Hydrogen (Hydrogen Peroxide And Heavy Water)

Hydrogen Peroxide ($H_2O_2$)

Hydrogen peroxide is a chemical compound with the formula $H_2O_2$. It is a weak acid and a strong oxidizing agent.

Preparation

1. From Barium Peroxide: This is a common laboratory method. Barium peroxide ($BaO_2$) reacts with cold dilute sulfuric acid to form hydrogen peroxide and insoluble barium sulfate.

$$BaO_2(s) + H_2SO_4(aq) \rightarrow BaSO_4(s) + H_2O_2(aq)$$

2. From Sodium Peroxide: Sodium peroxide ($Na_2O_2$) reacts with water to produce hydrogen peroxide and sodium hydroxide.

$$Na_2O_2(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2O_2(aq)$$

3. Commercial Production (Electrolytic Method): On a large scale, hydrogen peroxide is produced by the electrolysis of cold, concentrated sulfuric acid ($H_2SO_4$). This process initially forms peroxodisulfuric acid ($H_2S_2O_8$), which is then hydrolyzed.

Step 1: Electrolysis of cold concentrated $H_2SO_4$.

$$2H_2SO_4(l) \xrightarrow{electrolysis} H_2S_2O_8(l) + H_2(g)$$

Step 2: Hydrolysis of peroxodisulfuric acid.

$$H_2S_2O_8(l) + 2H_2O(l) \rightarrow 2H_2SO_4(aq) + H_2O_2(aq)$$

4. Anthraquinone Process: This is the most important industrial method. It involves the catalytic hydrogenation of an anthraquinone derivative, followed by oxidation of the resulting anthrahydroquinone with air.

Physical Properties

Appearance: Pure hydrogen peroxide is a colorless, viscous liquid (almost as viscous as glycerol).

Odor: It has a faint sharp odor.

Density: It is denser than water (density $\approx$ 1.47 g/mL at 20°C).

Boiling Point: It has a high boiling point (150.2°C), which is much higher than water due to strong hydrogen bonding. However, it tends to decompose rapidly at this temperature.

Freezing Point: -0.43°C.

Acidity: It is a weak acid ($pK_a \approx 11.6$).

Stability: Aqueous solutions of $H_2O_2$ are unstable and decompose over time, especially in the presence of light, heat, or impurities (metals like $Fe$, $Cu$, $Pb$, $Pt$).

$$2H_2O_2(aq) \rightarrow 2H_2O(l) + O_2(g)$$

Structure

Molecular Structure: The molecule $H_2O_2$ has a non-planar structure.

Non-planarity: The two hydrogen atoms are not in the same plane as the two oxygen atoms. The molecule has a 'open book' like structure.

Bonding: It consists of a single covalent bond between the two oxygen atoms ($O-O$) and two covalent bonds between oxygen and hydrogen atoms ($O-H$). The $O-O$ bond is relatively weak (bond enthalpy $\approx$ 146 kJ/mol).

Dihedral Angle: The dihedral angle (the angle between the two $H-O-O$ planes) is about 111.5° in the gas phase and 90.2° in the solid phase.

Diagram:

Chemical Properties

Hydrogen peroxide exhibits properties of both an acid and an oxidizing/reducing agent.

1. Oxidizing Action: $H_2O_2$ is a strong oxidizing agent, especially in acidic solution. It is oxidized to $O_2$ in these reactions, while the species it reacts with is reduced.

With Halides: Oxidizes iodide ions to iodine, bromide ions to bromine, but chloride ions are not oxidized.

$2I^-(aq) + H_2O_2(aq) + 2H^+(aq) \rightarrow 2H_2O(l) + I_2(aq)$

With Sulfides: Oxidizes sulfides to sulfates or elemental sulfur.

$H_2S(aq) + H_2O_2(aq) \rightarrow 2H_2O(l) + S(aq)$

With Nitrites: Oxidizes nitrite ions to nitrate ions.

$2NO_2^-(aq) + 2H^+(aq) + H_2O_2(aq) \rightarrow 2NO_3^-(aq) + 2H_2O(l)$

With $Fe^{2+}$ ions: Oxidizes ferrous ions to ferric ions.

$2Fe^{2+}(aq) + H_2O_2(aq) + 2H^+(aq) \rightarrow 2Fe^{3+}(aq) + 2H_2O(l)$

With $SO_2$: Oxidizes sulfur dioxide to sulfuric acid.

$SO_2(g) + H_2O_2(aq) \rightarrow H_2SO_4(aq)$

2. Reducing Action: $H_2O_2$ can also act as a reducing agent, particularly in alkaline or neutral solutions, where it is oxidized to $O_2$, while the species it reacts with is reduced.

With Permanganate Ions ($MnO_4^-$): Reduces permanganate ions (purple) to manganese(II) ions (pale pink/colorless) in acidic solution.

$2MnO_4^-(aq) + 5H_2O_2(aq) + 6H^+(aq) \rightarrow 2Mn^{2+}(aq) + 5O_2(g) + 8H_2O(l)$

With Dichromate Ions ($Cr_2O_7^{2-}$): Reduces dichromate ions (orange) to chromium(III) ions (green) in acidic solution.

$Cr_2O_7^{2-}(aq) + 3H_2O_2(aq) + 8H^+(aq) \rightarrow 2Cr^{3+}(aq) + 3O_2(g) + 8H_2O(l)$

With Hypochlorite Ions ($ClO^-$): Reduces hypochlorite ions to chloride ions in alkaline solution.

$ClO^-(aq) + H_2O_2(aq) \rightarrow Cl^-(aq) + O_2(g) + H_2O(l)$

3. Acidic Nature: $H_2O_2$ is a weak acid and reacts with strong bases to form hydrogen peroxide salts called peroxohydrates or peroxides.

$H_2O_2(aq) + OH^-(aq) \rightarrow HO_2^-(aq) + H_2O(l)$
$H_2O_2(aq) + 2OH^-(aq) \rightarrow O_2^{2-}(aq) + 2H_2O(l)$

4. Decomposition: As mentioned earlier, $H_2O_2$ decomposes into water and oxygen. This decomposition is catalyzed by metals ($Fe, Cu, Pt$), metal oxides ($MnO_2$), and even light.

5. Catalase Enzyme: Enzymes like catalase present in living cells rapidly decompose $H_2O_2$ into water and oxygen.

Storage

Instability: Hydrogen peroxide solutions are unstable and tend to decompose.

Storage Conditions: To minimize decomposition:

Dark Colored Bottles: Store in dark-colored glass or plastic bottles to prevent decomposition by light.
Absence of Impurities: Keep away from metal contaminants, dust, acids, and alkalis, which catalyze decomposition.
Stabilizers: Small amounts of stabilizers like stannic acid ($H_2SnO_3$) or phosphoric acid are often added to commercial samples.
Cool Place: Store in a cool place.

Uses

Hydrogen peroxide has a variety of uses due to its oxidizing and bleaching properties:

Antiseptic and Disinfectant: Used in dilute solutions (e.g., 3-6%) to clean wounds and sterilize medical equipment. It kills bacteria by oxidation.
Bleaching Agent: Used as a bleaching agent for textiles, paper pulp, hair, and in the cosmetic industry. Its bleaching action is due to oxidation.
Oxidizing Agent in Chemical Synthesis: Used in various organic and inorganic syntheses.
Propellant: Highly concentrated hydrogen peroxide solutions can be used as rocket propellants, either alone (as a monopropellant) or with a fuel.
Water Treatment: Used to remove pollutants from wastewater by oxidation.
Food Industry: Used as a bleaching agent for food products and as a sterilizing agent for packaging.
Source of Oxygen: Can decompose to release oxygen.

Heavy Water, D2O

Heavy Water ($D_2O$): Heavy water is water in which the hydrogen atoms are replaced by deuterium ($^2H$), an isotope of hydrogen that contains one proton and one neutron.

Discovery: Discovered by Harold Urey in 1932.

Occurrence:

Deuterium is present naturally in water to the extent of about 1 part in 6420 parts of ordinary water ($H_2O$).
Thus, natural water contains small amounts of $HDO$ and $D_2O$.

Preparation: Heavy water is prepared by the fractional electrolysis of ordinary water or by fractional distillation of liquid hydrogen.

Separation: The separation is based on the slight differences in physical properties between $H_2O$ and $D_2O$ due to the mass difference. For example, $D_2O$ has a higher boiling point ($101.4^\circ C$) and freezing point ($3.82^\circ C$) than ordinary water ($H_2O$, b.p. $100^\circ C$, f.p. $0^\circ C$).

Physical Properties of $D_2O$ (compared to $H_2O$):

Density: Higher density ($\approx 1.107 \text{ g/cm}^3$ at 20°C).
Boiling Point: Higher ($101.4^\circ C$).
Freezing Point: Higher ($3.82^\circ C$).
Viscosity: Higher viscosity.
Surface Tension: Higher surface tension.
Dielectric Constant: Slightly higher dielectric constant.

Chemical Properties:

Similar to $H_2O$: Chemically, $D_2O$ behaves very similarly to $H_2O$. It undergoes similar reactions like autoionization, acts as an acid and a base, and forms hydrates.
Differences due to Mass: However, due to the higher mass of deuterium, the $O-D$ bond is stronger than the $O-H$ bond. This leads to slight differences in reaction rates (kinetic isotope effect).
Reactions:

With Active Metals: Reacts with active metals like sodium to produce hydrogen deuteride ($D_2$) gas (and $NaOD$).

$2Na(s) + 2D_2O(l) \rightarrow 2NaOD(aq) + D_2(g)$

Reactions with Metal Oxides: Forms metal deuteroxides.

$CaO(s) + D_2O(l) \rightarrow Ca(OD)_2(aq)$

Reactions with Non-metallic Oxides: Forms deuterated acids.

$SO_3(g) + D_2O(l) \rightarrow D_2SO_4(aq)$

Uses:

Nuclear Reactors: Used as a moderator in nuclear reactors. Neutrons are slowed down by collision with deuterium nuclei, making them more likely to cause fission in uranium.
Tracers: Used as tracers in biological and chemical research due to the slight differences in its physical properties and its unique mass for detection.
Research: Used in studies related to the kinetic isotope effect and hydrogen bonding.
Not Suitable for Drinking (in large quantities): While consuming small amounts of heavy water is not harmful, consuming large quantities can be detrimental to living organisms because the deuterium-based biochemical reactions proceed at different rates, potentially disrupting metabolism.