Question

Para hydrogen is:
(A) Less stable than ortho hydrogen
(B) More stable than ortho hydrogen
(C) As stable as ortho hydrogen
(D) None of these

Answer 1

Hint: Ortho hydrogen and para hydrogen are the isomers of molecular hydrogen in which ortho is in triplet state and para isomer is singlet state. The alignment of the proton nuclear spins of the two H-atoms will decide the stability of the isomers.

Complete step by step answer:
Here, we are being asked to compare the stability of the para and ortho hydrogen.
- Molecular hydrogen exists in two isomeric forms namely ortho hydrogen and para hydrogen.
- In the structure of ortho hydrogen, the proton nuclear spins are aligned parallel to each other. While in para hydrogen, the proton nuclear spins are aligned antiparallel to each other.
- These two forms of molecular hydrogen are also referred to as spin isomers.
- Now, due to antiparallel spin arrangement, para hydrogen has less energy and thus, they are more stable than ortho hydrogen.
- It has been found that at room temperature, due to thermal excitation, the ratio of isomers of molecular hydrogen is 3:1 (Ortho hydrogen : Para hydrogen).
- We can also say that a triplet state of molecular hydrogen is known as ortho hydrogen and singlet state is known as para hydrogen.
- It has been found by the experiments that it is not possible to have % orthohydrogen in the mixture more than 75% by heating.
- The existence of para hydrogen increases when the temperatures are low.
- Thus, we can conclude that ortho hydrogen is less stable than para hydrogen.
So, the correct answer is (B).

Note: Do not consider these as the ortho and para hydrogens of the mono substituted benzene ring. Here, they are talking about the isomers of the molecular hydrogen. Remember that in ortho, nuclear spins are parallel and in para, nuclear spins as antiparallel.