Given below are two statements :
One is labelled as Assertion A and other is labelled as Reason R.
A

Q: Why doesn't $\text{H}_2\text{O}_2$ just have a flat, planar structure?

The oxygen atoms have two lone pairs each. To minimize the strong repulsion between these lone pairs, the O-H bonds are pushed into different planes, creating the non-planar open-book shape.

Q: How can the phase of a substance change its internal bond angles?

In the gas phase, molecules are isolated. But in the solid phase, they are packed tightly in a crystal lattice. The strong intermolecular hydrogen bonds formed with neighboring molecules exert physical forces that twist the molecule, altering its dihedral angle.

Q: Is it easy to confuse the gas and solid phase angles?

Yes, it's a classic trap! Just remember that in the gas phase, the molecule is 'free' and relaxed, so the angle is wider ($111.5^\circ$). In the solid phase, it's 'squeezed' by neighbors, so the angle is smaller ($90.2^\circ$).

Q: Does the O-O bond length change as well?

Yes, slightly. The O-O bond length is $147.5 \text{ pm}$ in the gas phase and decreases to $145.8 \text{ pm}$ in the solid phase, again due to the influence of crystal packing and hydrogen bonding.

Q: What exactly is a dihedral angle?

Imagine two intersecting planes (like the two covers of a book). The dihedral angle is the angle between these two planes. In $\text{H}_2\text{O}_2$, it's the angle between the plane containing the first O-H bond and the plane containing the second O-H bond.

Question

Given below are two statements :
One is labelled as Assertion A and other is labelled as Reason R.
**Assertion (A)** The dihedral angles in $	ext{H}_2	ext{O}_2$ in gaseous phase is $90.2^\circ$ and in solid phase is $111.5^\circ$.
**Reason (R)** The change in dihedral angle in solid and gaseous phase is due to the difference in the intermolecular forces.
Choose the most appropriate answer from the options given below for A and R.

Options:

(A) A is correct but R is not correct.

(B) Both A and R are correct but R is not the correct explanation of A.

(C) Both A and R are correct and R is the correct explanation of A.

(D) A is not correct but R is correct.

Sigma Percentile · Accepted Answer

### The Open-Book Mystery of Hydrogen Peroxide

Hydrogen peroxide ($	ext{H}_2	ext{O}_2$) is a fascinating molecule that often defies our simple two-dimensional expectations. If you were to draw it on a piece of paper, you might be tempted to sketch a flat, linear, or zig-zag chain. However, reality is much more three-dimensional.

Due to the presence of two lone pairs of electrons on each oxygen atom, there is significant lone pair-lone pair repulsion. To minimize this repulsive energy, the molecule twists itself into a non-planar geometry. Imagine a slightly opened book: the two oxygen atoms lie along the spine, and the two hydrogen atoms lie on the separate pages. The angle between these two "pages" is known as the **dihedral angle**.

### Gas Phase: The Relaxed State

When $	ext{H}_2	ext{O}_2$ is in the gaseous phase, the molecules are isolated and far apart from one another. They are free from the influence of neighboring molecules. In this relaxed, unperturbed state, the dihedral angle naturally settles at **$111.5^\circ$**.

This wide angle is the molecule's preferred geometry when it only has to worry about its own internal electron repulsions.

### Solid Phase: The Squeeze of Hydrogen Bonding

Now, imagine cooling the gas down until it freezes into a solid crystal lattice. The environment changes drastically. The $	ext{H}_2	ext{O}_2$ molecules are now packed tightly together. Because hydrogen peroxide has highly electronegative oxygen atoms bonded to hydrogen, it forms extensive and strong **intermolecular hydrogen bonds** with its neighbors.

These external forces act like invisible hands, squeezing and twisting the molecule to fit optimally into the crystal structure. As a result of this intense intermolecular hydrogen bonding, the dihedral angle compresses significantly, dropping down to **$90.2^\circ$**.

### Decoding the Assertion and Reason

Let's look at the statements provided in the question.

**Assertion (A)** claims that the dihedral angle in the gaseous phase is $90.2^\circ$ and in the solid phase is $111.5^\circ$.

As we just explored, this is a classic trap! The examiners have deliberately swapped the values. The gas phase is the wider angle ($111.5^\circ$), and the solid phase is the compressed angle ($90.2^\circ$). Therefore, **Assertion (A) is absolutely false**.

**Reason (R)** states that the change in the dihedral angle between the solid and gaseous phases is due to the difference in intermolecular forces.

This is a perfectly accurate scientific explanation. The strong hydrogen bonding present in the solid phase (which is absent in the gas phase) is exactly what causes the structural deformation. Therefore, **Reason (R) is true**.

### The Final Verdict

By carefully analyzing the physical chemistry behind the molecular structure, we can confidently conclude that the Assertion is incorrect, but the Reason provides a valid scientific fact. This leads us directly to the correct option: **A is not correct but R is correct**.

Select Answer:

Visualized Solution

$Structure of H_{2} O_{2}$

$Gas Phase Structure$

$Solid Phase Structure$

$Evaluating Assertion (A)$

$Evaluating Reason (R)$

$Final Conclusion$

The Sigma Insight: Hydrogen, Hydrides and Water

The Open-Book Mystery of Hydrogen Peroxide

Gas Phase

Solid Phase

Decoding the Assertion and Reason

The Final Verdict

Similar Questions

Which one of the following statements is incorrect ?

Which one of the following statements about water is false?

The correct statements about $H_{2} O_{2}$ are (A) used in the treatment of effluents. (B) used as both oxidising and reducing agents. (C) the two hydroxyl groups lie in the same plane. (D) miscible with water. Choose the correct answer from the options given below.

The volume strength of $1 M H_{2} O_{2}$ is (Molar mass of $H_{2} O_{2} = 34 g mol^{- 1}$ )

Hydrogen peroxide, in the pure state, is