PREPARATION OF DIHYDROGEN, $\mathrm{H}_2$

Laboratory Preparation of Dihydrogen

(i) By the reaction of granulated zinc with dilute hydrochloric acid.

$$ \mathrm{Zn}+2 \mathrm{H}^{+} \rightarrow \mathrm{Zn}^{2+}+\mathrm{H}_2 $$

(ii) By the reaction of zinc with aqueous alkali.

$$ \mathrm{Zn}+2 \mathrm{NaOH} \rightarrow \mathrm{Na}_2 \mathrm{ZnO}_2+\mathrm{H}_2 $$

Commercial Production of Dihydrogen

(i) Electrolysis of acidified water using platinum electrodes gives hydrogen.

$$ 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \xrightarrow[\text { Tracs of acdid/hase }]{\text { Eletrals }} 2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2(\mathrm{~g}) $$

(ii) High purity ( $>99.95 %$ ) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes. (iii) By the electrolysis of brine solution

At anode: $2 \mathrm{Cl}^{-}(\mathrm{aq}) \rightarrow \mathrm{Cl}_2(\mathrm{~g})+2 \mathrm{e}^{-}$ At cathode: $2 \mathrm{H}_2 \mathrm{O}$ (l) $+2 \mathrm{e}^{-} \rightarrow \mathrm{H}_2(\mathrm{~g})+2 \mathrm{OH}^{-}$(aq) The overall reaction is

$$ \begin{gathered} 2 \mathrm{Na}^{+}(\mathrm{aq})+2 \mathrm{Cl}^{-}(\mathrm{aq})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \ \downarrow \ \mathrm{Cl}_2(\mathrm{~g})+\mathrm{H}_2(\mathrm{~g})+2 \mathrm{Na}^{+}(\mathrm{aq})+2 \mathrm{OH}^{-}(\mathrm{aq}) \end{gathered} $$

(iv) Reaction of steam on hydrocarbons or coke at high temperatures in the presence of catalyst yields hydrogen.

Physical Properties

Dihydrogen is a colourless, odourless,tasteless, combustible gas. It is lighter than air and insoluble in water. Its other physical properties alongwith those of deuterium are

Chemical Properties

• The H–H bond dissociation enthalpy is the highest for a single bond between two atoms of any element because the dissociation of dihydrogen into its atoms is only ~0.081% around 2000K which increases to 95.5% at 5000K Reaction with halogens: It reacts with halogens, $\mathrm{X}_2$ to give hydrogen halides, $\mathrm{HX}$,

$$ \mathrm{H}_2(\mathrm{~g})+\mathrm{X}_2(\mathrm{~g}) \rightarrow 2 \mathrm{HX}(\mathrm{g}) \quad(\mathrm{X}=\mathrm{F}, \mathrm{Cl}, \mathrm{Br}, \mathrm{I}) $$

While the reaction with fluorine occurs even in the dark, with iodine it requires a catalyst. Reaction with dioxygen: It reacts with dioxygen to form water. The reaction is highly exothermic.

$$ \begin{aligned} 2 \mathrm{H}_2(\mathrm{~g})+\mathrm{O}_2 \text { (g) } \xrightarrow{\text { catalyst or hesting }} & 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) ; \ \Delta H^{\ominus} & =-285.9 \mathrm{~kJ} \mathrm{~mol}^{-1} \end{aligned} $$

Reaction with dinitrogen: With dinitrogen it forms ammonia.

$$ \begin{aligned} & 3 \mathrm{H}_2(\mathrm{~g})+\mathrm{N}_2(\mathrm{~g}) \xrightarrow[\mathrm{Fe}]{673 \mathrm{~K}, 200 \mathrm{~atm}} 2 \mathrm{NH}_3(\mathrm{~g}) \text {; } \ & \Delta H^{\ominus}=-92.6 \mathrm{~kJ} \mathrm{~mol}^{-1} \ & \end{aligned} $$

This is the method for the manufacture of ammonia by the Haber process. Reactions with metals: With many metals it combines at a high temperature to yield the corresponding hydrides

$$ \mathrm{H}_2(\mathrm{~g})+2 \mathrm{M}(\mathrm{g}) \rightarrow 2 \mathrm{MH}(\mathrm{s}) \text {; } $$

where $\mathrm{M}$ is an alkali metal Reactions with metal ions and metal oxides: It reduces some metal ions in aqueous solution and oxides of metals (less active than iron) into corresponding metals.

$$ \begin{aligned} & \mathrm{H} _2(\mathrm{~g})+\mathrm{Pd}^{2+}(\mathrm{aq}) \rightarrow \mathrm{Pd}(\mathrm{s})+2 \mathrm{H}^{+}(\mathrm{aq}) \ & \mathrm{yH} _2(\mathrm{~g})+\mathrm{M} _{\mathrm{x}} \mathrm{O} _{\mathrm{y}}(\mathrm{s}) \rightarrow \mathrm{xM}(\mathrm{s})+\mathrm{yH} _2 \mathrm{O}(\mathrm{l}) \end{aligned} $$

Reactions with organic compounds: It reacts with many organic compounds in the presence of catalysts to give useful hydrogenated products of commercial importance. For example :

(i) Hydrogenation of vegetable oils using nickel as catalyst gives edible fats (margarine and vanaspati ghee) (ii) Hydroformylation of olefins yields aldehydes which further undergo reduction to give alcohols.

$$ \begin{aligned} & \mathrm{H}_2+\mathrm{CO}+\mathrm{RCH}=\mathrm{CH}_2 \rightarrow \mathrm{RCH}_2 \mathrm{CH}_2 \mathrm{CHO} \ & \mathrm{H}_2+\mathrm{RCH}_2 \mathrm{CH}_2 \mathrm{CHO} \rightarrow \mathrm{RCH}_2 \mathrm{CH}_2 \mathrm{CH}_2 \mathrm{OH} \end{aligned} $$

Uses of Dihydrogen

• In the synthesis of ammonia which is used in the manufacture of nitric acid and nitrogenous fertilizers.
• In the manufacture of vanaspati fat by the hydrogenation of polyunsaturated vegetable oils like soyabean, cotton seeds etc.
• It is used in the manufacture of bulk organic chemicals, particularly methanol.

$$ \mathrm{CO}(\mathrm{g})+2 \mathrm{H}_2(\mathrm{~g}) \xrightarrow[\text { catalyst }]{\text { cobalt }} \mathrm{CH}_3 \mathrm{OH}(1) $$

• It is widely used for the manufacture of metal hydrides
• It is used for the preparation of hydrogen chloride, a highly useful chemical.
• In metallurgical processes, it is used to reduce heavy metal oxides to metals.
• It is used as a rocket fuel in space research.
• Dihydrogen is used in fuel cells for generating electrical energy.

HYDRIDES

Under certain reaction conditions, $\mathrm H _2$ combines with almost all elements, except noble gases and form hydrides. $\mathrm{EH} _{\mathrm{x}}$ (e.g., $\mathrm{MgH} _2$ ) or $\mathrm{E} _{\mathrm{m}} \mathrm{H} _{\mathrm{n}}$ (e.g., $\mathrm{B} _2 \mathrm{H} _6$ ).

Types:

(i) Ionic or saline or saltlike hydrides (ii) Covalent or molecular hydrides (iii) Metallic or non-stoichiometric hydrides

WATER

Physical Properties of Water

• It is a colourless and tasteless liquid. Its physical properties along with the physical properties of heavy water.
• In comparison to other liquids, water has a higher specific heat, thermal conductivity, surface tension, dipole moment and dielectric constant, etc.

Structure of Ice

Ice has a highly ordered three dimensional hydrogen bonded structure as shown Examination of ice crystals with

X-rays shows that each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm. Hydrogen bonding gives ice a rather open type structure with wide holes. These holes can hold some other molecules of appropriate size interstitially

Chemical Properties of Water

Amphoteric Nature:

It has the ability to act as an acid as well as a base i.e., it behaves as an amphoteric substance .

$$ \begin{array}{lll} \mathrm{H}_2 \mathrm{O}(1)+\mathrm{NH}_3(\mathrm{aq}) \to & \mathrm{OH}^{-}(\mathrm{aq})+\mathrm{NH}_4^{+}(\mathrm{aq}) \ \mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{H}_2 \mathrm{~S}(\mathrm{aq}) \to & \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{HS}^{-}(\mathrm{aq}) \end{array} $$

The auto-protolysis (self-ionization) of water takes place as follows :

$$ \mathrm{H}_2 \mathrm{O}(1)+\mathrm{H}_2 \mathrm{O}(1) \to \quad \mathrm{H}_3 \mathrm{O}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq}) $$

acid-1 base-2 acid-2 base-1 (acid) (base) (conjugate (conjugate acid) base)

Redox Reactions Involving Water:

Water can be easily reduced to dihydrogen by highly electropositive metals.

$$ 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})+2 \mathrm{Na}(\mathrm{s}) \rightarrow 2 \mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_2(\mathrm{~g}) $$

Thus, it is a great source of dihydrogen. Water is oxidised to $\mathrm{O}_2$ during photosynthesis.

$$ \begin{aligned} 6 \mathrm{CO} _2(\mathrm{~g})+12 \mathrm{H} _2 \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{C} _6 \mathrm{H} _{12} \mathrm{O} _6(\mathrm{aq})+ & 6 \mathrm{H} _2 \mathrm{O}(\mathrm{l}) \ & +6 \mathrm{O} _2(\mathrm{~g}) \end{aligned} $$

With fluorine also it is oxidised to $\mathrm{O}_2$.

$$ 2 \mathrm{~F}_2(\mathrm{~g})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow 4 \mathrm{H}^*(\mathrm{aq})+4 \mathrm{~F}^{-}(\mathrm{aq})+\mathrm{O}_2(\mathrm{~g}) $$

Hydrolysis Reaction:

Due to high dielectric constant, it has a very strong hydrating tendency. It dissolves many ionic compounds. However, certain covalent and some ionic compounds are hydrolysed in water.

$$ \begin{aligned} & \mathrm{P} _4 \mathrm{O} _{10}(\mathrm{~s})+6 \mathrm{H} _2 \mathrm{O}(1) \rightarrow 4 \mathrm{H} _3 \mathrm{PO} _4(\mathrm{aq}) \ & \mathrm{SiCl} _4(1)+2 \mathrm{H} _2 \mathrm{O}(1) \rightarrow \mathrm{SiO} _2(\mathrm{~s})+4 \mathrm{HCl}(\mathrm{aq}) \end{aligned} $$

HYDROGEN PEROXIDE $\left(\mathrm{H}_2 \mathrm{O}_2\right)$

Hydrogen peroxide is an important chemical used in pollution control treatment of domestic and industrial effluents.

Preparation

(i) Acidifying barium peroxide and removing excess water by evaporation under reduced pressure gives hydrogen peroxide.

$$ \begin{array}{r} \mathrm{BaO}_2 \cdot 8 \mathrm{H}_2 \mathrm{O}(\mathrm{s})+\mathrm{H}_2 \mathrm{SO}_4(\mathrm{aq}) \rightarrow \mathrm{BaSO}_4(\mathrm{~s})+ \ \mathrm{H}_2 \mathrm{O}_2(\mathrm{aq})+8 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \end{array} $$

(ii) Peroxodisulphate, obtained by electrolytic oxidation of acidified sulphate solutions at high current density, on hydrolysis yields hydrogen peroxide.

$$ \begin{aligned} & 2 \mathrm{HSO}_4^{-}(\mathrm{aq}) \xrightarrow{\text { Electrolysis }} \mathrm{HO}_3 \mathrm{SOOSO}_3 \mathrm{H}(\mathrm{aq}) \ & \xrightarrow{\text { Hydrolysis }} 2 \mathrm{HSO}_4^{-}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{H}_2 \mathrm{O}_2(\mathrm{aq}) \ & \end{aligned} $$

This method is now used for the laboratory preparation of $\mathrm{D}_2 \mathrm{O}_2$.

$$ \mathrm{K}_2 \mathrm{~S}_2 \mathrm{O}_8(\mathrm{~s})+2 \mathrm{D}_2 \mathrm{O}(\mathrm{l}) \rightarrow 2 \mathrm{KDSO}_4(\mathrm{aq})+\mathrm{D}_2 \mathrm{O}_2(\mathrm{l}) $$

(iii) Industrially it is prepared by the autooxidation of 2 -alklylanthraquinols.

Physical Properties

• In the pure state $\mathrm{H}_2 \mathrm{O}_2$ is an almost colourless (very pale blue) liquid.
• $\mathrm{H}_2 \mathrm{O}_2$ is miscible with water in all proportions and forms a hydrate $\mathrm{H}_2 \mathrm{O}_2 \cdot \mathrm{H}_2 \mathrm{O}$ (mp $221 \mathrm{~K}$ ).
• A $30 %$ solution of $\mathrm{H}_2 \mathrm{O}_2$ is marketed as ’ 100 volume’ hydrogen peroxide. It means that one millilitre of $30 % \mathrm{H}_2 \mathrm{O}_2$ solution will give $100 \mathrm{~mL}$ of oxygen at STP. Commercially marketed sample is $10 \mathrm{~V}$, which means that the sample contains $3 % \mathrm{H}_2 \mathrm{O}_2$.

Structure

It has a non-planar structure.

Chemical Properties

• It acts as an oxidising as well as reducing agent in both acidic and alkaline media. (i) Oxidising action in acidic medium

$$ \begin{aligned} 2 \mathrm{Fe}^{2+}(\mathrm{aq})+2 \mathrm{H}^{+}(\mathrm{aq})+ & \mathrm{H}_2 \mathrm{O}_2(\mathrm{aq}) \rightarrow \ & 2 \mathrm{Fe}^{3+}(\mathrm{aq})+2 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \ \mathrm{PbS}(\mathrm{s})+4 \mathrm{H}_2 \mathrm{O}_2(\mathrm{aq}) \rightarrow & \mathrm{PbSO}_4(\mathrm{~s})+4 \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \end{aligned} $$

(ii) Reducing action in acidic medium

$$ \begin{aligned} & 2 \mathrm{MnO}_4^{-}+6 \mathrm{H}^{+}+5 \mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{Mn}^{2+}+8 \mathrm{H}_2 \mathrm{O}+5 \mathrm{O}_2 \ & \mathrm{HOCl}+\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{H}_3 \mathrm{O}^{+}+\mathrm{Cl}^{-}+\mathrm{O}_2 \end{aligned} $$

(iii) Oxidising action in basic medium

$$ \begin{aligned} & 2 \mathrm{Fe}^{2+}+\mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{Fe}^{3+}+2 \mathrm{OH}^{-} \ & \mathrm{Mn}^{2+}+\mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{Mn}^{4+}+2 \mathrm{OH}^{-} \end{aligned} $$

(iv) Reducing action in basic medium

$$ \begin{aligned} & \mathrm{I}_2+\mathrm{H}_2 \mathrm{O}_2+2 \mathrm{OH}^{-} \rightarrow 2 \mathrm{I}^{-}+2 \mathrm{H}_2 \mathrm{O}+\mathrm{O}_2 \ & 2 \mathrm{MnO}_4^{-}+3 \mathrm{H}_2 \mathrm{O}_2 \rightarrow 2 \mathrm{MnO}_2+3 \mathrm{O}_2+ \ & 2 \mathrm{H}_2 \mathrm{O}+2 \mathrm{OH}^{-} \end{aligned} $$

Storage

$\mathrm{H}_2 \mathrm{O}_2$ decomposes slowly on exposure to light.

$$ 2 \mathrm{H}_2 \mathrm{O}_2(\mathrm{l}) \rightarrow 2 \mathrm{H}_2 \mathrm{O}(\mathrm{l})+\mathrm{O}_2(\mathrm{~g}) $$

In the presence of metal surfaces or traces of alkali (present in glass containers), the above reaction is catalysed. It is, therefore, stored in wax-lined glass or plastic vessels in dark. Urea can be added as a stabiliser. It is kept away from dust because dust can induce explosive decomposition of the compound.

Uses

• In daily life it is used as a hair bleach and as a mild disinfectant. As an antiseptic it is sold in the market as perhydrol.
• It is used to manufacture chemicals like sodium perborate and per-carbonate, which are used in high quality detergents.
• It is used in the synthesis of hydroquinone, tartaric acid and certain food products and pharmaceuticals (cephalosporin) etc.

HEAVY WATER, $\mathrm{D}_2 \mathrm{O}$

• It is extensively used as a moderator in nuclear reactors and in exchange reactions for the study of reaction mechanisms.
• It can be prepared by exhaustive electrolysis of water or as a by-product in some fertilizer industries.
• It is used for the preparation of other deuterium compounds, for example:

$$ \begin{aligned} & \mathrm{CaC}_2+2 \mathrm{D}_2 \mathrm{O} \rightarrow \mathrm{C}_2 \mathrm{D}_2+\mathrm{Ca}(\mathrm{OD})_2 \ & \mathrm{SO}_3+\mathrm{D}_2 \mathrm{O} \rightarrow \mathrm{D}_2 \mathrm{SO}_4 \ & \mathrm{Al}_4 \mathrm{C}_3+12 \mathrm{D}_2 \mathrm{O} \rightarrow 3 \mathrm{CD}_4+4 \mathrm{Al}(\mathrm{OD})_3 \end{aligned} $$