16Chalkogens, Sulfur Chemistry, and Acid-Base Titrations

Acid-Base Titrations and Indicators

• Titration Principles: An unknown amount of acid (base) is determined by adding a base (acid) of known concentration.

• Equivalence Point (Äquivalenzpunkt): The point at which the equivalent amount of substance required for neutralization has just been added. This is identified by the color change of an indicator.

• Half-Equivalence Point (Halbäquivalenzpunkt): At this specific point, the following relationships apply:

  • $pH = pK_s$

  • $[A^-] = [HA]$

• Selection of Indicators:

  • Strong Acids and Strong Bases: Any indicator with a color change between $pH = 3$ and $pH = 11$ is suitable.

  • Weak Acids with Strong Bases: The indicator's transition range must be in the weakly alkaline range (e.g., titration of acetic acid $\text{CH}_3\text{COOH}$ with $\text{NaOH}$; indicator: Phenolphthalein).

  • Weak Bases with Strong Bases: The indicator's transition range must be in the weakly acidic range (e.g., titration of ammonia $\text{NH}_3$ with $\text{HCl}$).

Titration of Phosphoric Acid ($\text{H}_3\text{PO}_4$)

• Characteristics: Phosphoric acid is a triprotic acid, resulting in three distinct equivalence points ($ÄP_1, ÄP_2, ÄP_3$) on a titration curve with $\text{NaOH}$.

• Key Values:

  • $pK_{a,1}$ occurs during the first buffering stage leading to $ÄP_1$.

  • $pK_{a,2}$ occurs during the second buffering stage leading to $ÄP_2$.

  • $pK_{a,3}$ occurs during the third buffering stage leading to $ÄP_3$.

• Curve Observations: The graph shows pH levels rising from approximately $2$ to $12$ as milliliters of $\text{NaOH}$ are added.

Group 16: The Chalkogens (O, S, Se, Te, Po)

• Elements and Properties:

  • Oxygen (O): Atomic Number $Z = 8$; Configuration: $\text{[He]} 2s^2 2p^4$; Ionization Energy: $13.6 \text{ eV}$; Electronegativity (EN): $3.5$. Character: Non-metal.

  • Sulfur (S): Atomic Number $Z = 16$; Configuration: $\text{[Ne]} 3s^2 3p^4$; Ionization Energy: $10.4 \text{ eV}$; EN: $2.4$. Character: Non-metal.

  • Selenium (Se): Atomic Number $Z = 34$; Configuration: $\text{[Ar]} 3d^{10} 4s^2 4p^4$; Ionization Energy: $9.8 \text{ eV}$; EN: $2.5$. Character: Non-metal.

  • Tellurium (Te): Atomic Number $Z = 52$; Configuration: $\text{[Kr]} 4d^{10} 5s^2 5p^4$; Ionization Energy: $9.0 \text{ eV}$; EN: $2.0$. Character: Metalloid.

  • Polonium (Po): Atomic Number $Z = 84$; Configuration: $\text{[Xe]} 5d^{10} 6s^2 6p^4$; Ionization Energy: $8.4 \text{ eV}$; EN: $1.8$. Character: Metal.

• Group Trends:

  • Ionization energy and electronegativity decrease down the group.

  • Affinity to electropositive elements decreases down the group.

  • Affinity to electronegative elements increases down the group.

Hydrogen Peroxide ($\text{H}_2\text{O}_2$)

• Physical Properties: A syrupy, nearly colorless liquid (appears bluish in thick layers).

  • Boiling Point (Sdp.): $150\,^\circ\text{C}$

  • Melting Point (Smp.): $-0.4\,^\circ\text{C}$

  • Commercial Form: A $30\%$ solution known as Perhydrol.

• Chemical Decomposition: The $\text{O-O}$ bond is weak, leading to a strong tendency to decompose into water and oxygen with high heat release.

  • $2\text{H}_2\text{O}_2 \rightarrow 2\text{H}_2\text{O} + \text{O}_2$ with $\Delta H = -196.2\text{ kJ/mol}$.

  • Decomposition is catalyzed by heavy metal ions ($\text{Fe}^{3+}, \text{Cu}^{2+}, \text{MnO}_2$), platinum ($\text{Pt}$), or alkaline substances.

• Molecular Structure: The molecule is twisted (verdrillt) to minimize the repulsion of the free electron pairs on the oxygen atoms. This remaining repulsion is the reason for the low bond energy of the $\text{O-O}$ bond.

• Synthesis:

  • Industrial: Anthraquinone autoxidation process (approx. $1,900,000 \text{ t/y}$).

  • Laboratory: $\text{BaO}_2 + \text{H}_2\text{SO}_4 \rightarrow \text{H}_2\text{O}_2 + \text{BaSO}_4$.

• Applications:

  • Bleaching agent for textiles, paper, oils, and fats.

  • Production of perborates and percarbonates (detergents, $15-25\%$ content).

  • Wastewater treatment: Oxidation of cyanide ($\text{CN}^- \rightarrow \text{OCN}^-$) and sulfides ($\text{RS}^- \rightarrow \text{RSO}_3^-$).

• Redox Reactions:

  • Acidic Medium: $2\text{MnO}_4^- + 6\text{H}^+ + 5\text{H}_2\text{O}_2 \rightarrow 2\text{Mn}^{2+} + 5\text{O}_2 + 8\text{H}_2\text{O}$

  • Basic Medium: $2\text{MnO}_4^- + 3\text{H}_2\text{O}_2 \rightarrow 2\text{MnO}_2 + 3\text{O}_2 + 2\text{H}_2\text{O} + 2\text{OH}^-$

• Biological Role: Bombardier beetles use a defense system involving hydrogen peroxide and hydroquinone. A catalytic reaction shoots toxic $1,4$-benzoquinone, water, and oxygen at attackers.

Sulfur (S): Occurrence and Extraction

• Natural Occurrence:

  • Elemental (Native): Found in Sicily, North/Central/South America, Japan, and Poland. Examples include the Kawah Ijen mines.

  • Sulfidic Minerals (Kiese, Glanze, Blenden):

  • Cinnabar ($\text{HgS}$): Mercury(II) sulfide.

  • Galena (Bleiglanz, $\text{PbS}$): Lead(II) sulfide.

  • Pyrite (Katzengold, $\text{FeS}_2$): Iron(II) disulfide.

  • Zinc blende (Zinkblende, $\text{ZnS}$): Zinc sulfide.

  • Orpiment (Auripigment, $\text{As}_2\text{S}_3$): Arsenic(III) sulfide.

  • Stibnite (Antimonit, $\text{Sb}_2\text{S}_3$): Antimony(III) sulfide.

  • Chalcopyrite (Kupferkies, $\text{CuFeS}_2$): Copper(I) iron(III) sulfide.

  • Sulfatic Minerals:

  • Gypsum ($\text{CaSO}_4 \times 2\text{H}_2\text{O}$).

  • Anhydrite ($\text{CaSO}_4$).

  • Epsom salt (Bittersalz, $\text{MgSO}_4 \times 7\text{H}_2\text{O}$).

  • Barium sulfate (Baryt/Schwerspat, $\text{BaSO}_4$).

• Extraction Methods:

  • Frasch Process: For native sulfur. Superheated water ($155\,^\circ\text{C}$) is pressed into the deposit; hot compressed air forces liquid sulfur to the surface.

  • Claus Process: For extracting sulfur from $\text{H}_2\text{S}$ in natural gas (up to $20\%$).

  1. Thermal Stage: $2\text{H}_2\text{S} + 3\text{O}_2 \rightarrow 2\text{SO}_2 + 2\text{H}_2\text{O}$ ($\Delta H = -518\text{ kJ/mol}$).

  2. Catalytic Stage (Co/Mo Catalyst on $\text{Al}_2\text{O}_3$): $4\text{H}_2\text{S} + 2\text{SO}_2 \rightarrow 6\text{S} + 4\text{H}_2\text{O}$ ($\Delta H = -146\text{ kJ/mol}$).

  • Net Reaction: $6\text{H}_2\text{S} + 3\text{O}_2 \rightarrow 6\text{S} + 6\text{H}_2\text{O}$ ($\Delta H = -664\text{ kJ/mol}$).

Sulfur Modifications and Allotropes

• Sulfur Modalities: Sulfur has the largest number of allotropic modifications ($6$ to $20$ membered rings).

  • $\alpha$-Sulfur: Orthorhombic, $\rho = 2.17\text{ g/cm}^3$, stable at room temperature. Consists of $\text{S}_8$ rings ($\text{S-S}$ bond length $204.8\text{ pm}$, angle $108\,^\circ$).

  • $\beta$-Sulfur: Monoclinic, $\rho = 2.00\text{ g/cm}^3$.

  • $\gamma$-Sulfur (Rosickýit): Monoclinic, $\rho = 2.03\text{ g/cm}^3$.

• Temperature Effects on Sulfur:

  • Melting point involves transition to liquid $\text{S}_{\lambda}$ (yellow, low viscosity, $\text{S}_8$ rings).

  • At $159\,^\circ\text{C}$, ring opening occurs, transition to $\text{S}_{\pi}$ and then to $\text{S}_{\mu}$ (rotbraun, high viscosity), forming chains up to $10^6$ atoms.

  • Plastic Sulfur: Formed by rapid cooling of viscous sulfur.

  • Gas Phase: Equilibrium of $\text{S}_n$ ($n=1-8$). $\text{S}_2$ is paramagnetically violet (>1250\,^\circ\text{C}), and atomic $\text{S}$ exists at >2500\,^\circ\text{C}.

Hydrogen Sulfide ($\text{H}_2\text{S}$) and Metal Sulfides

• Structure: V-shaped molecule; Bond angle: $92.3\,^\circ$. Electronegativity difference: $\text{S}(2.4)$, $\text{H}(2.2)$.

• Synthesis:

  • Technical: From elements at $600\,^\circ\text{C}$ or desulfurization of fossil fuels.

  • Laboratory: Kipp’s apparatus: $\text{FeS} + 2\text{HCl} \rightarrow \text{H}_2\text{S} + \text{FeCl}_2$.

• Properties:

  • Highly toxic, colorless gas with a rotten egg smell.

  • Weak dibasic acid:

  • $pK_{s1} = 7$ ($\text{H}_2\text{S} + \text{H}_2\text{O} \rightleftharpoons \text{HS}^- + \text{H}_3\text{O}^+$)

  • $pK_{s2} = 13$ ($\text{HS}^- + \text{H}_2\text{O} \rightleftharpoons \text{S}^{2-} + \text{H}_3\text{O}^+$)

• Sulfidic Precipitates (Fällungsmittel):

  • Acidic Medium (pH < 7): $\text{HgS}$ (black), $\text{PbS}$ (brown), $\text{CuS}$ (black), $\text{CdS}$ (yellow), $\text{As}_2\text{S}_3$ (yellow), $\text{Sb}_2\text{S}_3$ (orange), $\text{SnS}$ (brown).

  • Basic Medium (pH > 7): $\text{NiS}$ (black), $\text{CoS}$ (black), $\text{MnS}$ (flesh-colored), $\text{ZnS}$ (white).

• Crystal Structures of AB-type Sulfides:

  • Zinc Blende: Cubic close-packed ($ccp$) of $\text{S}^{2-}$ ions with half the tetrahedral gaps filled by $\text{Zn}^{2+}$.

  • Wurtzite: Hexagonal close-packed ($hcp$) of $\text{S}^{2-}$ ions with half the tetrahedral gaps filled by $\text{Zn}^{2+}$.

Polysulfides and Lapis Lazuli

• Sulfanes and Polysulfides: $\text{H}_2\text{S}$ dissolves in liquid sulfur to form Sulfanes ($\text{H}_2\text{S}_n$). Alkalimetal sulfides react with sulfur to form polysulfides ($\text{M}_2\text{S}_n$ ). These are chain-like and thermodynamically unstable.

• Lapislazuli (Lazurite): A blue semi-precious stone containing the mineral Lasurit.

  • Formula: $(\text{Na,Ca})_8[\text{Al}_6\text{Si}_6\text{O}_{24}](\text{S, Cl, SO}_4, \text{OH})_2$.

  • Color Basis: Due to radical ions trapped in the alumosilicate framework.

  • $\text{S}_2^{\cdot -}$: Yellow-green

  • $\text{S}_3^{\cdot -}$: Blue (Trisulfur radical anion, stable at high pressure/temperature in geofluids).

  • $\text{S}_4^{\cdot -}$: Red

• History: Major source was the Sar-e-Sang Mine (Afghanistan). Used in the mask of Tutankhamun and pigments by artists like Giotto.