15.4 Main Groups 13–16 & Hydrogen: Characteristic Compounds and Reaction Products

15.4 Main Groups 13–16 & Hydrogen: Characteristic Compounds and Reaction Products

Quick Summary: The CLEP outline lists "chemical reactivity and products of chemical reactions" and "chemistry of the main groups ... including typical examples of each" under Descriptive Chemistry (about 14% of the exam). Section 15.2 covered Groups 1, 2, 17, and 18. This section covers hydrogen and Groups 13–16 (boron and aluminum, carbon and silicon, nitrogen and phosphorus, oxygen and sulfur), then turns their typical reactions into a product-prediction checklist.

1. Hydrogen

• Laboratory preparation: an active metal plus a non-oxidizing acid, $\text{Zn}(s) + 2\,\text{HCl}(aq) \to \text{ZnCl}_2(aq) + \text{H}_2(g)$, or electrolysis of water.
• Industrial preparation: steam reforming of natural gas over a nickel catalyst, $\text{CH}_4 + \text{H}_2\text{O} \to \text{CO} + 3\,\text{H}_2$.
• Hydride types:
  • Ionic (saline) hydrides of Groups 1 and 2 contain $\text{H}^-$ and react with water to release hydrogen: $\text{CaH}_2(s) + 2\,\text{H}_2\text{O}(l) \to \text{Ca(OH)}_2(aq) + 2\,\text{H}_2(g)$.
  • Covalent (molecular) hydrides of nonmetals: $\text{CH}_4$, $\text{NH}_3$, $\text{H}_2\text{O}$, $\text{HF}$, $\text{H}_2\text{S}$.
  • Metallic (interstitial) hydrides: palladium absorbs large volumes of $\text{H}_2$ into its lattice.
• Hydrogen burns to water ($2\,\text{H}_2 + \text{O}_2 \to 2\,\text{H}_2\text{O}$) and reduces some metal oxides when heated ($\text{CuO} + \text{H}_2 \to \text{Cu} + \text{H}_2\text{O}$).

2. Group 13: Boron and Aluminum

• Boron is a metalloid whose compounds are covalent and electron-deficient. $\text{BF}_3$ is a Lewis acid that accepts a lone pair from $\text{NH}_3$. Boric acid is a weak Lewis acid that accepts hydroxide rather than donating its own proton: $\text{B(OH)}_3 + \text{H}_2\text{O} \rightleftharpoons \text{B(OH)}_4^- + \text{H}^+$.
• Aluminum is a reactive metal protected by a thin, tough $\text{Al}_2\text{O}_3$ film. It is amphoteric and releases hydrogen with both acids and strong bases:
  • $2\,\text{Al} + 6\,\text{H}^+ \to 2\,\text{Al}^{3+} + 3\,\text{H}_2$
  • $2\,\text{Al} + 2\,\text{OH}^- + 6\,\text{H}_2\text{O} \to 2\,[\text{Al(OH)}_4]^- + 3\,\text{H}_2$
• The thermite reaction, $2\,\text{Al} + \text{Fe}_2\text{O}_3 \to \text{Al}_2\text{O}_3 + 2\,\text{Fe}$, is strongly exothermic because $\text{Al}_2\text{O}_3$ is so stable.

3. Group 14: Carbon, Silicon, Tin, and Lead

• Carbon allotropes: diamond (network, $sp^3$), graphite (layers, $sp^2$, conducts), and fullerenes such as $\text{C}_{60}$.
• Carbon monoxide forms when carbon burns in limited oxygen. It is toxic because it binds hemoglobin, and it reduces metal ores in a blast furnace: $\text{Fe}_2\text{O}_3 + 3\,\text{CO} \to 2\,\text{Fe} + 3\,\text{CO}_2$.
• Carbon dioxide is an acidic oxide: $\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3$. It turns limewater cloudy by precipitating $\text{CaCO}_3$.
• Carbonates decompose on heating ($\text{CaCO}_3 \to \text{CaO} + \text{CO}_2$) and release $\text{CO}_2$ with acids.
• Silicon forms network $\text{SiO}_2$ and silicate minerals. Hydrofluoric acid etches glass: $\text{SiO}_2 + 4\,\text{HF} \to \text{SiF}_4 + 2\,\text{H}_2\text{O}$.
• Tin and lead show the inert-pair trend: $\text{Sn}^{2+}$ is a reducing agent, while $\text{PbO}_2$ (lead +4) is an oxidizing agent.

4. Group 15: Nitrogen and Phosphorus

• Nitrogen ($\text{N}_2$) is unreactive because of its $945\text{ kJ/mol}$ triple bond. Ammonia forms by the Haber-Bosch process and is a weak base that neutralizes acids: $\text{NH}_3 + \text{HCl} \to \text{NH}_4\text{Cl}$.
• Nitrogen oxides: $\text{N}_2\text{O}$ (+1), NO (+2, colorless), $\text{NO}_2$ (+4, brown, dimerizes to colorless $\text{N}_2\text{O}_4$), and $\text{N}_2\text{O}_5$ (+5, the anhydride of nitric acid).
• Nitric acid is a strong acid and a strong oxidizing agent, so it dissolves copper even though copper lies below hydrogen. The gas depends on concentration:
  • Concentrated: $\text{Cu} + 4\,\text{HNO}_3 \to \text{Cu(NO}_3)_2 + 2\,\text{NO}_2 + 2\,\text{H}_2\text{O}$ (brown gas)
  • Dilute: $3\,\text{Cu} + 8\,\text{HNO}_3 \to 3\,\text{Cu(NO}_3)_2 + 2\,\text{NO} + 4\,\text{H}_2\text{O}$
• Phosphorus: white $\text{P}_4$ ignites spontaneously in air and is stored under water; red phosphorus is far less reactive. Phosphorus burns to $\text{P}_4\text{O}_{10}$, which reacts with water to give phosphoric acid, a weak triprotic acid: $\text{P}_4\text{O}_{10} + 6\,\text{H}_2\text{O} \to 4\,\text{H}_3\text{PO}_4$.

5. Group 16: Oxygen and Sulfur

• Oxygen: $\text{O}_2$ is paramagnetic; ozone ($\text{O}_3$) is a bent, strongly oxidizing allotrope that absorbs ultraviolet light in the stratosphere.
• Hydrogen peroxide disproportionates to water and oxygen, a reaction catalyzed by $\text{MnO}_2$: $2\,\text{H}_2\text{O}_2 \to 2\,\text{H}_2\text{O} + \text{O}_2$.
• Sulfur exists as $\text{S}_8$ rings. It burns to $\text{SO}_2$, which dissolves to sulfurous acid ($\text{SO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{SO}_3$) and contributes to acid rain.
• Sulfuric acid (contact process): $2\,\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\,\text{SO}_3$ over a $\text{V}_2\text{O}_5$ catalyst; $\text{SO}_3$ is absorbed into concentrated sulfuric acid and diluted, because adding it directly to water produces a corrosive mist. Concentrated $\text{H}_2\text{SO}_4$ is a strong dehydrating agent that chars sugar ($\text{C}_{12}\text{H}_{22}\text{O}_{11} \to 12\,\text{C} + 11\,\text{H}_2\text{O}$).
• Hydrogen sulfide ($\text{H}_2\text{S}$) is a toxic weak acid with a rotten-egg odor, released when metal sulfides react with acids.

6. Predicting Products: A Reactivity Checklist

Worked Example: Predict the Products

1. Sulfur dioxide bubbled into water: a nonmetal oxide plus water gives an oxoacid, so the product is $\text{H}_2\text{SO}_3$, and the solution turns acidic.
2. Aluminum foil in hot NaOH solution: amphoteric aluminum gives $[\text{Al(OH)}_4]^-$ and $\text{H}_2$ gas.
3. Copper in dilute nitric acid: nitrate, not $\text{H}^+$, is the oxidizing agent, so the gas is NO rather than $\text{H}_2$, and the solution turns blue from $\text{Cu}^{2+}$.
4. Calcium hydride in water: the hydride ion $\text{H}^-$ is a strong base and reducing agent, so the products are $\text{Ca(OH)}_2$ and $\text{H}_2$.