General Consideration
In atomic form it consists of only one proton and one electron.
In elemental form it exists as a diatomic (H2) molecule and is called dihydrogen.
Hydrogen has similarity with alkali metals and halogens
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Similarity with alkali metals
1. Reducing nature
2. Electronic configuration
3. Electropositive nature
4. Oxidation state
Similarity with halogens
1. Electronic configuration
2. Electronegative nature
3. Oxidation state
4. I.P.
5. Formation of covalent compounds
Isotopes of Hydrogen
1. Isotopes have the same electronic configuration,
2. They have almost the same chemical properties.
3. The only difference is in their rates of reactions, mainly due to their different enthalpy of bond dissociation T2 > D2 > H2
4. Physical properties these isotopes differ considerably due to their large mass differences.
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| Properties | Hydrogen | Deuterium | Tritium |
|---|---|---|---|
| Relative abundance(%) | 99.985 | 0.0156 | 10⁻¹⁵ |
| Relative Atomic Mass(g mol⁻¹) | 1.008 | 2.014 | 3.016 |
| Nuclear spin quantum number | 1/2 | 1 | 1/2 |
| Radioactive/Stability | Non radioactive, Stable | Non radioactive, Stable | Radioactive, Unstable(t1/2=12.33y) |
Dihydrogen, H₂
Laboratory Preparation
By the reaction of granulated zinc with dilute hydrochloric acid.
Zn + 2H+ → Zn2+ + H2
By the reaction of zinc with aqueous alkali.
Zn + 2NaOH → Na2ZnO2 + H2
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Commercial Preparation
Electrolysis of acidified water using platinum electrodes gives hydrogen
2H2O(l) → 2H2(g) + O2(g)
High purity (>99.95%) dihydrogen is obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes
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Chemical
Properties
The H–H bond
dissociation enthalpy is the highest for a single bond between two atoms of any
element.
Reaction with
halogens, dioxygen, metal, dinitrogen
Uses of
Dihydrogen
1.
Synthesis
of ammonia, methanol, HCl
2.
Manufacture
of vanaspati fat
3.
In
metallurgical processes
4.
Fuel
cells for generating electrical energy
5.
Cutting
and welding purposes.
6.
Rocket
fuel in space research.
Hydrides
The hydrides are classified into three categories:
- Ionic or saline or salt-like hydrides
- Covalent or molecular hydrides
- Metallic or non-stoichiometric hydride
Ionic or saline or salt like hydrides
Most of the s-block elements
LiH, BeH2 and MgH2
LiH, BeH2 are covalent character
Lithium hydride is rather unreactive
Rest of are ionic hydrides are crystalline, non-volatile and non-conducting in solid state. However, their melts conduct electricity.
Covalent or Molecular Hydride
1. p-block elements form this types of hydride
2. Most familiar examples are CH4, NH3, H2O and HF
3. Being covalent, they are volatile compounds
(i) Electron-deficient
1. In fact all elements of group 13 will form electron-deficient compounds.
2. They act as Lewis acids
3. Diborane (B2H6) is an example.
(ii) Electron-precise
1. All elements of group 14 form such compounds (CH4)
2. Electron-precise compounds have the required number of electrons to write their conventional Lewis structures.
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(iii) Electron-rich hydrides
1. Electron-rich hydrides have excess electrons which are present as lone pairs.
2. Elements of group 15-17 form such compounds. (NH3, H2O and HF, has one, two and there lone pair respectively).
3. They will behave as Lewis bases
Metallic or Non-stoichiometric (or Interstitial ) Hydrides
1. These are formed by many d-block and f-block elements.
2. The metals of group 7, 8 and 9 do not form hydride.
3. From group 6, only Chromium forms CrH.
4. They are almost always nonstoichiometric, being deficient in hydrogen
5. They were termed as interstitial hydride
6. The property of absorption of hydrogen on transition metals is widely used in catalytic reduction / hydrogenation reactions
LaH2.87 YbH2.55 YiH1.5-1.8 ZrH1.3-1.75
Water
Human body has about 65% and some plants have as much as 95% water
Extensive hydrogen bonding
High freezing point, high boiling point, high heat of vaporisation and high heat of fusion in comparison to H2S and H2Se
Heavy Water (D₂O):
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.
Dielectric constant of H2O is more than D2O
How many hydrogen-bonded water molecule(s) are associated in CuSO4.5H2O
Density of ice is less than that of water. Therefore, an ice cube floats on water.
Ice has a highly ordered three dimensional hydrogen bonded structure
Each oxygen atom is surrounded tetrahedrally by four other oxygen atoms at a distance of 276 pm
Chemical Properties of Water
1. Amphoteric Nature
2. Redox Reactions
3. Hydrolysis
4. Hydrates
Hard and Soft Water
Hard water:
Presence of calcium and magnesium salts in the form of
hydrogen carbonate, chloride and sulphate in water makes water ‘hard’.
Hard water does not give lather with soap.
Soft water
Water free from soluble salts of calcium and magnesium is called Soft water. It gives lather with soap easily.
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Hardness of water is of two types:
(i) Temporary hardness (ii) Permanent hardness.
Temporary hardness is due to the presence of magnesium and calcium hydrogen carbonates.
It is due to the presence of soluble salts of magnesium and calcium in the form of chlorides and sulphates in water.
Hydrogen Peroxide (H₂O₂)
Preparation
Barium peroxide
BaO2.8H2O(s) + H2SO4(aq) → BaSO4(s) + H2O2(aq) + 8H2O(l)
Peroxo Disulphate:
2HSO–4(aq) – Electrolysis → HO3SOOSO3H (aq) – Hydrolysis → 2HSO–4(aq) + 2H+ (aq) + H2O2 (aq)
Industrial method: by oxidation of 2-alklylanthraquinols.
2 – ethylanthraquinol ⇌ H2O2 + Oxidised Product
