Unit 12: Comprehensive Summary

Non-metals are generally poor conductors, brittle, and form covalent bonds. Noble Gases (Group 18) are inert, monatomic gases with very low boiling points due to their full valence shells. Halogens (Group 17) are reactive, diatomic non-metals.

• Physical Trends (Halogens): Down the group, boiling points increase due to stronger van der Waals forces. The state changes from gas (F₂, Cl₂) to liquid (Br₂) to solid (I₂).
• Chemical Trends (Halogens): They are strong oxidising agents (power decreases down the group). Halide ions are reducing agents (power increases down the group). A more reactive halogen will displace a less reactive halide from solution.
• Reactions with Conc. H₂SO₄: This reaction highlights the reducing power of halides. $Cl^-$ undergoes an acid-base reaction. $Br^-$ is oxidized to $Br_2$. $I^-$ is a strong enough reducing agent to produce $I_2$, S, and $H_2S$.
• Disproportionation: Chlorine reacts with water and cold, dilute alkali in disproportionation reactions, where it is both oxidised and reduced. This forms the basis for bleach (NaClO) and water disinfection (HOCl).

• Water ($H_2O$): Has unusually high melting/boiling points and a low density of ice due to hydrogen bonding. It's a polar molecule and an excellent solvent. Water treatment involves coagulation, sedimentation, filtration, and disinfection.
• Hydrogen & Oxygen: Hydrogen ($H_2$) is a flammable gas tested by a "squeaky pop". Oxygen ($O_2$) supports combustion and relights a glowing splint. Oxides are classified as acidic (non-metal oxides), basic (metal oxides), amphoteric (e.g., Al₂O₃, ZnO), or neutral (e.g., CO).
• Sulphur: The Contact Process manufactures sulfuric acid ($H_2SO_4$) from sulfur. Key step: $2SO_2 + O_2 \rightleftharpoons 2SO_3$ using a V₂O₅ catalyst at 450°C and 1-2 atm. $H_2SO_4$ is a strong acid, dehydrating agent, and oxidizing agent.
• Carbon: Exists as allotropes (diamond, graphite). Coal is an impure form used to make coke for the blast furnace. Carbon dioxide ($CO_2$) is an acidic oxide, while carbon monoxide (CO) is a neutral, toxic reducing agent.
• Nitrogen: The Haber Process manufactures ammonia ($NH_3$) from nitrogen and hydrogen ($N_2 + 3H_2 \rightleftharpoons 2NH_3$) using an iron catalyst, 450°C, and 200 atm. Ammonia is a weak base used for fertilizers. Nitrates decompose on heating, with products depending on the metal's reactivity.

The method of collecting a gas depends on its properties:

• Collection over water: For gases insoluble in water (e.g., H₂, O₂, N₂).
• Upward delivery: For gases less dense than air (e.g., NH₃).
• Downward delivery: For gases denser than air (e.g., CO₂, Cl₂, HCl, SO₂).

Key preparations include reacting a metal with acid (for H₂), a carbonate with acid (for CO₂), and an ammonium salt with a base (for NH₃).

A mineral is a pure, naturally occurring compound, while a rock is a mixture of minerals.

• Rock Types: Igneous (from cooled magma/lava), Sedimentary (from compacted sediments), and Metamorphic (changed by heat and pressure).
• Weathering: The breakdown of rocks. It can be physical (e.g., freeze-thaw), biological (e.g., plant roots), or chemical (e.g., acid rain on limestone).