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Hydrogen is the lightest atom on earth. It has only one electron. Henry Cavendish first discovered hydrogen in 1766. At first it was named 'Inflammable Air'. Later, Lavoisier named it 'Hydrogen'. Hydrogen can be found in free as well as in combined states. Protium, Deuterium and Tritium are the three stable isotopes of Hydrogen.
Keyterms: Atom, Earth, Electron, Protium, Deuterium, Tritium, Inflammable Air, Hydrogen, Group 17 elements, Halogens
Also read: Colloids
Hydrogen in the Periodic Table
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The first element in the periodic table is Hydrogen. It is placed in 'Group 1' of the periodic table on the basis of its configuration (1s¹). Hydrogen resembles Alkali Metals of 'Group 1'. But it resembles the Halogens of 'Group 17' as well. Though Hydrogen resembles Alkali Metals and Halogens it has some unique properties too.
Physical Properties of Hydrogen
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- Hydrogen is a colourless and odourless non-toxic gas.
- It has the lowest density among all the other gases.
- Hydrogen is the most abundant element in the universe. It is present in almost all molecules of living things.
- Hydrogen bonds with oxygen to form water (H2O).
- Hydrogen stores energy.
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Chemical Properties of Hydrogen
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- Hydrogen is the first element of the periodic table. It belongs to Group 1 and Period 1.
- The atomic number of Hydrogen is 1.
- Under standard conditions for temperature and pressure it is found in gaseous state.
- The electronic components configuration of Hydrogen is 1s1.
- Melting point of Hydrogen is -259.16° C.
- Boiling point of Hydrogen is -252.879° C.
- Density of Hydrogen is 0.000082 gram cm-3.
- Atomic mass of Hydrogen is 1.008.
Isotopes of Hydrogen
Hydrogen has three isotopes. They are: Protium (H), Deuterium (D) and Tritium (T) among which Tritium is a radioactive isotope.
Preparation of Hydrogen
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As mentioned above, though Hydrogen resembles Alkali Metals and Halogens, it has some unique properties too. Generally, Hydrogen is a very reactive element. The preparation of Hydrogen can be classified into following categories.
Preparation of Hydrogen in Laboratory
Hydrogen is obtained when granulated Zinc (Zn) reacts with dilute Hydrochloric Acid (HCl).
Hydrogen can also be obtained when Zinc (Zn) reacts with aqueous alkali (NaOH).
Zn + 2NaOH → Na2ZnO2 + H2
Preparation of Hydrogen Commercially
There are several ways to prepare Hydrogen commercially. Some of those are:
- Hydrogen is obtained when acidified water is electrolysed using platinum electrodes.
- Pure Hydrogen can be obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
- Hydrogen is formed when steam reacts with hydrocarbons at very high temperatures in the presence of some catalyst.
Preparation of Dihydrogen
Some of the methods by which Dihydrogen is obtained are mentioned below:
- Dihydrogen can be obtained when active metals like Sodium (Na), Potassium (K) react at room temperature. The general form of the reaction can be expressed as:
2(M) + 2H2O → 2 (M) OH + H2
Here, M = Na, K etc.
- Dihydrogen can be obtained by the electrolysis of acidified water.
2H2O → 2H2 + O2
- Dihydrogen can be obtained when metals like Nickel (Ni), Cobalt (Co), Iron (Fe) react with steam passed over red hot metals.
3Fe + 4H2O (Steam) → Fe3O4 + 4H2
- Dihydrogen can be obtained when water reacts with alkali and alkaline earth metal hydrides.
CaH2 + 2H2O → Ca(OH)2 + 2H2
- Dihydrogen can be obtained when metals like Aluminium (Al), Zinc (Zn) react with alkalies (NaOH or KOH).
Al + 2NaOH + H2O → 2NaAlO2 + 2H2
- Dihydrogen can be obtained by heating less active metals like Zinc (Zn), Magnesium (Mg), Aluminium (Al), Calcium (Ca), etc.
2Mg + 3H2O → Mg2O2 + H2
- Dihydrogen can be obtained when granulated Zinc (Zn) reacts with dilute H2SO4.
Zn + dil. H2SO4 → ZnSO4 + H2
Hydrides
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Hydrides are the compounds of hydrogen with less electronegative elements. The product formed as a result of reaction of hydrogen with any other element is said to be a hydride. There are three types of hydrides.
(i) Ionic Hydrides: Ionic Hydrides are formed when elements of Group 1 and Group 2 (Except Be and Mg) react with Hydrogen.
Example:
CaH2 + 2CO2 → (HCOO)2Ca
CaH2 + 2H2O → Ca(OH)2 + 2H2
(ii) Covalent or Molecular Hydrides: When hydrogen reacts with electronegative elements, covalent or molecular hydrides are formed. Covalent hydrides are generally found as liquid or gas. NH3, CH4 are some of the most common examples of molecular hydrides.
(iii) Metallic Hydrides: When a hydrogen molecule reacts with the d-block and f-block elements, metallic hydrides are formed. Metals of Group 7, 8 and 9 don't form hydrides. So this region of the periodic table is called the 'hydride gap'.
Magnesium (Mg), Cadmium (Cd) are some examples of metallic hydrides. Metallic hydrides are also known as interstitial hydrides.
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Things To Remember
- Hydrogen has the lowest density among all the other gases.
- Hydrogen is a colourless and odourless non-toxic gas.
- Hydrogen has three isotopes. They are: Protium (H), Deuterium (D) and Tritium (T)
- Hydrogen resembles Alkali Metals and Halogens.
- Hydrides are the compounds of hydrogen with less electronegative elements. Ionic, metallic and covalent hydrides are the three types of hydrides.
- Metallic hydrides are also known as interstitial hydrides.
Previous years questions
- Syngas is a mixture of
- When H2O2 is added to an acidified K2Cr2O7 solution
- The boiling point of methanol is greater than methyl thiol because
- The boiling point of para nitrophenol is greater than ortho nitrophenol, because:
- Heavy water is used as a
- The volume strength of 3.0NH2O2 is
- The strength of H2O2 (in g/L) in 11.2 volume solution of H2O2 is
- Water gas is a mixture of
- Para hydrogen is
- Which is the shortest wavelength line in the Lyman series of the hydrogen spectrum?
- H2O is liquid while H2S is a gas due to
- Hardness of water is due to presence of salts of:
- The angular momentum of electron in 3d orbital of an atom is
- What are the products of auto-photolysis of water?
- The hydrides of the first elements in groups 15−17, namely NH3,H2O and HF respectively show abnormally high values for melting and boiling points. This is due to
- In transforming 0.01 mole of PbS to PbSO4, the volume of ' 10 volume' H2O2 required will be
Sample Questions
Ques. Justify the position of hydrogen in the periodic table on the basis of its electronic configuration. (2 marks)
Ans. The electronic configuration of hydrogen is 1s1. Hydrogen exhibits dual behaviour i.e. it resembles both alkali metal and halogen, as it has only one electron in its outer shell. So it is placed in the first position of the periodic table.
Ques. Why does hydrogen occur in a diatomic form rather than in a monoatomic form under normal conditions? (2 marks)
Ans. The ionization enthalpy of H-H bond is very high. This indicates that hydrogen has a low tendency to form H+ ions. Hence its tendency to exist in a monatomic form is low. So hydrogen forms a covalent bond with another hydrogen atom and exists as a diatomic molecule.
Ques. How many isotopes are there of hydrogen? Name them. (2 marks)
Ans. There are three isotopes of hydrogen. These are :
Protium (H),
Deuterium (D), and
Tritium (T).
Ques. What are the commercial methods of preparing hydrogen? (2 marks)
Ans. The commercial methods of preparing hydrogen are:
- By electrolysing acidified water using platinum electrodes.
- By electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
- By letting steam react with hydrocarbons at very high temperatures in the presence of some catalyst.
Ques. How can pure hydrogen be obtained? (2 marks)
Ans. Pure Hydrogen can be obtained by electrolysing warm aqueous barium hydroxide solution between nickel electrodes.
Ques. Give the formula of dihydrogen preparation by electrolysis of acidified water. (2 marks)
Ans. The formula of dihydrogen preparation by electrolysis of acidified water is:
2H2O → 2H2 + O2
Ques.How many types of hydrides are there? Name them. (2 marks)
Ans. There are three types of hydrides. These are
- Ionic Hydrides
- Covalent or Molecular Hydrides
- Metallic Hydrides
Ques.Write the names of isotopes of hydrogen. What is the mass ratio of these isotopes? (2 marks)
Ans. Protium – 11h
Deuterium – 21H or D
Tritium – 31H or T
Mass ratio of Protium : Deuterium : Tritium
= 1:2:3
Ques. How can the production of dihydrogen obtained from ‘Coal gasification’ be increased? (2 marks)
Ans. The production of dihydrogen in coal gasification can be increased by reacting CO(y) present in syngas with steam in the presence of iron chromate catalysts.
CO(g) + H2O(g) →(FeCrO4 (catalyst))(673K) CO2(g) + H2(g)
With the removal of CO2 the reaction shifts in the forward direction and thus, the production of dihydrogen will be increased.
Ques. Describe the bulk preparation of dihydrogen by electrolytic method. What is the role of an electrolyte in this process? (2 marks)
Ans. In bulk, hydrogen can be produced by electrolysis of acidified water using Pt electrodes.
2H2O(l) →(Electrolysis)(Traces of acid or base) 2H2(g) + O2(g)
Electrolyte is added to increase the dissociation of water.
Ques. What do you understand by (i) Electron-deficient (ii) Electron-precise (iii) Electron-rich compounds of hydrogen? Provide justification with suitable examples. (2 marks)
Ans. (i) Electron deficient hydrides: Compounds in which the central atom has incomplete octet, are called electron deficient hydrides. For example, BeH2, BH3 are electron deficient hydrides.
(ii) Electron precise hydrides: Those compounds in which exact number of electrons are present in central atom or the central atom contains complete octet are called precise hydrides e.g., CH4, SiH4, GeH4 etc. are precise hydrides.
(iii) Electron rich hydrides: Those compounds in which the central atom has one or more lone pair of excess electrons are called electron rich hydrides, e.g.,NH3, H2O.
Ques. What characteristics do you expect from an electron-deficient hydride with respect to its structure and chemical reaction? (2 marks)
Ans. It is expected to be a Lewis acid. They are likely to accept electrons to become stable. They can form a coordinate bond with electron rich compounds.
2NaH(s) + B2H6(g) → (Diethyl ether) 2Na+ [BH4]- (s) (Sod. borohydride)
Ques. What do you understand about the term ‘non-stoichiometric hydrides’? Do you expect this type of hydrides to be formed by alkali metals? Justify your answer. (2 marks)
Ans. Those hydrides which do not have fixed composition are called non-stoichiometric hydrides, and the composition varies with temperature and pressure. This type of hydrides are formed by d and /block elements. They cannot be formed by alkali metals because alkali metal hydrides form ionic hydrides.
Ques How do you expect the metallic hydrides to be useful for hydrogen storage? Explain. (2 marks)
Ans. In metallic hydrides, hydrogen is adsorbed as H-atoms. Due to the adsorption of H atoms the metal Lattice expands and becomes unstable. Thus, when metallic hydride is heated, it decomposes to form hydrogen and finely divided metal. The hydrogen evolved can be used as fuel.
Ques. How does the atomic hydrogen or oxy-hydrogen torch function for cutting and welding purposes ? Explain. (2 marks)
Ans. When hydrogen is burnt in oxygen the reaction is highly exothermic, it produces a very high temperature nearly 4000°C which is used for cutting and welding purposes.
Ques. Saline hydrides are known to react with water violently producing fire. Can CO2, a well known fire extinguisher, be used in this case? Explain. (2 marks)
Ans. No. Because if saline hydrides react with water the reaction will be highly exothermic thus the hydrogen evolved in this case can catch fire. CO2 cannot be used as a fire extinguisher because CO2 will get absorbed in alkali metal hydroxides.
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