JEE Important Chapter - Hydrogen

Hydrogen is the most prevalent element in the cosmos and the third most plentiful element on the surface of the earth. It is also the simplest element, comprising only one proton and having only one electron in its orbit around the nucleus. In its elemental form, this element occurs as a diatomic molecule, which is H2. 

The primary worldwide issue about clean energy necessitates a thorough examination of hydrogen as a separate concept from that of other elements. The usage of hydrogen as an energy source can alleviate this worry to a larger extent.

It also has properties that are similar to both halogens and alkali metals. It has occupied a separate position in the periodic table because of its unique features. It also cannot be found in its natural state in the Earth's atmosphere.

JEE Main Chemistry Chapter-wise Solutions 2022-23 

Important Topics for Hydrogen

• Hydrogen

• Properties of Hydrogen

Important Concepts for Hydrogen

The Element - Hydrogen

• The first element in the periodic table is hydrogen. 

• Due to its property of supplying electrons (when H+ is generated) and removing electrons (when H- is formed), hydrogen is put in no distinct group.

• Group I (Alkali metals) includes hydrogen as follows:
  - Like other alkali metals with a (inert gas) 𝜈𝜎1 configuration, it has one electron in its (Outer) shell - 1𝜎1.
  - It produces monovalent H+ ions such as Li+ and Na+.
  - It has a valency of 1 as well.
  - Its oxide (H2O) has the same stability as Na2O.
  - It is a good reducing agent (in both the atomic and molecule states), similar to Na and Li.

• Hydrogen resembles halogens (Group VII A) in the following ways:
  - It is also diatomic Xi2.
  - By gaining one electron, it also produces anion H- like F- and Cl-.
  - H has a stable inert gas (He) configuration as XH4, and X2H6 has halogens and others.
  - Like F Cl, which is also one electron deficient than octet, H is one electron short of duplet (Stable configuration).
  - The (IE) of  H  is comparable to that of halogens.

• In comparison to alkali metals, H has an extremely high level (IE). In addition, the size of H+ is quite small when compared to the size of an alkali metal ion.
  - Because of its lower electron affinity, H forms stable hydrides only with strongly electropositive metals.

• Because of hydrogen's peculiar behaviour, it's difficult to assign it a specific location in the periodic table.
  - As a result, it's common to put it in both group I (together with alkali metals) and group VII (Along with halogens).

Physical Properties of Hydrogen

• It's a tasteless, odourless, and colourless gas. 

• It is water soluble to a degree. 

• It is extremely flammable.

• Atomic radius (pm) – 37
  - Ionic radius of H- ion (pm) – 210
  - Ionisation energy – 1312 kJ/mol.
  - Electron affinity – 72.8 kJ/mol.
  - Electronegativity – 2.1

Chemical Properties of Hydrogen

• Dihydrogen is a relatively stable gas that only dissociates into hydrogen atoms at temperatures above 2000 K.
  H2 → H + H.

• It has a very high bond dissociation energy of H, ΔH = 435.9 kJ/mol.

• It is not highly reactive due to its high bond dissociation energy. It does, however, mix with a variety of elements or compounds.

• Dihydrogen produces interstitial hydrides with transition metals (d-block elements) such as Pd, Ni, and Pt, in which the small molecules of hydrogen occupy the interstitial spaces in the crystal lattices of these hydrides.

• Saturated hydrocarbons are formed when H2 reacts with unsaturated hydrocarbons like ethylene and acetylene.

Hydrogen's Applications

• It is used as a reducing agent.

• Vegetable oils are hydrogenated in this process.

• In the form of liquid H2 used as rocket fuel.

• In the production of synthetic gasoline.

• Many compounds are made in this way.

Isotopes of Hydrogen

• The other two isotopes of hydrogen are (i) Hydrogen, (ii) Deuterium, and (iii) Tritium.

• Representation is given as:
  -  Hydrogen: 11H or H,
  -  Deuterium: 21H or D,
  - Tritium: 31H or T. 

• Of the three isotopes, only the Tritium is a radioactive isotope, the other two are non-radioactive. 

• Isotopes have similar chemical properties in general, but there are quantitative differences between them.

Different Forms of Hydrogen

• The dissociation of hydrogen molecules produces it. 

• Atomic hydrogen is extremely reactive and only stays stable for a fraction of a second. 

• It's made by passing dihydrogen gas through an electric arc formed between two tungsten rods at atmospheric pressure.

• The hydrogen gas formed in the reaction mixture when it comes into contact with the substance to be reacted with is referred to as nascent hydrogen. 

• It's also referred to as "newly born hydrogen." It has higher reactivity than regular hydrogen.

• Two atoms make up a hydrogen molecule. The nuclei of both atoms in each hydrogen molecule are spinning.

• Ortho hydrogen is a type of hydrogen molecule in which the spins of both nuclei are in the same direction.

• Para hydrogen is a type of hydrogen molecule in which the spins of each nuclei are in opposing orientations.

• Hydrogen is mostly para hydrogen at 0°K, which is more stable.

• The ratio of ortho and para hydrogen is 1:1 at the temperature of air liquefaction.

• The ratio of ortho to para hydrogen is 3:1 at ambient temperature.

• The ratio of ortho to para hydrogen can never exceed 3:1, even at extremely high temperatures.

Solved Examples from the Chapter

Question 1: Under typical circumstances, why does hydrogen exist in a diatomic form rather than a mono-atomic one?

Solution: The K-shell of hydrogen is complete (1s2) in diatomic form, making it highly stable.

Key Points to remember: Hydrogen has 1 electron.

Question 2: Discuss the chemical reactivity of dihydrogen as a result of the high enthalpy of the H-H bond.

Ans: The low chemical reactivity of dihydrogen is a result of the high enthalpy of the H-H bond. This is due to its small atomic size and the H-H bond's short bond length (74 pm). It will take high energy to break the H-H bond in a hydrogen molecule.

Key points to remember: Hydrogen atoms are small in size.

Solved Questions from the Previous Year Question Papers

Question 1: Identify the incorrect statement regarding heavy water.

(1) It reacts with SO3 to form deuterated sulphuric acid (D2SO4).

(2) It is used as a coolant in nuclear reactors.

(3) It reacts with CaC2 to produce C2D2 and Ca(OD)2.

(4) It reacts with Al4C3 to produce CD4 and Al(OD)3.

Solution: In nuclear reactors, heavy water is employed as a moderator to control the speed of neutrons. It isn't used to keep things cool.

As a result, option (2) is the proper response.

Question 2: Calculate the total number of neutrons in three hydrogen isotopes.

(1) One

(2) Two

(3) Three

(4) Four

Solution: Number of neutrons in three hydrogen isotopes = 0 + 1 + 2 = 3.

As a result, option (3) is the correct answer.

Question 3: Hydrogen peroxide acts both as an oxidising and as a reducing agent depending upon the nature of the reacting species. In which of the following cases does H2O2 act as a reducing agent in acid medium?

(1) MnO4–

(2) SO32-

(3) KI

(4) Cr2O72-

Solution: In a redox chemical process, a reducing agent is an element or compound that loses or gives an electron to an electron recipient, the oxidising agent.

H2O2 + MnO4– → Mn+2 + O2

As a result, option 1 is the correct answer.

Practice Questions

Question 1: Which hydrogen generators are used? 

(1) NaH

(2) HI

(3) S6H3

(4) None of the preceding

Answer: (1) NaH

Question 2: When metal hydride is treated with water, it produces

(1) H2O2 

(2) H2O 

(3) Acid 

(4) Hydrogen.

Answer: (4) Hydrogen.

Conclusion

Thus, among all the elements found in nature, hydrogen has the simplest atomic structure. It is made up of simply one proton and one electron in its atomic form. It is known as hydrogen in its elemental form, where it occurs as a diatomic (H2) molecule. It has the most number of compounds of any element. Do you know that the usage of hydrogen as a source of energy can help to alleviate some of the world's energy concerns? In fact, as known from this article, hydrogen is extremely important in the industry.

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