Hydrogen

Hydrogen is said to be an analogous element when its position in the periodic table is considered. This is because it shares its properties both with alkali metals as well as with halogens in the field of sharing or gaining electrons or in other words it has its electronic configuration similar to these. That is why its position in the periodic table is not justified.

Hydrogen (1s¹) shares the similar electronic configuration with halogens where both of them require to gain only one electron to obtain the stable state configuration. Eg:

F : 1s ² 2s ² 2p ⁵

Cl : 1s ² 2s ³ 2p ⁶ 3s ² 3p ⁵

So, we can see that according to this electronic configuration, both hydrogen and halogens require to gain only one electron to acquire the stable duplet(for hydrogen) and octet(for halogens) states or a uni-negative ion.

Also the electronic configuration of hydrogen (1s¹) is similar to that of alkali metals (ns¹) which shows that only one electron is present in their valence shell. So if they lose that single electron, a uni-positive ion would be formed.

Also, the size of a halogen atom does not remain the same all throughout the periodic table. As we go down a group, in a periodic table, more number of electrons are added to the last shell. Thus the nuclear charge cannot reach to the last shells due to which, atomic radius increases.

The reason why any atom or molecule is formed is because the ionic forms cannot remain stable.

When H⁺ ion is formed, the size of the ion is very small (near about 10^-3 pm. The normal atomic and ionic sizes vary from 50 to 230pm. Due to which, it cannot exist freely and it has to be always associated with other atoms or molecules to get stability.

When we go down the Group 1 or the alkali metals group, the elements we observe in order are Li, Na, K, Rb, Cs. As we go down the group, metallic character increases.

Metallic character of something can be expressed as the reluctance to lose electrons. A molecule is said to be more metallic than the other if it loses an electron more easily than the other.

Now, an atom will lose an electron more easily if the nuclear attraction on the electrons of valence shells would be as low as possible to facilitate the loss.

As we go down a group in a periodic table, electrons get added to the last shells. As a result of which the elements which are situated at the bottom of a group do not feel the nuclear attraction which the elements at the top of the group feel. This is because in the elements at the bottom of a group, we see that the valence electrons are very far from the nucleus, due to which the nuclear attraction that these electrons receive is quite negligible. So, these electrons can be removed very easily and the formation of cations is preferred. Therefore, these metals are said to be more metallic than the ones at the top of the group.

So the metallic order is as follows:-

LiH < NaH < KH < RbH < CsH

An electron-precise hydride is a type of hydride which has the exact number of electrons required to form normal covalent bonds, no excess no deficient. These are formed generally by group 14 elements like Si and C which have 4 valence electrons.

Eg. CH₄ , SiH₄, etc.

B₂H₆ is an electron deficient hydride. These have an incomplete octet. They are formed generally by the group 13 elements like B, Al, etc.

NH₃, H₂O are examples of electron rich hydrides. These contain excess number of electrons on the central atom called lone pairs. These are formed by group 15,16,17 elements like N, O, F, etc.

Structure of CH₄

Structures of NH₃ and H₂O

Structure of B₂H₆

Hydrogen has three isotopes – Protium (¹H₁), Deuterium (²H₁), Tritium (³H₁). The number of neutrons is the reason why these isotopes differ from each other in spite of having the same atomic number.

Among these isotopes, the only tritium is radioactive isotope and it emits low energy β particles as it has more number of neutrons.

An oxidising agent is one which oxidises(oxidation number increases) others and itself gets reduced. Whereas a reducing agent reduces (oxidation number decreases) others and itself gets oxidised.

So, we can see that H₂O₂ acts as an oxidising agent in (A) as it increases the oxidation number of I^- in HI from -1 to 0 in I_2.

Similarly, H₂O₂ acts as a reducing agent in (B) as it decreases the oxidation number of Cl⁺ in HOCl from +1 to -1 in Cl^-.

Barium peroxide (BaO₂) as we can get from its name, has a per-oxy linkage in it. Due to which its treatment with H₂SO₄ gives H₂O₂ . Whereas, the other oxides don’t have the per-oxy linkage due to which hydrogen peroxide cannot be produced.

Whereas, all the other oxides give only water instead of hydrogen peroxide due to the absence of the per-oxy linkage in them.

In the reaction where ferrocyanide ion reacts with hydrogen peroxide, the oxidation state on ferrocyanide ion is +2 and it increases to +3 in ferricyanide ion in the product. So, here hydrogen peroxide acts as an oxidising agent.

Note:- The given reaction is not possible. The actual reaction could have been

In this case, I₂ have oxidation number of 0 which reduces to -1 in I^-.

This reaction depicts the reducing nature of H₂O_2.

The oxidising state of the oxygen atom on H₂O₂ is -1 which is an intermediate state. It means that it can either reduce itself to H₂O where oxidation state of O is -2 or it can oxidise itself to O₂, where the oxidation state of O is 0.

In the first case, H₂O₂ acts as an oxidising agent and in the second case, it acts as a reducing agent.

The water shift gas reaction is used in the formation of synthesis gas (hydrogen and carbon monoxide or carbon dioxide mixture. Here CO is converted to CO₂ and H₂ is also obtained from water.

From the name itself, we get to know that sodium peroxide has a per-oxy linkage in it due to which the formation of hydrogen peroxide is always preferred. So, when it reacts with dilute sulphuric acid, we get hydrogen peroxide along with sodium sulphate.

Peroxide linkage in Sodium peroxide

From the electrolysis of sulphuric acid (H₂SO₄), we obtain hydrogen peroxide. We can observe it from the following reactions:

Water-gas is nothing but a mixture of carbon monoxide (CO) and hydrogen (H₂) gases.

Here, in the reaction, water gas is used in the synthesis of methanol (CH₃OH).

Permanent hardness in water is caused due to the presence of soluble salts of magnesium and calcium in the form of chlorides or sulphates in water. So Ca²⁺ will cause hardness in water sample.

Permanent hardness cannot be removed only by simple boiling. One method of removing permanent harness is by Calgon’s method.

Here, calgon, which is sodium hexametaphosphate(Na₆P₆O₁₈), when added to hard water following reactions take place :

When the metals of group 7,8,9 react with Hydrogen, they accept the only negative electron of H atom and the metal ion becomes negatively charged and a ligand is formed in this way.

If the opposite would have occurred, like if the metal lost an electron and became positively charged then the compound formed with hydrogen would be known as a hydride.

This trait seen is known as hydride gap.

We know metals of group 7,8,9 cannot form hydrides. This is because these metals accept the only negative electron of H atom and the metal ion becomes negatively charged and a ligand is formed in this way.

If the opposite would have occurred, like if the metal lost an electron and became positively charged then the compound formed with hydrogen would be known as a hydride.

This trait seen is known as hydride gap.

So, only group 6 elements form hydrides.

When a hydrogen atom loses an electron, it forms a cation H⁺. But it is not at all stable to exist freely. The reason why any atom or molecule is formed is because the ionic forms cannot remain stable.

When H⁺ ion is formed, the size of the ion is very small (near about 10^-3 pm. The normal atomic and ionic sizes vary from 50 to 230pm. Due to which, it cannot exist freely and it has to be always associated with other atoms or molecules to get stability.

Hydrogen exists as a diatomic molecule like H₂ and has only one electron in the valance shell.

Water gas is nothing but a mixture of carbon monoxide (CO) and hydrogen (H₂) gases.

The water shift gas reaction is used in the formation of synthesis gas (hydrogen and carbon monoxide or carbon dioxide mixture. Here CO is converted to CO₂ and H₂ is also obtained from water. The production of dihydrogen can be increased by reacting carbon monoxide of syngas mixtures with steam and using iron chromate as a catalyst.

Heavy water D₂O is used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms.

Also there is stronger hydrogen bonding in heavy water than in ordinary water due to which it is more associated.

Hydrogen has three isotopes – Protium (¹H₁), Deuterium (²H₁), Tritium (³H₁). The number of neutrons is the reason why these isotopes differ from each other in spite of having the same atomic number.

Ordinary hydrogen, protium has no neutrons and thus the predominant form is protium.

When a hydrogen atom loses an electron, it forms a cation H⁺. But it is not at all stable to exist freely. The reason why any atom or molecule is formed is because the ionic forms cannot remain stable.

When H⁺ ion is formed, the size of the ion is very small (near about 10^-3 pm. The normal atomic and ionic sizes vary from 50 to 230pm. Due to which, it cannot exist freely and it has to be always associated with other atoms or molecules to get stability. So it never acts as cation in ionic salt.

In water due to the electronegative oxygen atom, we can see hydrogen bonding in water which is even present in the frozen state of water.

Since water expands on freezing, so the volume of ice for the same mass of water is more than water. In other words, density of ice is lower than water and hence ice floats on water.

As , thus for a substance with lower density, its mass is also less.

Permanent hardness in water is caused due to the presence of soluble salts of magnesium and calcium in the form of chlorides or sulphates in water.

So the chlorides as well as sulphates are responsible for causing permanent hardness in water.

An electron-precise hydride is a type of hydride which has the exact number of electrons required to form normal covalent bonds, no excess no deficient. These are formed generally by group 14 elements like Si and C which have 4 valence electrons.

Eg. CH₄ , SiH₄, etc.

Due to the sp³ hybridisation configuration, the molecular geometry is tetrahedral.

sp³ hybridisation is tetrahedral in geometry.

Tetrahedral geometries of CH₄ and SiCl₄.

B₂H₆ is an electron deficient hydride. These have an incomplete octet. They are formed generally by the group 13 elements like B, Al, etc.

Electron deficient hydrides are those which have an incomplete octet. They are formed generally by the group 13 elements like B, Al, etc.

Electron rich hydrides are those which contain excess number of electrons on the central atom called lone pairs. These are formed by group 15,16,17 elements like N, O, F, etc.

Now, lewis acids are electron deficient species. They contain an empty orbital which are capable of accepting an electron pair, from Lewis base(electron enriched species) thereby forming Lewis adduct. Eg. FeCl₃, FeBr_3, BF₃, anhydrous AlCl₃, etc. Therefore we can see that hydrides of group 13 can act as Lewis acid as they are electron deficient species.

Likewise, hydrides of group 15 can act as Lewis base as they are electron rich species.

Metallic hydrides are non-stoichiometric hydrides. They conduct heat and electricity.

Whereas, ionic hydrides conduct electricity only in molten or in aqueous state. This is because charge dissociation is not possible in the solid state. It is only possible when the sample is either electrolysed or is in aqueous state.

The water shift gas reaction is used in the formation of synthesis gas (hydrogen and carbon monoxide or carbon dioxide mixture. Here CO is converted to CO₂ and H₂ is also obtained from water.

Water gas is produced when super-heated steam is passed over red hot coke at very high temperature (around 1300K) in the presence of a Ni catalyst.

The H₂ obtained from this reaction is difficult to obtain due to the difficulty in the removal of poisonous CO gas which is formed along. To fix this difficulty, we oxidise the poisonous CO gas to CO₂ gas in another reaction where the products obtained previously is mixed with more steam over heated FeCrO₄ catalyst at 673K.

Now, this obtained CO₂ gas can be removed easily by passing it through sodium arsenite solution. In this way, production of H₂ gas can be increased.

Metallic hydrides -

Metallic hydrides are those compounds which are formed when transition element react with hydrogen. Eg. CaH₂ , LiBH₄

Molecular Hydrides -

Molecular hydrides are formed by the combination of an element which is having comparable higher electronegativity with hydrogen. Eg. H₂O , B₂H₆

Metal hydrides have high thermal and electrical conductivity whereas the molecular hydrides have low thermal and electrical conductivity.

Metallic hydrides are found in solid state whereas the molecular hydrides are comparatively soft & volatile.

H2O - Electron rich hydride.

H₂O is an example of electron rich hydride. It contains excess number of electrons on the central atom called lone pairs. These are formed by group 15,16,17 elements like N, O, F, etc.
B2H6 - Electron deficient hydride.

B₂H₆ is an electron deficient hydride. These have an incomplete octet. They are formed generally by the group 13 elements like B, Al, etc.

NaH - Ionic hydride.

Hydrides formed with alkali and alkaline-earth metals. These compounds are ionic in nature.

Since water expands on freezing, so the volume of ice for the same mass of water is more than water. In other words, density of ice is lower than water and hence ice floats on water.

As , thus for a substance with lower density, its mass is also less.

(i) This is an example of a redox reaction. Here, hydrogen peroxide is acting as an oxidising agent, due to which PbS (oxidation state of S is -2) is oxidised to PbSO₄(oxidation state of S is +6) and H₂O₂(oxidation state of O is -1). itself gets reduced to H₂O(oxidation state of O is -2).

(ii) This reaction yields methanol at 340K

(i) As we know that, , thus for a substance with lower density, its mass is also less. The density of ice is less than water. Cold water starts expanding, and becomes less dense than what it should be when temperature drops to 4°C. As a result cold water being less dense, floats on to the surface and when it comes to the chilly freezing weather conditions outside, it gets frozen.

(ii) Since water expands on freezing, so the volume of ice for the same mass of water is more than water. In other words, density of ice is lower than water and hence ice floats on water.

As , thus for a substance with lower density, its mass is also less.

Auto protolysis of water is basically self ionisation of water. It is one of the chemical properties of water. Here, two same molecules react to give ions involving proton transfer.

This reaction also shows the amphoteric nature, which means water can act both as an acid as base.

Demineralised water is free from all soluble mineral salts. It is obtained by passing water successively through a cation exchange and an anion exchange resin. Resins have giant molecules with -SO₃H or -COOH groups.

First, the hard water is passed through the cation exchanger, which removes cations like Na⁺, Mg²⁺, Ca²⁺,etc. by exchanging them with H⁺ ions.

Now, this water coming out from cation exchanger is acidic, therefore it is passed through the anionic exchanger which removes anions like OH^-, NO₃^-, etc. by exchanging with OH^- ions.

At the end, the H⁺ and OH^- thus produced, combine amongst themselves to produce water

This is demineralisation of water by ion exchange resin.

Molecular hydrides are classified into electron deficient, electron precise and electron rich compounds.

B₂H₆ is an electron deficient hydride. These have an incomplete octet. They are formed generally by the group 13 elements like B, Al, etc.

Structure of B₂H₆

An electron-precise hydride is a type of hydride which has the exact number of electrons required to form normal covalent bonds, no excess no deficient. These are formed generally by group 14 elements like Si and C which have 4 valence electrons.

Eg. CH₄ , SiH₄, etc.

Structure of CH₄

NH₃, H₂O are examples of electron rich hydrides. These contain excess number of electrons on the central atom called lone pairs. These are formed by group 15,16,17 elements like N, O, F, etc.

Structures of NH₃ and H₂O

Heavy water, which is deuterium oxide (D₂O) is mainly prepared by electrolysis of ordinary water.

This method involves multistage electrolysis of ordinary water containing alkali like NaOH. The cylindrical vessels made of steel which act as cathode and cylindrical sheets of nickel acts as anode having a number of holes punched in it. The electrolysis carried out in different stages as we mentioned earlier. Large number of electrolytic cell are also used for this process.

When D₂SO₄ reacts with BaO_2, D₂O₂ is formed along with barium sulphate (BaSO₄).

We know, molar mass of H₂O₂ is 34gm. According to this reaction, 2 moles of peroxide is involved, so mass becomes 68gm.

Also, we know that 1mole of any gas occupies a volume of 22.4L at NTP.

Now, 5 volume H₂O₂ solution means that 1 L of 5 volume H₂O₂ on decomposition gives 5L of O₂ at NTP.

So we can see from the reaction that, from 22.4L at NTP, we get 68gm H₂O_2.

Now to get the required 5L of O₂ at NTP, we need () gm H₂O₂

= 15.179gm

But we know that 5L of O₂ at NTP is produced from 1L of 5 volume H₂O_{2 ,} as we have to find strength of 5 volume H₂O₂

∴ Strength of H₂O₂ in 5 volumes of it is 15.179gm/L

∴ Percentage strength = ()%

= 1.518%

(i)

Structure of H₂O₂ in gas phase

Structure of H₂O₂ in solid phase

(ii) An oxidising agent is one which oxidises(oxidation number increases) others and itself gets reduced.

The oxidising state of the oxygen atom on H₂O₂ is -1 which is an intermediate state. It means that it can reduce itself to H₂O where oxidation state of O is -2 . So here, H₂O₂ acts as an oxidising agent. This is not possible for H₂O so we can say that H₂O₂ is a better oxidising agent than water.

We know, melting point, enthalpy of vapourisation, viscosity all these physical properties depend on the intermolecular forces of attraction. From the data supplied above, we can see that all the values corresponding to the properties are higher for D₂O than that of H₂O. So, we can say that the intermolecular forces in D₂O are stronger.

Hydrogen has three isotopes – Protium (¹H₁), Deuterium (²H₁), Tritium (³H₁). The number of neutrons is the reason why these isotopes differ from each other in spite of having the same atomic number.

Among these isotopes, only deuterium has one proton and one neutron in its nucleus. When deuterium reacts with oxygen, deuterium oxide which is heavy water (D₂O), is produced.

The D – D bond is stronger than H - H bond, therefore, H₂ is more reactive than D₂ towards reaction with oxygen. So, they would not have the same reactivity.

HF has high boiling point than HCl because of the reason that in HF the strength of hydrogen bond is stronger than the strength of hydrogen bond in HCl.

This happens because the electronegativity of F atom is much more than that of Cl along with that F has a much smaller size. All these factors favour the formation of strong intermolecular hydrogen bonding with F atom. Due to the formation of strong and stable hydrogen bonding, high energy is required to break the H-F bond. So the boiling point of HF is more than HCl and due to which HF is a liquid and HCl is a gas.

Note – Both HCl and HF are gases at room temperature.

The first element of the periodic table is hydrogen. When hydrogen reacts with dioxygen, the product formed is water. The solid state of water is ice floats on water due to its lower density. This is because water expands on freezing, so the volume of ice for the same mass of water is more than water. In other words, density of ice is lower than water and hence ice floats on water.

Water has an amphoteric nature which means that it can act both as an acid as well as a base.

Auto protolysis or auto ionisation of water is basically self ionisation of water. It is one of the chemical properties of water. Here, two same molecules react to give ions involving proton transfer.

This reaction also shows the amphoteric nature, which means water can act both as an acid as base.

(i) The compound told in this passage is hydrogen peroxide (H₂O₂). It can act both as oxidising and reducing agent in acidic and alkaline media. H₂O₂ is important for use in the pollution control treatment of domestic and industrial effluents.

(ii) H₂O₂ may undergo decomposition into H₂Oif it is placed in contact with sunlight or with dust particles. So it is kept in wax lined coloured bottles.

Another measure taken to prevent dissociation is by adding small amounts of negative catalyst like urea, glycerol, etc.

Hydrogen is said to be an analogous element when its position in the periodic table is considered. This is because it shares its properties both with alkali metals as well as with halogens in the field of sharing or gaining electrons or in other words it has its electronic configuration similar to these. That is why its position in the periodic table is not justified.

The electronic configuration of hydrogen (1s¹) is similar to that of alkali metals (ns¹) which shows that only one electron is present in their valence shell. So if they lose that single electron, a uni-positive ion would be formed.

Hydrogen has only one electron in its valence shell. So it can either lose that electron to form H⁺ or it can gain one electron to attain the noble gas configuration of Helium. So, it can form compounds with both covalent and ionic bonds.

But, the ionisation enthalpy of H is very high and electron gain enthalpy is slightly negative. Therefore, ionic bonds are not preferred, instead H likes to form compounds with covalent bonding.

We know, the size of H atom is much smaller than that of Na atom. Due to which electrons are situated at a distance in Na from the nucleus as compared to that of H atom. As a result nuclear attraction received by the valence electrons is much higher for H atom than Na atom. So, there would be difficulty in removing a valence electron from H atom than Na atom due to higher nuclear attraction, so more energy would be required. Thus, ionisation enthalpy of hydrogen is higher than sodium.

Hydrogen is in gaseous form at room temperature and due to the bulk and leakage factors, it is difficult to transport it. So, if pressure is applied and cooling is done then the gaseous hydrogen can be converted to liquid form.

Being liquid, less consumption of volume takes place, so transportation can be done easily. Thus, the basic property of hydrogen which is useful for hydrogen economy is that it can be converted into a liquid - by cooling under high pressure.

Heavy water is water that contains heavy hydrogen , also known as deuterium in place of regular hydrogen. It can also be written as D₂O.

Heavy water D₂O is used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms. In nuclear fission reactors, the neutrons must be slowed down to ensure an effective fission chain reaction occurs. This process of slowing neutrons down is known as moderation, and the material that slows down these neutrons is known as a neutron moderator. Heavy water acts as a moderator.

In hydrogen peroxide, the peroxide bond(O-O) is present and on both ends, hydrogen is bonded covalently.

An oxidising agent is one which oxidises(oxidation number increases) others and itself gets reduced. Whereas a reducing agent reduces (oxidation number decreases) others and itself gets oxidised.

The oxidising state of the oxygen atom on H₂O₂ is -1 which is an intermediate state. It means that it can either reduce itself to H₂O where oxidation state of O is -2 or it can oxidise itself to O₂, where the oxidation state of O is 0.

So, we can see that H₂O₂ acts as an oxidising agent in the first reaction as it increases the oxidation number of I^- in HI from -1 to 0 in I_2.

Similarly, H₂O₂ acts as a reducing agent in the second reaction as it decreases the oxidation number of Cl⁺ in HOCl from +1 to -1 in Cl^-.

H2O2 or hydrogen peroxide acts as a strong oxidizing agent both in acidic and basic media. It acts as a bleaching agent due to the release of nascent oxygen.The nascent oxygen combines with colouring matter breaks the chemical bonds present, which in turn gets oxidised. Hence, the visible light is not absorbed and the cloth gets whitened.

A polar molecule is a molecule that has a slight δ positive and a slight δ negative charge distribution. Due to the high electronegativity difference between oxygen and hydrogen atoms, polarity in a molecule arises.

This is due to the formation of very stable intermolecular hydrogen bonding(bonding with other molecules of the same or different species) because of the presence of electronegative elements like oxygen, fluorine, nitrogen.

We know, the electronegativity of oxygen is much more than sulphur atom. So, it is expected that we would find intermolecular hydrogen bonding in water than hydrogen sulphide.

So, due to intermolecular hydrogen bonding, many similar molecules join due to the formation of these strong bonds. As a result of which, a large amount of energy is required to break these bonds. Therefore, boiling point of water is higher than that of hydrogen sulphide.

The dilute solutions of hydrogen peroxide cannot be concentrated by simple heating. This is because hydrogen peroxide readily decomposes below its boiling point.So, to obtain a concentrated solution of hydrogen peroxide, the following steps are needed to done – •The diluted solution is carefully heated on a water bath. Doing slow evaporation, helps the release of water molecules and we obtain 50% solution of H₂O₂.•The 50% solution is placed in vacuum desiccator over concentrated sulphuric acid. The dehydrating nature of sulphuric acid absorbs water vapours and we get 90% solution of H₂O₂. •The 90% solution of H₂O₂ is then subjected to distillation under reduced pressure. Water distils over at 303 – 313 K and we get 99% pure H₂O₂ solution.

H₂O₂ may undergo decomposition into H₂Oif it is placed in contact with sunlight or with dust particles. So it is kept in wax lined coloured bottles.

As we know, soaps are cleaning agents. They are usually made by reacting alkali (eg. sodium hydroxide) with naturally occurring fat or fatty acids. Therefore we get salts of fatty acids.

Soaps have the general formula (RCO₂⁻)_nMⁿ⁺ (Where R is an alkyl, M is a metal and n is the charge of the cation). Hardness in water is caused due to the presence of soluble salts of magnesium and calcium in the form of chlorides or sulphates in water.

Lather is formed when the sodium or potassium salts dissolve in water. When soap is added to hard water, calcium and magnesium ions of hard water replaces so

dium or potassium ions in the soap and gets precipitated hence lather formation is hindered. Therefore, hard water does not form lather with soap.

In this reaction, sulphuric acid catalyses the decomposition of hydrogen peroxide formed. So different acids like phosphoric acid, carbonic acids are preferred where this decomposition of the produced hydrogen peroxide does not take place.

Oxygen is sp³ hybridised in H₂O molecule. Two hybrid orbitals are occupied by lone pairs and two are used in sigma bonding with hydrogen atoms. Now, the expected tetrahedral angle is 109.5° but the actual bond angle observed is 104.5°. This because from VSEPR theory, we know that lone pair-lone pair repulsions are greater than bond pair-bond pair repulsions and the same situation we observe here. As a result, bond angle in water is 104.5°.

This is the oxidation reaction as electrons are released.

This is the reduction reaction as electrons are consumed.

Now, adding both the reactions, we get

So we can see that oxidation and reduction taking place simultaneously.

Auto protolysis of water is basically self ionisation of water. It is one of the chemical properties of water. Here, two same molecules react to give ions involving proton transfer.

This reaction also shows the amphoteric nature, which means water can act both as an acid as base.

(i) Water acting as an acid

Here, water acts as Bronsted acid and donates protons.

(ii) Water acting as a base

Here, water acts as Bronsted base and accepts protons.

(i) Statements A and R both are correct and R is the correct explanation of A.

Permanent hardness of water is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.

Permanent hardness can be removed by treatment of water with washing soda which contains sodium carbonate. Washing soda reacts with soluble calcium and magnesium chlorides and sulphates in hard water to form insoluble carbonates.

(i) Statements A and R both are correct and R is the correct explanation of A.

Some metals like platinum and palladium can be used as storage media for hydrogen as they will not react with the atomic hydrogen due to their inertness.

They absorb large volumes of hydrogen as they have high surface area which allows hydrogen to get adsorbed over their surface.

These properties are utilized in many chemical reactions where hydrogen adsorbed over Pt or Pd is used as a catalyst.

Atomic hydrogen is very reactive whereas molecular hydrogen is stable. This can be understood by the dissociation enthalpy of H₂ which is very high (435.88 kJ mol^–1).

So, from the dissociation enthalpy of H-H bond it is clear that H₂ bond will not easily break and it is relatively inert at room temperature. Thus it will only combine with few elements.

On the other hand atomic hydrogen being a single species can combine with other elements easily at room temperature.

D₂O can be prepared by exhaustive or prolonged electrolysis of water or as a by-product in some fertilizer industries.

Some of the physical properties in which D₂O differs from H₂O are

Reactions of D₂O showing the exchange of hydrogen with deuterium are

HCl + D₂O → HOD + DCl

NaOH + D₂O → NaOD + HOD

NH₄Cl + D₂O → NH₃DCl + HOD

H₂O₂ is concentrated by distillation under reduced pressure. It can be further concentrated by careful distillation under low pressure.

H₂O₂ has a non polar structure whereas H₂O has a polar structure which can be drawn as given below-

3 important uses of H₂O are-

1. It is often used in bleach for hair and it is also used as an antiseptic and is old in market as perhydrol.

2. It is used to manufacture chemicals like sodium perborate and per-carbonate, which are present in high quality detergents.

3. It is used in industry as a bleaching agent for textiles, paper pulp, leather, etc.

(i)It can be prepared by acidification of barium peroxide which is done by adding H₂SO₄. The excess water molecules then produced are evaporated under reduced pressure. The reaction for which is as follows-

BaO.8H₂O (s) + H₂SO₄ (aq) → BaSO₄(s) + H₂O₂ (aq) + 8H₂O (l)

H₂O₂ acts as an oxidizing as well as reducing agent in both acidic and alkaline medium. This can be illustrated by following examples-

A. Oxidizing action in acidic medium-

2Fe²⁺ + 2H⁺ + H₂O₂ → 2Fe³⁺ + 2H₂O

B. reducing agent in acidic medium-

2MnO₄^- + 6H⁺ + 5H₂O₂ → 2Mn²⁺ + 8H₂O + 5O₂

C. oxidizing agent in basic medium-

2Fe²⁺ + H₂O₂ → 2Fe³⁺ + 2OH^-

D. reducing agent in basic medium-

I₂ + H₂O₂ + OH^-→ 2I^- +2H₂O + O₂

(ii) Acidic property of Hydrogen peroxide-

It turns blue litmus red but its dilute solution is almost neutral to litmus which means it is a weak acid.

Its acidic nature can be further supported by its reaction with NaOH (base).

NaOH + H₂O₂ → NaHO₂ + 2H₂O

Since, H₂O₂ is a weak acid therefore CO₂ is not produced as final product.

Given, V = 2L and Molarity = 5M

We know that Molar mass of H₂O₂ = 34g

Molarity (M) =

=

5 =

W= 340g

Therefore, mass of hydrogen peroxide will be present in 2 litres of a 5 molar solution is 340g.

Mass of oxygen which will be liberated by the decomposition of 200 mL of this solution can be calculated as shown below.

No. of moles =

Molar volume = volume at STP = 22.4 L or 22400ml

No. of moles = = 0.0089

2H₂O₂ → 2H₂O + O₂

From the above equation it is clear that,

2 moles of H₂O₂ → 1 mole of O₂

0.0089 moles of H₂O₂ → moles of O₂

Now, we know mass = no. of moles of O₂ × molecular mass of O₂

= = 0.15g

Mass of oxygen which will be liberated by the decomposition of 200 mL of this solution is 0.15g.

‘A’ is Hydrogen peroxide i.e. H₂O_2.

Hydrogen peroxide contains H and O elements only. It decomposes slowly on exposure of light whose equation is as follows- 2H₂O₂ (l)→ 2H₂O (l) + O₂ (g)

And that’s why it is stored in wax-lined glass or plastic vessels in dark or urea can be added as a stabilizer.

(i) Possible structure of hydrogen peroxide is

(ii) Chemical equations for its decomposition reaction in light is as follows-

2H₂O₂ (l)→ 2H₂O (l) + O₂ (g)

The ionic hydride of an alkali metal which has a significant covalent character and is almost unreactive towards oxygen and chlorine at moderate temperatures is Lithium hydride (LiH).

It is therefore used in synthesis of other useful hydrides.

Reaction of LiH with Al₂Cl₆ is- 8LiH + Al₂Cl₆→ 2LiAlH₄ + 6LiCl

In this way LiH was useful in preparation of LiAlH₄ which is an important hydride (often used as reducing agent in many organic reactions).

The ionic solid crystal which is formed is non-volatile and non- conducting in nature. It reacts violently with water to produce dihydrogen gas.

The formula of this compound is NaH and its reaction with water is given below

NaH (s) + H₂O (aq) →NaOH (aq) +H₂ (g)

On electrolysis of the melt of this solid, following reaction will take place-

2H^- (melt) → H₂ (g) + 2e^-

The melt is able to conduct electricity and liberate Dihydrogen gas at anode on electrolysis.