Preparation, Properties and Structure — Revision Notes

⚡ 30-Second Revision

Formula: — $\text{H}_2\text{O}_2$
Structure: — Non-planar, 'open book' shape. Dihedral angle: $\approx 111.5^circ$ (gas), $\approx 90.2^circ$ (solid).
Oxidation State of O: — $-1$.
Preparation (Industrial): — Auto-oxidation of 2-ethylanthraquinol.
Preparation (Lab): — $\text{BaO}_2 \cdot 8\text{H}_2\text{O} + \text{H}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + \text{H}_2\text{O}_2 + 8\text{H}_2\text{O}$.
Decomposition: — $2\text{H}_2\text{O}_2 (l) \rightarrow 2\text{H}_2\text{O} (l) + \text{O}_2 (g)$ (accelerated by light, heat, catalysts like $\text{MnO}_2$).
Oxidizing Agent: — O ($-1$) $\rightarrow$ O ($-2$) (e.g., $2\text{Fe}^{2+} + \text{H}_2\text{O}_2 + 2\text{H}^+ \rightarrow 2\text{Fe}^{3+} + 2\text{H}_2\text{O}$).
Reducing Agent: — O ($-1$) $\rightarrow$ O ($0$) (e.g., $2\text{MnO}_4^- + 5\text{H}_2\text{O}_2 + 6\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 5\text{O}_2 + 8\text{H}_2\text{O}$).
Bleaching: — Due to nascent oxygen release.
Storage: — Dark, plastic bottles, cool temperature, stabilizers.

2-Minute Revision

Hydrogen peroxide ($\text{H}_2\text{O}_2$) is a pale blue, viscous liquid with a unique non-planar 'open book' structure, characterized by a dihedral angle (approx. $111.5^circ$ in gas phase). Oxygen in $\text{H}_2\text{O}_2$ is in the $-1$ oxidation state, allowing it to exhibit dual redox properties. It acts as an oxidizing agent by getting reduced to water (oxygen $-2$) and as a reducing agent by getting oxidized to oxygen gas (oxygen $0$).

Industrially, it's primarily produced via the auto-oxidation of 2-ethylanthraquinol. Laboratory preparation often involves reacting hydrated barium peroxide with dilute sulfuric acid. $\text{H}_2\text{O}_2$ is thermodynamically unstable, decomposing into water and oxygen.

This decomposition is accelerated by light, heat, rough surfaces, and catalysts like $\text{MnO}_2$. Hence, it's stored in dark, plastic bottles in cool conditions, often with stabilizers. Its bleaching action is due to the release of nascent oxygen.

Remember key reactions where it acts as an oxidant (e.g., with $\text{Fe}^{2+}$, $\text{PbS}$) and a reductant (e.g., with $\text{KMnO}_4$, $\text{Cl}_2$).

5-Minute Revision

Hydrogen peroxide ($\text{H}_2\text{O}_2$) is a crucial compound with distinct physical and chemical characteristics. Physically, it's a pale blue, syrupy liquid, denser and more viscous than water, and miscible with water due to extensive hydrogen bonding.

Structurally, it adopts a non-planar 'open book' conformation, unlike water. This is defined by a dihedral angle of about $111.5^circ$ in the gas phase and $90.2^circ$ in the solid state, arising from repulsion between lone pairs and hydrogen atoms.

The most significant chemical property is its dual redox nature. With oxygen in the $-1$ oxidation state, $\text{H}_2\text{O}_2$ can be reduced to $\text{H}_2\text{O}$ (oxygen $-2$), acting as an oxidizing agent, or oxidized to $\text{O}_2$ (oxygen $0$), acting as a reducing agent.

For example, it oxidizes $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$ in acidic medium ($2\text{Fe}^{2+} + \text{H}_2\text{O}_2 + 2\text{H}^+ \rightarrow 2\text{Fe}^{3+} + 2\text{H}_2\text{O}$) and reduces $\text{MnO}_4^-$ to $\text{Mn}^{2+}$ in acidic medium ($2\text{MnO}_4^- + 5\text{H}_2\text{O}_2 + 6\text{H}^+ \rightarrow 2\text{Mn}^{2+} + 5\text{O}_2 + 8\text{H}_2\text{O}$).

Its bleaching action is attributed to nascent oxygen release. It's also thermodynamically unstable, decomposing into water and oxygen, a process catalyzed by light, heat, and impurities. Therefore, it's stored in dark, cool, plastic containers with stabilizers.

Industrial preparation primarily uses the auto-oxidation of 2-ethylanthraquinol, a cyclic process. Laboratory methods include reacting $\text{BaO}_2 \cdot 8\text{H}_2\text{O}$ with dilute $\text{H}_2\text{SO}_4$ at low temperatures to prevent decomposition and precipitate $\text{BaSO}_4$. Understanding these preparation methods, the factors affecting its stability, and its versatile redox chemistry are crucial for NEET.

Prelims Revision Notes

Hydrogen Peroxide ($\text{H}_2\text{O}_2$) - NEET Revision Notes

1. Preparation Methods:

* Industrial (Anthraquinone Process): Most common. Auto-oxidation of 2-ethylanthraquinol by air/oxygen to 2-ethylanthraquinone and $\text{H}_2\text{O}_2$. 2-ethylanthraquinone is then reduced back and recycled.

This is a cyclic, efficient process. * Laboratory: * From Barium Peroxide: $\text{BaO}_2 \cdot 8\text{H}_2\text{O} (s) + \text{H}_2\text{SO}_4 (aq) \xrightarrow{\text{cold}} \text{BaSO}_4 (s) \downarrow + \text{H}_2\text{O}_2 (aq) + 8\text{H}_2\text{O} (l)$.

Reaction kept cold to prevent $\text{H}_2\text{O}_2$ decomposition. $\text{BaSO}_4$ is insoluble. * From Sodium Peroxide: $\text{Na}_2\text{O}_2 (s) + \text{H}_2\text{SO}_4 (aq) \rightarrow \text{Na}_2\text{SO}_4 (aq) + \text{H}_2\text{O}_2 (aq)$.

Vigorous reaction. * Electrolytic (Older Industrial): Electrolysis of $50\%$ $\text{H}_2\text{SO}_4$ or ammonium sulfate solution to form peroxodisulfuric acid ($\text{H}_2\text{S}_2\text{O}_8$), followed by hydrolysis: $\text{H}_2\text{S}_2\text{O}_8 + 2\text{H}_2\text{O} \rightarrow 2\text{H}_2\text{SO}_4 + \text{H}_2\text{O}_2$.

2. Physical Properties:

* Pure $\text{H}_2\text{O}_2$: Pale blue, syrupy liquid. Dilute solutions are colorless. * Denser and more viscous than water. * Miscible with water in all proportions (due to strong hydrogen bonding). * High dielectric constant.

3. Chemical Properties:

* Oxidation State of Oxygen: $-1$. * Acidic Nature: Very weak acid ($K_a = 2.4 \times 10^{-12}$). Forms $\text{HO}_2^-$ ion. * Decomposition: Thermodynamically unstable. $2\text{H}_2\text{O}_2 (l) \rightarrow 2\text{H}_2\text{O} (l) + \text{O}_2 (g)$.

* Accelerated by: Light (UV), heat, rough surfaces, metal ions ($\text{Fe}^{2+}$, $\text{Cu}^{2+}$), catalysts ($\text{MnO}_2$, enzymes). * Storage: Dark, plastic bottles, cool place, stabilizers (e.g.

, urea, phosphoric acid). * Dual Redox Nature: * As Oxidizing Agent: Oxygen ($-1$) $\rightarrow$ Oxygen ($-2$) (forms $\text{H}_2\text{O}$ or $\text{OH}^-$). * Acidic: $\text{H}_2\text{O}_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow 2\text{H}_2\text{O}$ * Basic: $\text{H}_2\text{O}_2 + 2\text{e}^- \rightarrow 2\text{OH}^-$ * Examples: Oxidizes $\text{Fe}^{2+}$ to $\text{Fe}^{3+}$, $\text{PbS}$ (black) to $\text{PbSO}_4$ (white - used to restore old paintings), $\text{SO}_2$ to $\text{H}_2\text{SO}_4$.

* As Reducing Agent: Oxygen ($-1$) $\rightarrow$ Oxygen ($0$) (forms $\text{O}_2$). Occurs with strong oxidizing agents. * Acidic: $\text{H}_2\text{O}_2 \rightarrow \text{O}_2 + 2\text{H}^+ + 2\text{e}^-$ * Basic: $\text{H}_2\text{O}_2 + 2\text{OH}^- \rightarrow \text{O}_2 + 2\text{H}_2\text{O} + 2\text{e}^-$ * Examples: Reduces $\text{KMnO}_4$ to $\text{Mn}^{2+}$, $\text{Cl}_2$ to $\text{HCl}$, $\text{O}_3$ to $\text{O}_2$, $\text{Ag}_2\text{O}$ to $\text{Ag}$.

* Bleaching Action: Due to nascent oxygen release: $\text{H}_2\text{O}_2 \rightarrow \text{H}_2\text{O} + [\text{O}]$. Oxidizes colored substances to colorless ones.

4. Structure:

* Non-planar, 'open book' structure. * Dihedral Angle: Angle between the two $\text{H-O-O}$ planes. * Gas phase: $\approx 111.5^circ$ * Solid phase: $\approx 90.2^circ$ * Bond Lengths: $\text{O-O} \approx 147.5 \text{ pm}$, $\text{O-H} \approx 95.0 \text{ pm}$ (gas phase). * Bond Angles: $\text{H-O-O} \approx 94.8^circ$ (gas phase). * Reason for non-planarity: Repulsion between lone pairs on oxygen atoms and hydrogen atoms.

Vyyuha Quick Recall

To remember $\text{H}_2\text{O}_2$'s dual nature and decomposition:

Hydrogen Peroxide Often Reacts As Oxidizer Reducer, Decomposing Lightly.

Hydrogen Peroxide
Oxidizer Reducer: Dual redox nature
As Oxidizer: Oxygen goes from $-1$ to $-2$ (forms $\text{H}_2\text{O}$)
Reducer: Oxygen goes from $-1$ to $0$ (forms $\text{O}_2$)
Decomposing Lightly: Decomposes easily, especially with light.