NCERT Solutions for Class 11 Chemistry Chapter 9
Hydrogen Class 11
Chapter 9 Hydrogen Exercise Solutions
Exercise : Solutions of Questions on Page Number : 288
Justify the position of hydrogen in the periodic table on the basis of its electronicconfiguration.
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.
1. Like alkali metals, hydrogen contains one valence electron in its valency shell. H : 1s1
Li : [He] 2s1
Hence, it can lose one electron to form a unipositive ion.
2. Like alkali metals, hydrogen combines with electronegative elements to form oxides, halides, and sulphides.
1. Both hydrogen and halogens require one electron to complete their octets. H : 1s1
F : 1s2 2s2 2p5
Cl : 1s2 2s2 2p6 3s2 3p5
Hence, hydrogen can gain one electron to form a uninegative ion.
2. Like halogens, it forms a diatomic molecule and several covalent compounds.
Though hydrogen shows some similarity with both alkali metals and halogens, it differs from them on some grounds. Unlike alkali metals, hydrogen does not possess metallic characteristics. On the other hand, it possesses a high ionization enthalpy. Also, it is less reactive than halogens.
Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes?
Hydrogen has three isotopes. They are:
1. Protium, ,
2. Deuterium, or D, and
3. Tritium, or T
Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions?
How can the production of dihydrogen, obtained from 'coal gasification', beincreased?
Dihydrogenis produced by coal gasification method as:
This reaction is called the water-gas shift reaction. Carbon dioxide is removed by scrubbing it with a solution of sodium arsenite.
Describe the bulk preparation of dihydrogen by electrolytic method. What is the role of an electrolyte in this process?
Dihydrogen is prepared by the electrolysis of acidified or alkaline water using platinum electrodes. Generally, 15 – 20% of an acid (H2SO4) or a base (NaOH) is used.
Atthe anode, oxidation of OH–ions takes place as:
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. The addition of the electrolyte makes the ions available in the process for the conduction of electricity and for electrolysis to take place.
Complete the following reactions:
(ii)
(iv) Answer :
(iii)
Discuss the consequences of high enthalpy of H-H bond in terms of chemical reactivity of dihydrogen.
The ionization 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 ionization enthalpy value is comparable to that of halogens. Hence, it forms diatomic molecules (H2), hydrides with elements, and a large number of covalent bonds.
What do you understand by (i) electron-deficient, (ii) electron-precise, and (iii)electron-rich compounds of hydrogen? Provide justification with suitable examples.
Molecular hydrides are classified on the basis of the presence of the total number of electrons and bonds in their Lewis structures as:
1. Electron-deficient hydrides
2. Electron-precise hydrides
3. Electron-rich hydrides
An electron-deficient hydride has very few electrons, less than that required for representing its conventional Lewis structure e.g. diborane (B2H6). In B2H6, there are six bonds in all, out of which only four bonds are regular two centered-two electron bonds. The remaining two bonds are three centered-two electron bonds i.e., two electrons are shared by three atoms. Hence, its conventional Lewis structure cannot be drawn.
An electron-precise hydride has a sufficient number of electrons to be represented by its conventional Lewis structure
Four regular bonds are formed where two electrons are shared by two atoms. An electron-rich hydride contains excess electrons as lone pairs e.g. NH3.
What characteristics do you expect from an electron-deficient hydride with respect to its structure and chemical reactions?
An electron-deficient hydride does not have sufficient electrons to form a regular bond in which two electrons are shared by two atoms e.g., B2H6, Al2H6 etc.
Since these hydrides are electron-deficient, they have a tendency to accept electrons. Hence, they act as Lewis acids.
Do you expect the carbon hydrides of the type (CnH2n+ 2) to act as 'Lewis' acid or base? Justify your answer.
For carbon hydrides of type CnH2n+ 2, the following hydrides are possible for
For a hydride to act as a Lewis acid i.e., electron accepting, it should be electron-deficient. Also, for it to act as a Lewis base i.e., electron donating, it should be electron-rich.
Hence, hydride C2H6has sufficient electrons to be represented by a conventional Lewis structure. Therefore, it is an electron-precise hydride, having all atoms with complete octets. Thus, it can neither donate nor accept electrons to act as a Lewis acid or Lewis base.
What do you understand by the term “non-stoichiometric hydrides”? Do you expect this type of the hydrides to be formed by alkali metals? Justify your answer.
Non-Stoichiometric hydrides are hydrogen-deficient compounds formed by the reaction of dihydrogen with d-block and f-block elements. These hydrides do not follow the law of constant composition. For example: LaH2.87, YbH2.55, TiH1.5 - 1.8etc.
Alkali metals form stoichiometric hydrides. These hydrides are ionic in nature. Hydride ions have comparable sizes (208 pm) with alkali metal ions. Hence, strong binding forces exist between the constituting metal and hydride ion. As a result, stoichiometric hydrides are formed.
How do you expect the metallic hydrides to be useful for hydrogen storage? Explain.
How does the atomic hydrogen or oxy-hydrogen torch function for cutting and welding purposes? Explain.
Among NH3, H2O and HF, which would you expect to have highest magnitude of hydrogen bonding and why?
The extent of hydrogen bonding depends upon electronegativity and the number of hydrogen atoms available for bonding. Among nitrogen, fluorine, and oxygen, the increasing order of their electronegativities are N < O < F.
Hence, the expected order of the extent of hydrogen bonding is HF > H2O > NH3. But, the actual order is H2O > HF > NH3.
Although fluorine is more electronegative than oxygen, the extent of hydrogen bonding is higher in water. There is a shortage of hydrogens in HF, whereas there are exactly the right numbers of hydrogens in water. As a result, only straight chain bonding takes place. On the other hand, oxygen forms a huge ring-like structure through its high ability of hydrogen bonding.
Saline hydrides are known to react with water violently producing fire. Can CO2, a well known fire extinguisher, be used in this case? Explain.
The reaction is violent and produces fire.
CO2is heavier than dioxygen. It is used as a fire extinguisher because it covers the fire as a blanket and inhibits the supply of dioxygen, thereby dousing the fire.
Arrange the following
(ii) LiH, NaH and CsH in order of increasing ionic character.
(iv) NaH, MgH2and H2O in order of increasing reducing property.
(i) The electrical conductance of a molecule depends upon its ionic or covalent nature. Ionic compounds conduct, whereas covalent compounds do not.
BeH2 is a covalent hydride. Hence, it does not conduct. CaH2 is an ionic hydride, which conducts electricity in the molten state. Titanium hydride, TiH2 is metallic in nature and conducts electricity at room temperature. Hence, the increasing order of electrical conductance is as follows:
BeH2 < CaH2 < TiH2
(ii) The ionic character of a bond is dependent on the electronegativities of the atoms involved. The higher the difference between the electronegativities of atoms, the smaller is the ionic character.
Electronegativity decreases down the group from Lithium to Caesium. Hence, the ionic character of their hydrides will increase (as shown below).
LiH < NaH < CsH
(iii) Bond dissociation energy depends upon the bond strength of a molecule, which in turn depends upon the attractive and repulsive forces present in a molecule.
The bond pair in D-D bond is more strongly attracted by the nucleus than the bond pair in H-H bond. This is because of the higher nuclear mass of D2. The stronger the attraction, the greater will be the bond strength and the higher is the bond dissociation enthalpy. Hence, the bond dissociation enthalpy of D-D is higher than H-H.
However, bond dissociation enthalpy is the minimum in the case of F-F. The bond pair experiences strong repulsion from the lone pairs present on each F-centre.
Therefore, the increasing order of bond dissociation enthalpy is as follows:
F-F < H-H < D-D
(iv) Ionic hydrides are strong reducing agents. NaH can easily donate its electrons. Hence, it is most reducing in nature.
Both, MgH2 and H2O are covalent hydrides. H2O is less reducing than MgH2 since the bond dissociation energy of H2O is higher than MgH2.
Compare the structures of H2O and H2O2.
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.
What do you understand by the term 'auto-protolysis' of water? What is its significance?
Auto-protolysis (self-ionization) of water is a chemical reaction in which two water molecules react to produce a hydroxide ion (OH–) and a hydronium ion (H3O+).
Auto-protolysis of water indicates its amphoteric nature i.e., its ability to act as an acid as well as a base. The acid-base reaction can be written as:
Consider the reaction of water with F2 and suggest, in terms of oxidation and reduction, which species are oxidized/reduced.
This is an example of a redox reaction as water is getting oxidized to oxygen, while fluorine is being reduced to fluoride ion.
Fluorine is reduced from zero to (– 1) oxidation state. A decrease in oxidation state indicates the reduction of fluorine.
Complete the following chemical reactions.
(ii)
(iv)
(v)
Answer :
H2O2is acting as an oxidizing agent in the reaction. Hence, it is a redox reaction.
is acting as a reducing agent in the acidic medium, thereby oxidizing . Hence, the given reaction is a redox reaction.
The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction is hydrolysis.
The reactions in which a compound reacts with water to produce other compounds are called hydrolysis reactions. The given reaction represents hydrolysis of AlCl3.
Describe the structure of the common form of ice.
Ice is the crystalline form of water. It takes a hexagonal form if crystallized at atmospheric pressure, but condenses to cubic form if the temperature is very low.
The three-dimensional structure of ice is represented as:
What causes the temporary and permanent hardness of water?
Temporary hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of hydrogen carbonates (MHCO3, where M = Mg, Ca) in water.
Discuss the principle and method of softening of hard water by synthetic ion-exchange resins.
The process of treating permanent hardness of water using synthetic resins is based on the exchange of cations (e.g., Na+, Ca2+, Mg2+ etc) and anions (e.g., Cl–, SO42–, HCO3– etc) present in water by H+ and OH– ions respectively.
Synthetic resins are of two types:
1) Cation exchange resins
2) Anion exchange resins
There are cation exchange resins in H+ form. The resins exchange H+ ions for Na+, Ca2+, and Mg2+ ions.
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.
Write chemical reactions to show the amphoteric nature of water.
The amphoteric nature of water can be described on the basis of the following reactions:
In the forward reaction, accepts a proton from . Hence, it acts as a Lewis base.
In the forward reaction, denotes its proton to . Hence, it acts as a Lewis acid.
Write chemical reactions to justify that hydrogen peroxide can function as an oxidizing as well as reducing agent.
Hydrogen peroxide, H2O2acts as an oxidizing as well as a reducing agent in both acidic and alkaline media. Reactions involving oxidizing actions are:
2)
4)
Reactions involving reduction actions are:
2)
What is meant by 'demineralised' water and how can it be obtained?
Demineralised water is free from all soluble mineral salts. It does not contain any 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.
During the cation exchange process, H+exchanges for Na+, Mg2+, Ca2+, and other cations present in water.
……. (1)
In the anion exchange process, OH– exchanges for anions such as etc. present in water.
OH– ions liberated in reaction (2) neutralize H+ ions liberated in reaction (1), thereby forming water.
Is demineralised or distilled water useful for drinking purposes? If not, how can it be made useful?
Water is an important part of life. It contains several dissolved nutrients that are required by human beings, plants, and animals for survival. Demineralised water is free of all soluble minerals. Hence, it is not fit for drinking.
Describe the usefulness of water in biosphere and biological systems.
Wateris essential for all forms of life. It constitutes around 65% of the human body and 95% of plants. 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 vapourization and heat of capacity of water helps in moderating the climate and body temperature of all living beings.
What properties of water make it useful as a solvent? What types of compound can it (i) dissolve, and (ii) hydrolyse?
A high value of dielectric constants (78.39 C2/Nm2) and dipole moment make water a universal solvent.
Water is able to dissolve most ionic and covalent compounds. Ionic compounds dissolve in water because of the ion- dipole interaction, whereas covalent compounds form hydrogen bonding and dissolve in water.
Water can hydrolyze metallic and non-metallic oxides, hydrides, carbides, phosphides, nitrides and various other salts. During hydrolysis, H+ and OH– ions of water interact with the reacting molecule.
Some reactions are:
Knowing the properties of H2O and D2O, do you think that D2O can be used for drinking purposes?
What is the difference between the terms 'hydrolysis' and 'hydration'?
Hydrolysis is defined as a chemical reaction in which hydrogen and hydroxide ions (H+ and OH– ions) of water molecule react with a compound to form products. For example:
Hydration is defined as the addition of one or more water molecules to ions or molecules to form hydrated compounds. For example:
How can saline hydrides remove traces of water from organic compounds?
Saline hydrides areionic 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:
(where, A = Na, Ca,……)
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.
The elements of atomic numbers 15, 19, 23, and 44 are nitrogen, potassium, vanadium, and ruthenium respectively.
Dihydrogen forms an ionic hydride with potassium owing to the high electropositive nature of potassium. It is crystalline and non-volatile in nature.
Both vanadium and ruthenium belong to the d–block of the periodic table. The metals of d–block form metallic or non–stoichiometric hydrides. Hydrides of vanadium and ruthenium are therefore, metallic in nature having a deficiency of hydrogen.
Hydrides of vanadium and Ruthenium do not react with water. Hence, the increasing order of reactivity of the hydrides is (V, Ru) H < NH3 < KH.
Do you expect different products in solution when aluminium(III) chloride and potassium chloride treated separately with (i) normal water (ii) acidified water, and (iii) alkaline water? Write equations wherever necessary.
In acidified and alkaline water, the ions do not react and remain as such.
In acidified water, H+ions react with Al(OH)3forming water and giving Al3+ions. Hence, in acidified water, AlCl3will exist as and ions.
In alkaline water, the following reaction takes place:
How does H2O2 behave as a bleaching agent?
H2O2 or hydrogen peroxide acts as a strong oxidizing agent both in acidic and basic media.
What do you understand by the terms:
Answer :
Hydrogen economy is a technique of using dihydrogen in an efficient way. It involves transportation and storage of dihydrogen in the form of liquid or gas.
Dihydrogen releases more energy than petrol and is more eco–friendly. Hence, it can be used in fuel cells to generate electric power. Hydrogen economy is about the transmission of this energy in the form of dihydrogen.
Hydrogenation is the addition of dihydrogen to another reactant. This process is used to reduce a compound in the presence of a suitable catalyst. For example, hydrogenation of vegetable oil using nickel as a catalyst gives edible fats such as vanaspati, ghee etc.
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 on the action of steam with hydrocarbons or coke at a high temperature in the presence of a catalyst.
For example :
It is a reaction of carbon monoxide of syngas mixture with steam in the presence of a catalyst as:
This reaction is used to increase the yield of dihydrogen obtained from the coal gasification reaction as:
Fuel cells are devices for producing electricity from fuel in the presence of an electrolyte. Dihydrogen can be used as a fuel in these cells. It is preferred over other fuels because it is eco-friendly and releases greater energy per unit mass of fuel as compared to gasoline and other fuels.