Samacheer Class 11 Chemistry - Hydrogen

• Hydrogen resembles alkali metals as well as halogens.
• Hydrogen resembles more alkali metals than halogens.
• The electron affinity of hydrogen is much less than that of halogen atoms. Hence the tendency to form hydride ion is low compared to that of halogens.
• In most of its compounds hydrogen exists in a +1 oxidation state. Therefore it is reasonable to place the hydrogen in group 1 along with alkali metals as shown in the latest periodic table published by IUPAC.

In ice, each atom is surrounded tetrahedrally by four water molecules through hydrogen bonds. That is, the presence of two hydrogen atoms and two lone pairs of electrons on oxygen atoms in each water molecule allows the formation of a three-dimensional structure.
This arrangement creates an open structure, which accounts for the lower density of ice compared with water at 0°C. While in liquid water, unlike ice where hydrogen bonding occurs over a long-range, the strong hydrogen bonding prevails only in a short-range and therefore the denser packing. Hence, the ice cube sinks in water.

• They are the compounds in which hydrogen is attached to another element by sharing electrons.
• The most common examples of covalent hydrides are methane, ammonia, water, and hydrogen chloride.
• Molecular hydrides of hydrogen are further classified into three categories,
• Electron precise (CH 4, C 2 H 6, SiH 4, GeH 4 )
• Electron-deficient (B 2 H 6 ) and
• Electron-rich hydrides (NH 3, H 2 O)
• Since most of the covalent hydrides consist of discrete, small molecules that have relatively weak intermolecular forces, they are generally gases or volatile liquids.

The expected formulas for the hydrides of 4th-period elements MH 4 (electron precise). M 2 H 6 (electron-deficient) and MH 3 (electron-rich).
The trend in the formula is
* Electron precise hydrides – CH 4 C 2 H 6, SiH 4, GeH 4
* Electron deficient hydrides – B 2 H 6
* Electron rich hydrides – NH 3, H 2 O
The first two members of the series KH, CaH 2 are ionic hydrides whereas the other members of the series CH 4, C 2 H 6, SiH 4, B 2 H 6, NH 3 are covalent hydrides.

(i) the reaction of hydrogen with tungsten (VI) oxide NO 3 on heating.
(ii) hydrogen gas and chlorine gas.
(i) Hydrogen can be used to reduce metal oxide into metal. Hydrogen reduces tungsten(VI) oxide into tungsten.
WO 3 + 3H 2 → W + 3H 2 O
(ii) Hydrogen reacts with chlorine at room temperature under light gives hydrogen chloride.
H 2(g) + Cl 2(g) → 2HCl (g)

Hydrogen peroxide can act both as an oxidizing agent and a reducing agent. Oxidation is usually performed in an acidic medium while the reduction reactions are performed in a basic medium.
In acidic conditions:
H 2 O 2 + 2H + + 2e – → 2H 2 O (E° = +1.77 V)
For example
2FeSO 4 + H 2 SO 4 + H 2 O 2 → Fe 2 (SO 4 ) 3 + 2H 2 O
In basic conditions:
HO 2 – + OH – → O 2 + H 2 O + 2e – (E° = + 0.08V)
For Example,
2 KMnO 4(aq) + 3H 2 O 2(aq) → 2MnO 2 + 2KOH + 2H 2 O + 3O 2 (g)

• Heavy water (D 2 O) contains a proton and a neutron. This makes deuterium about twice as heavy as protium, but it is not radioactive. So heavy water is not radioactive.
• If you drink heavy water, you don’t need to worry about radiation poisoning. But it is not completely safe to drink, because the biochemical reaction in our cells is affected by the difference in the mass of hydrogen atoms.
• If you drink an appreciable volume of heavy water, you might feel dizzy because of the density difference. It would change the density of the fluid in your inner ear. So it is unlikely to drink heavy water.

The carbon monoxide of the water gas can be converted to carbon dioxide by mixing the gas mixture with more steam at 400°C and passed over a shift converter containing iron/copper catalyst. This reaction is called as water-gas shift reaction.
CO + H 2 O → CO 2 + H 2
The CO 2 formed in the above process is absorbed in a solution of potassium carbonate.
CO 2 + K 2 CO 3 + H 2 O → 2KHCO 3

Hydrogen resembles alkali metals in the following aspects.
* Electronic configuration 1s 1 as alkali metals have ns 1.
* Hydrogen forms unipositive H+ ion like alkali metals Na +, K +.
* Hydrogen form halides (HX), oxides (H 2 O) peroxide (H 2 O 2 ) like alkali metals (NaX. Na 2 O, Na 2 O 2 ).
* Hydrogen also acts as a reducing agent like alkali metals. Hydrogen resembles halogens in the following aspects.
* Hydrogen has a tendency to gain one electron to form hydride ion (H – ) as halogens to form halide ion. (X – ).
* Comparing the properties of hydrogen with alkali metals and with halogens, we can conclude that hydrogen resembles more alkali metals. In most of the compounds, hydrogen exists in a +1 oxidation state.
* Therefore, it is reasonable to place the hydrogen in group 1 along with alkali metals as shown in the latest periodic table published by IUPAC.

Atoms of the same element having same atomic number and different mass number are called isotopes.
Hydrogen has three naturally occurring isotopes, viz., protium ( 1 H 1 or H), deuterium ( 1 H 2 or D) and tritium ( 1 H 3 or T). Protium 1 H 1 is the predominant form (99.985 %) and it is the only isotope that does not contain a neutron. Deuterium, also known as heavy hydrogen, constitutes about 0.015 %. The third isotope, tritium is a radioactive isotope of hydrogen which occurs only in traces (~1 atom per 1018 hydrogen atoms). Due to the existence of these isotopes naturally occurring hydrogen exists as H 2, HD, D 2, HT, T 2, and DT.

• Heavy water is used as a moderator in nuclear reactors as it can lower the energies of fast-moving neutrons.
• D 2 O is commonly used as a tracer to study organic reaction mechanisms and the mechanism of metabolic reactions.
• It is also used as a coolant in nuclear reactors as it absorbs the heat generated.

When compounds containing hydrogen are treated with D 2 O, hydrogen undergoes an exchange for deuterium. This reaction is known as the exchange reaction of deuterium.
2NaOH + D 2 O → 2NaOD + HOD
HCl + D 2 O → DCl + HOD
NH 4 Cl + 4D 2 O → ND 4 Cl + 4HOD
These exchange reactions are useful in determining the number of ionic hydrogens present in a given compound. For example, when D 2 O is treated with hypo-phosphorus acid only one hydrogen atom is exchanged with deuterium. It indicates that it is a monobasic acid.
H 3 PO 2 + D 2 O → H 2 DPO 2 + HDO
It is also used to prepare some deuterium compounds:
Al 4 C 3 + 12D 2 O → 4Al(OD) 3 + 3CD 4
CaC 2 + 2D 2 O → Ca(OD) 2 + C 2 D 2
Mg 3 N 2 + 6D 2 O → 3Mg(OD) 2 + 2ND 3
Ca 3 P 2 + 6D 2 O → 3Ca(OD) 2 + 2PD 3

Para hydrogen can be converted into ortho hydrogen in the following ways:
* By treating with catalysts platinum or iron.
* Bypassing an electric discharge
* By heating > 800°C.
* By mixing with paramagnetic molecules such as O 2, NO, NO 2.
* By treating with nascent/atomic hydrogen.

• Deuterium is used to prepare heavy water which is used as a moderator in nuclear reactors.
• Deuterium exchange reactions are useful in determining the number of ionic hydrogens present in a given compound.
• It is also used to prepare some deuterium compounds.

High purity of hydrogen (>99.9%) is obtained by the electrolysis of water containing traces of acid or alkali or electrolysis of an aqueous solution of sodium hydroxide or potassium hydroxide using a nickel anode and iron cathode. This process is not economical for large-scale production.
At anode: 2OH – → H 2 O + ½ O 2 + 2e –
At cathode: 2H 2 O + 2e – → 2OH – + H 2
Overall reaction: H 2 O → H 2 + 14 O 2

When a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen, the bond is polarized. Due to this effect, the polarized hydrogen atom is able to form a weak electrostatic interaction with another electronegative atom present in the vicinity.
This interaction is called hydrogen bonding. Hence, NH 3 has an exceptionally high melting point and boiling point as compared to those of the hydrides of the remaining element of group 15 due to intermolecular hydrogen bonding.

• d block elements form metallic or interstitial hydrides, on heating with dihydrogen under pressure.
• Hydrogen atoms being small in size occupy some in the metallic lattice producing distortion without any change in its type.
• The densities of these hydrides are lower than those of metals from which they are formed since the crystal lattice expands due to the inclusion of dihydrogen.

Metallic hydrides are usually obtained by hydrogenation of metals and alloys in which hydrogen occupies the interstitial sites (voids). Most of the hydrides are non-stoichiometric with variable composition (TiH 1.5 – 1.8 and PdH 0.6 – 0.8 ), some are relatively light, inexpensive, and thermally unstable which makes them useful for hydrogen storage applications.

• The increasing magnitude of hydrogen bonding among NH 3, H 2 O, and HF is HF > H 2 O > NH 3
• The extent of hydrogen bonding depends upon electronegativity and the number of hydrogen atoms available for bonding.
• Among N, F, and O the increasing order of their electronegativities are N < O, H 2 O > NH 3.

Both in gas-phase and liquid-phase, the molecule adopts a skew confirmation due to repulsive interaction of the OH bonds with lone-pairs of electrons on each oxygen atom. Indeed, it is the smallest molecule known to show hindered rotation about a single bond.
H 2 O 2 has a non-polar structure. The molecular dimensions in the gas phase and solid phase differ as shown in the figure. Structurally, H 2 O 2 is represented by the dihydroxyl formula in which the two OH-groups do not lie in the same plane.
One way of explaining the shape of hydrogen peroxide is that the hydrogen atoms would lie on the pages of a partly opened book, and the oxygen atoms along the spine. In the solid phase of the molecule, the dihedral angle reduces to 90.2° due to hydrogen bonding and the O-O-H angle expands from 94.8° to 101.9°. Water has a bent structure and the H-O-H bond angle is 104.5°.