Water & Peroxide

The oxygen atom in water is sp³-hybridised. Of the four electron pairs around oxygen, two are bonding (O–H bonds) and two are lone pairs. The two lone pairs exert greater repulsion than bond pairs, compressing the H–O–H bond angle from the ideal tetrahedral 109.5° to approximately 104.5°. The result is a bent (V-shaped) molecule with a bond length of 96 pm. The large electronegativity difference between O (3.44) and H (2.20) makes each O–H bond strongly polar (dipole moment ≈ 1.85 D for the molecule). Intermolecular hydrogen bonds (O–H···O, strength ≈ 20 kJ mol⁻¹) link water molecules in a dynamic three-dimensional network, explaining its anomalously high melting and boiling points relative to H₂S or H₂Se.

Temporary hardness is caused by dissolved calcium and magnesium hydrogencarbonate (bicarbonate) salts: $\ce{Ca(HCO3)2}$ and $\ce{Mg(HCO3)2}$. These arise when rainwater (slightly acidic due to dissolved $\ce{CO2}$) percolates through limestone or dolomite: $\ce{CaCO3 + CO2 + H2O -> Ca^{2+} + 2HCO3^{-}}$. On boiling, the equilibrium reverses and calcium carbonate precipitates: \ce{Ca^{2+} + 2HCO3^{-} ->[$\Delta$] CaCO3 v + CO2 ^ + H2O}. This is why kettles develop a white scale ('limescale'). Temporary hardness can also be removed by adding slaked lime ($\ce{Ca(OH)2}$), which precipitates $\ce{CaCO3}$: $\ce{Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 v + 2H2O}$ — the Clark process.

Permanent hardness is caused by dissolved calcium sulfate ($\ce{CaSO4}$), calcium chloride ($\ce{CaCl2}$), magnesium sulfate ($\ce{MgSO4}$), and magnesium chloride ($\ce{MgCl2}$). These salts do NOT decompose on boiling and cannot be removed by heating alone. Removal methods include: (1) Addition of washing soda ($\ce{Na2CO3}$), which precipitates the calcium as $\ce{CaCO3}$: $\ce{CaSO4 + Na2CO3 -> CaCO3 v + Na2SO4}$; (2) Ion-exchange resins, where $\ce{Ca^{2+}}$ and $\ce{Mg^{2+}}$ ions are exchanged for $\ce{Na^+}$ or $\ce{H^+}$ ions; (3) Distillation or reverse osmosis for complete removal of dissolved salts.

$\ce{H2O2}$ has the structural formula H–O–O–H. Each oxygen is sp³-hybridised, and the molecule adopts a non-planar 'open-book' or 'skewed' conformation: the dihedral angle between the two H–O–O planes is about 111.5° in the gas phase (94.8° in the solid). The O–O bond length is 147.5 pm (longer than a typical O=O double bond at 121 pm, consistent with a single bond) and the O–H bond length is 97 pm. The O–O–H bond angle is approximately 100°. Pure $\ce{H2O2}$ is a pale blue, syrupy liquid (bp 150.2°C) denser than water (1.44 g cm⁻³). The O–O bond is relatively weak (bond enthalpy ≈ 146 kJ mol⁻¹), making $\ce{H2O2}$ thermodynamically unstable with respect to disproportionation.

Laboratory preparation: Barium peroxide reacts with cold dilute sulfuric acid: $\ce{BaO2 + H2SO4 -> BaSO4 v + H2O2}$. Barium sulfate precipitates, leaving a solution of $\ce{H2O2}$ that can be concentrated by careful evaporation under reduced pressure. Industrial preparation (anthraquinone process): 2-ethylanthraquinone is hydrogenated catalytically to 2-ethylanthrahydroquinone, which is then oxidised by oxygen: $\ce{AQ(ox) + H2 ->[cat] AQ(red)}$ then $\ce{AQ(red) + O2 -> AQ(ox) + H2O2}$. This cyclic process produces millions of tonnes per year. $\ce{H2O2}$ decomposes readily in light or in the presence of metal ions: $\ce{2H2O2 -> 2H2O + O2}$, so it is stored in dark bottles with stabilisers.

In $\ce{H2O2}$, oxygen has oxidation state −1 — intermediate between 0 (in $\ce{O2}$) and −2 (in $\ce{H2O}$). This means $\ce{H2O2}$ can be BOTH oxidised (O: −1 → 0) and reduced (O: −1 → −2), making it amphoteric in redox terms. As an OXIDISING agent (most common): $\ce{H2O2 + 2Fe^{2+} + 2H^+ -> 2Fe^{3+} + 2H2O}$ (Fenton's reaction) and $\ce{H2O2 + 2KI + H2SO4 -> I2 + K2SO4 + 2H2O}$. As a REDUCING agent (with a stronger oxidant like $\ce{KMnO4}$): $\ce{5H2O2 + 2KMnO4 + 3H2SO4 -> 2MnSO4 + K2SO4 + 8H2O + 5O2}$. In acidic solution, the standard electrode potential is $E^\circ = +1.77$ V ($\ce{H2O2/H2O}$), confirming its powerful oxidising ability. The disproportionation reaction $\ce{2H2O2 -> 2H2O + O2}$ ($\Delta G < 0$) is catalysed by MnO₂, blood (catalase enzyme), or heavy metal ions.

• \ce{BaO2} — Barium peroxide (—): Reactant; contains the peroxide ion $\ce{O2^{2-}}$. Barium is in oxidation state +2.
• \ce{H2SO4} — Sulfuric acid (dilute, cold) (—): Must be cold to prevent decomposition of the $\ce{H2O2}$ product; provides $\ce{H^+}$ ions and $\ce{SO4^{2-}}$.
• \ce{BaSO4} — Barium sulfate precipitate (—): Insoluble white solid (↓) that is filtered off, leaving pure $\ce{H2O2}$ in solution.
• \ce{H2O2} — Hydrogen peroxide (—): Product; oxygen in −1 oxidation state. Concentrated by vacuum distillation.

Problem. A sample of hard water contains 243 mg of calcium hydrogencarbonate, $\ce{Ca(HCO3)2}$, dissolved per litre. (a) Classify this hardness as temporary or permanent, and state how it can be removed. (b) Calculate the mass of calcium carbonate ($\ce{CaCO3}$, M = 100 g mol⁻¹) that would precipitate when 2.00 L of this water is boiled. [M(Ca(HCO3)₂) = 162 g mol⁻¹]

Answer. The hardness is temporary; it is removed by boiling. 300 mg of calcium carbonate precipitates.

• Hydrogen: Properties and Isotopes — Water and hydrogen peroxide are both binary hydrogen compounds; understanding hydrogen's properties (isotopes, electronegativity, bond formation) underpins the discussion of O–H bonding, polarity, and the anomalous behaviour of water.
• Oxidation States and Redox Reactions — The dual redox behaviour of H₂O₂ (oxidant and reductant) is fully explained only through oxidation state changes. This concept also underlies the analysis of permanganate and iodide reactions with H₂O₂.
• Ionic Equilibria and Solubility — The chemistry of hard water involves the solubility product of CaCO₃ and CaSO₄, carbonate–bicarbonate equilibria, and precipitation reactions — all central topics in ionic equilibria.
• Intermolecular Forces and Hydrogen Bonding — The exceptional physical properties of water (high boiling point, surface tension, solvent ability) arise directly from extensive O–H···O hydrogen bonding, making this a key related concept.