + 8H2O(l) + 5O2(g)
(iii) CaO(s) + H2O(g) ————-> Ca(OH)2(aq)
(iv) AlCl3(aq) + 3H2O(l) ————-> Al(OH)3(S) + 3HCl (aq)
(v) Ca3N2(s) + H2O(l) ————-> 3Ca(OH)2(aq) + 2NH3(aq)
(a) Hydrolysis reactions, (iii) (iv) and (v)
(b) Redox reactions (i) and (ii)
(c) Hydration reaction: (iii)
21. Describe the structure of the common form of ice.
Ans: Ice has crystalline structure which is highly ordered due to hydrogen bonding. It has hexagonal form at atmospheric pressure and cubic form at low temperature. Each O atom has tetrahedral geometry and is surrounded by 4 oxygen atoms each at a distance of 276 pm.
22. What causes the temporary and permanent hardness of water?
Ans: Temporary hardness of water is due to the presence of bicarbonates of calcium and magnesium in water i.e., Ca(HCO3)2 and Mg(HCO3) in water. The presence of soluble salts of calcium and magnesium, i.e., sulphates and chlorides of calcium and magnesium cause permanent hardness in water.
23. Discuss the principle and method of softening of hard water by synthetic ion-exchange resins.
Ans: Cation exchange resins are large organic molecules with insoluble -SO₃H groups. Upon treatment with NaCl, they convert to RNa by exchanging H⁺ ions for Na⁺ ions. The resin exchanges Na+ ions with Ca2+ and Mg2+ ions present in hard water and makes it soft.
2RNa(s) + M2 + (aq) ——> R2M(s) + 2Na + (aq)
Where, M = Mg, Ca.
During the complete process, water first passes through the cation exchange process. The water obtained after this process is free from mineral cations and is acidic in nature. This acidic water is then passed through the anion exchange process where OH> ions neutralise the H+ ions and de-ionize the water obtained.
24. Write chemical reactions to show the amphoteric nature of water.
Ans: (i) Reaction with H2S-
The reaction takes place as:
Water is amphoteric in nature because it acts as an acid
In the forward reaction, H2O(l) accepts a proton from H2S(aq). Hence, it acts as a Lewis base.
2. Reaction with NH3–
The reaction takes place as:
In the forward reaction, H2O(l) denotes its proton to NH3(aq). Hence, it acts as a Brønsted-Lowry acid.
3. Self-ionisation of water-
In the reaction, two water molecules react as:
In the forward reaction, H2O(l) denotes its proton to H2O(l). Hence, it acts as an amphoteric solution.
25. Write chemical reactions to justify that hydrogen peroxide can function as an oxidising as well as reducing agent.
Ans: As an oxidising agent
2Fe2+ (aq) + 2H+(aq) + H2O2(aq) → 2Fe3+ (aq) + 2 H2O(l)
As a reducing agent
I2(s) + H2O2 (aq) + 2OH– (aq) → 2I– (aq) + 2 H2O(l) + O2(g)
26. What is meant by ‘demineralised water’ and how can it be obtained?
Ans: Demineralized water is completely free of soluble mineral salts, containing no anions or cations. Demineralised water is obtained by passing water successively through a cation exchange (in the H+ form) and an anion exchange (in the OH− form) resin.
H+ exchanges for Na+, Ca2+, Mg2+ and other cations present in water. This process results in release of protons which makes the water acidic. OH⁻ ions in anion exchange resins replace anions like Cl⁻, HCO₃⁻, and SO₄²⁻. The released OH⁻ ions neutralise H⁺ ions from cation exchange, forming water:
H⁺(aq) + OH⁻(aq) → H₂O(l).
27. Is demineralised or distilled water useful for drinking purposes? If not, how can it be made useful?
Ans: No, demineralized or distilled water is not useful for drinking purposes because it lacks essential minerals. However, you can make it useful for drinking by adding the necessary minerals.
28. Describe the usefulness of water in biosphere and biological systems.
Ans: Water is vital for all forms of life, making up about 65% of the human body and 95% of plant composition. Water plays an important role in the biosphere owing to its high specific heat, thermal conductivity, surface tension, dipole moment, and dielectric constant.The high heat of vaporisation and heat of capacity of water helps in moderating the climate and body temperature of all living beings.
It serves as a carrier for various nutrients essential for plants and animals, facilitating numerous metabolic reactions.
29. What properties of water make it useful as a solvent? What types of compounds can it:
(i) dissolve.
(ii) hydrolyse?
Ans: Water has the ability to act as a solvent due to its polarity. Because water is composed of one oxygen atom (which are partially negative) and two hydrogen atoms (which are partially positive) water is considered to be a polar solvent.
The types of compounds that are soluble in water include ionic compounds and polar compounds.
(i) Dissolve: Ionic compounds, Polar covalent compounds.
(ii) Hydrolyse: Esters, Amides, Anhydrides.
30. Knowing the properties of H2O and D2O, do you think D2O can be used for drinking purposes?
Ans: No, D2O is injurious to human beings, plants and animals.
31. What is the difference between the terms ‘hydrolysis’ and ‘hydration’?
Ans: The difference between the terms ‘hydrolysis’ and ‘hydration’ are:
Hydration: The process of adding water molecules to a substance without splitting the water molecule. For example, in organic chemistry, hydration is the addition of a water molecule to an alkene or alkyne. In inorganic chemistry, hydration is the combination of water molecules without splitting them.
Hydrolysis: Hydration refers to the process of adding water molecules to form hydrated salts or ions. It’s crucial for biological reactions, which primarily occur in aqueous environments. Medically, hydration encompasses water intake through drinking or intravenously. When dehydration occurs, administering water helps restore normal hydration levels in the body.
32. How can saline hydrides remove traces of water from organic compounds?
Ans: Saline hydrides are ionic in nature. They react with water to form a metal hydroxide along with the liberation of hydrogen gas. The reaction of saline hydrides with water can be represented as:
NaH(s) + H2O(l) → NaOH(aq) + H2(g)
CaH2(s) + 2H2O(l) → Ca(OH)2(aq) + H2(g)
33. What do you expect the nature of hydrides is, if formed by elements of atomic numbers 15,19, 23 and 44 with dihydrogen? Compare their behaviour towards water.
Ans: Atomic No. 15 is of phosphorus. The hydride is PH3 and its nature is covalent. Atomic No. (Z = 19) is of potassium. Atomic number 19 is potassium, producing the ionic hydride KH. (Z = 23) is of vanadium. The hydride is VH. It is interstitial or metallic. Atomic No. 44 is of ruthenium, its hydride is interstitial or metallic.
34. Do you expect different products in solution when aluminium (III) chloride and potassium chloride treated separately with
(i) normal water.
(ii) acidified water.
(iii) alkaline water? Write equation wherever necessary.
Ans: Potassium chloride (KCl) is the salt of a strong acid (HCl) and strong base (KOH). Hence, it is neutral in nature and does not undergo hydrolysis in normal water. It dissociates into ions as follows:
In acidified or alkaline water, ions remain unchanged. Aluminium (III) chloride, being a salt of strong acid (HCl) and weak base [Al(OH)₃], undergoes hydrolysis in neutral water.
In acidified water, H⁺ ions react with Al(OH)₃, forming water and releasing Al³⁺ ions. Thus, in acidified water, AlCl₃ will exist as Al³⁺(aq) and Cl⁻(aq) ions.
In alkaline water, the following reaction takes place:
35. How does H2O2 behave as a bleaching agent?
Ans: The bleaching action of H₂O₂ is due to the oxidation of colouring matter by nascent oxygen.
H2O2(Z) → H2O(Z) + O(g)
36. What do you understand by the terms:
(i) Hydrogen economy.
Ans: Hydrogen economy: The basic principle of hydrogen economy is the storage and transportation of energy in the form of liquid or gaseous dihydrogen.
(ii) Hydrogenation
Ans: Hydrogenation: The process of adding hydrogen atoms to unsaturated fats and oils. Unsaturated molecules contain double bonds that can accept hydrogen atoms and become hydrogenated. For example, hydrogenation of vegetable oil with nickel as a catalyst produces edible fats like ghee and vanaspati.
(iii) syngas
Ans: Syngas: Syngas is a mixture of carbon monoxide and dihydrogen. Since the mixture of the two gases is used for the synthesis of methanol, it is called syngas, synthesis gas, or water gas. Syngas is produced by reacting steam with hydrocarbons or coke at high temperatures in the presence of a catalyst.
(iv) water-gas shift reaction.
Ans: Water-gas shift reaction: The amount of hydrogen in the syngas can be increased by the action of CO of syngas mixture with steam in the presence of iron chromate as catalyst.This is called water-gas shift reaction.
(v) fuel-cell
Ans: Fuel Cell: A fuel cell is an electrochemical device that converts chemical energy from a fuel, typically hydrogen, into electrical energy through a reaction with oxygen. It produces electricity with water and heat as byproducts, offering a clean and efficient energy source for power generation and vehicles.
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