Ch-9. Hydrogen (Exercise) Page 290

Question 26:	What is meant by ‘demineralised’ water and how can it be obtained?
Answer :	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.
Question 27:	Is demineralised or distilled water useful for drinking purposes? If not, how can it be made useful?
Answer :	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. It can be made useful only after the addition of desired minerals in specific amounts, which are important for growth.
Question 28:	Describe the usefulness of water in biosphere and biological systems.
Answer :	Water is 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. It acts as a carrier of various nutrients required by plants and animals for various metabolic reactions.
Question 29:	What properties of water make it useful as a solvent? What types of compound can it (i) dissolve, and (ii) hydrolyse?
Answer :	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:
Question 30:	Knowing the properties of H2O and D2O, do you think that D2O can be used for drinking purposes?
Answer :	Heavy water (D2O) acts as a moderator, i.e., it slows the rate of a reaction. Due to this property of D2O, it cannot be used for drinking purposes because it will slow down anabolic and catabolic reactions taking place in the body and lead to a casualty.
Question 31:	What is the difference between the terms ‘hydrolysis’ and ‘hydration’?
Answer :	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:
Question 32:	How can saline hydrides remove traces of water from organic compounds?
Answer :	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: (where, A = Na, Ca,……) When added to an organic solvent, they react with water present in it. Hydrogen escapes into the atmosphere leaving behind the metallic hydroxide. The dry organic solvent distills over.
Question 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.
Answer :	The elements of atomic numbers 15, 19, 23, and 44 are nitrogen, potassium, vanadium, and ruthenium respectively. 1) Hydride of nitrogen Hydride of nitrogen (NH3) is a covalent molecule. It is an electron-rich hydride owing to the presence of excess electrons as a lone pair on nitrogen. 2) Hydride of potassium Dihydrogen forms an ionic hydride with potassium owing to the high electropositive nature of potassium. It is crystalline and non-volatile in nature. 3) Hydrides of Vanadium and Ruthenium 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. 4) Behaviour of hydrides towards water Potassium hydride reacts violently with water as:  Ammonia (NH3) behaves as a Lewis base and reacts with water as: 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.
Question 34:	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.
Answer :	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 and alkaline water, the ions do not react and remain as such. Aluminium (III) chloride is the salt of a strong acid (HCl) and weak base [Al(OH)3]. Hence, it undergoes hydrolysis in normal water. In acidified water, H+ ions react with Al(OH)3 forming water and giving Al3+ ions. Hence, in acidified water, AlCl3 will exist as  and ions. In alkaline water, the following reaction takes place:
Question 35:	How does H2O2 behave as a bleaching agent?
Answer :	H2O2 or hydrogen peroxide acts as a strong oxidizing agent both in acidic and basic media. When added to a cloth, it breaks the chemical bonds of the chromophores (colour producing agents). Hence, the visible light is not absorbed and the cloth gets whitened.
Question 36:	What do you understand by the terms: (i) hydrogen economy (ii) hydrogenation (iii) ‘syngas’ (iv) water-gas shift reaction (v) fuel-cell ?
Answer :	(i) Hydrogen economy 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. (ii) Hydrogenation 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. (iii) 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 on the action of steam with hydrocarbons or coke at a high temperature in the presence of a catalyst. (iv) Water shift reaction 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: (v) Fuel cells 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.