9.4 Metals, Non-metals & Carbon Compounds
Key Takeaways
- Metals are lustrous, malleable, ductile, good conductors of heat and electricity and lose electrons to form positive ions; non-metals (except graphite) are poor conductors and gain or share electrons to form negative ions.
- The reactivity series arranges metals from most reactive (K, Na, Ca, Mg, Al) to least reactive (Au, Pt) and predicts displacement, reaction with acid/water and ease of extraction.
- Ionic compounds form by electron transfer (metal + non-metal) and have high melting points; covalent compounds form by electron sharing and usually have low melting points.
- Rusting of iron (Fe₂O₃·nH₂O) requires both air and water; it is prevented by painting, oiling, greasing, galvanisation (zinc coating) or alloying.
- Hydrocarbons are classified by C–C bonds: alkanes (single, CₙH₂ₙ₊₂), alkenes (double, CₙH₂ₙ) and alkynes (triple, CₙH₂ₙ₋₂); ethanol (C₂H₅OH) and ethanoic acid (CH₃COOH) are the two carbon compounds RRB tests most.
This section spans the largest part of the chemistry syllabus. RRB tests (a) physical and chemical properties of metals and non-metals, (b) the reactivity series and what it predicts, (c) ionic vs covalent bonding, (d) corrosion and its prevention, and (e) carbon compounds including hydrocarbons, functional groups, ethanol, ethanoic acid, soaps and detergents.
Metals vs Non-metals
| Property | Metals | Non-metals |
|---|---|---|
| State (room temp.) | Solid (except Hg — liquid) | Solid, liquid (Br₂) or gas |
| Lustre | Shiny when freshly cut | Dull (except iodine, diamond) |
| Malleability / ductility | High | Low / brittle |
| Conductivity | Good (heat + electricity) | Poor (except graphite) |
| Oxides | Basic (Na₂O, MgO) | Acidic (CO₂, SO₂); some neutral (CO, NO) |
| Ion formed | Cation (lose e⁻) | Anion (gain e⁻) |
Exceptions RRB loves: mercury is liquid, sodium is so soft it can be cut with a knife, lithium/sodium/potassium are kept under kerosene (react violently with water and air), graphite is a non-metal that conducts electricity, diamond is the hardest natural substance and iodine is a lustrous non-metal.
Chemical Properties of Metals
- Reaction with air: 2Mg + O₂ → 2MgO (basic oxide); 4Al + 3O₂ → 2Al₂O₃ (amphoteric — reacts with both acids and bases). Sodium and potassium burn with characteristic flame colours (Na: golden yellow; K: lilac). 2Cu + O₂ → 2CuO (black).
- Reaction with water:
- Cold water: K, Na → 2K + 2H₂O → 2KOH + H₂; Ca → Ca + 2H₂O → Ca(OH)₂ + H₂.
- Hot water: Mg → Mg + 2H₂O → Mg(OH)₂ + H₂.
- Steam: Al, Fe, Zn → 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
- No reaction: Pb, Cu, Ag, Au.
- Reaction with dilute acids (HCl, H₂SO₄): Zn + 2HCl → ZnCl₂ + H₂↑. Cu, Ag, Au do not react with dilute HCl/H₂SO₄ because they are below hydrogen in the reactivity series. Dilute HNO₃ is an exception — it oxidises H₂ to water and gives no H₂.
- Reaction with salt solutions of other metals: more reactive metal displaces less reactive (Zn + CuSO₄ → ZnSO₄ + Cu).
The Reactivity Series
K > Na > Ca > Mg > Al > Zn > Fe > Pb > [H] > Cu > Hg > Ag > Au
| Position | Reactivity | Notes |
|---|---|---|
| K, Na, Ca | Very high | React with cold water; kept under kerosene |
| Mg, Al, Zn, Fe | Moderate | React with steam; react with dilute acids |
| Pb, Cu | Low | No reaction with HCl; Pb reacts very slowly |
| Hg, Ag, Au | Very low | Found native; no reaction |
The series predicts displacement, the products of metal + acid, and the order of extraction.
Ionic and Covalent Compounds
- Ionic bond = electrostatic attraction between oppositely charged ions after electron transfer (metal gives e⁻ to non-metal). Examples: NaCl, MgCl₂, CaO, KBr. Properties: high melting/boiling points, solids at room temperature, conduct electricity in solution/molten state (ions free to move), hard and brittle.
- Covalent bond = sharing of electrons between atoms to complete octets. Examples: H₂, O₂, N₂, H₂O, CO₂, CH₄, NH₃, C₂H₆. Properties: low melting/boiling points, often gases/liquids at room temperature, do not conduct electricity (no ions), soft if solid.
Electron dot structures (Lewis) help RRB ask the bond type: Na → Na⁺ … Cl⁻ (ionic); shared pair in H:H (covalent).
Metallurgy and Corrosion
Occurrence: most metals are found as oxides, sulphides or carbonates in nature (minerals). The commercially exploitable form is an ore. Concentration → roasting/calcination → reduction (carbon, more reactive metal, or electrolysis) → refining. Examples: Zn from ZnS (roasted to ZnO, reduced by carbon); Al from bauxite Al₂O₃·2H₂O by electrolysis of molten Al₂O₃ (cryolite lowers melting point); Fe from haematite Fe₂O₃ in a blast furnace.
Corrosion is the slow attack of metals by air/moisture. Rusting of iron gives hydrated ferric oxide Fe₂O₃·nH₂O, requiring both air and water (sealed dry iron does not rust; underwater anoxic iron does not rust). Prevention: paint, oil/grease, galvanisation (zinc coating — zinc sacrificially oxidises), alloying (stainless steel contains Cr, Ni), electroplating with tin/nickel/chromium.
Carbon Compounds
Carbon has the unique ability to form long stable chains and rings (catenation) and to make four covalent bonds (tetravalency), giving rise to millions of organic compounds.
Allotropes of carbon: diamond (each C bonded to four others, tetrahedral, hardest natural substance, non-conductor), graphite (layers of hexagons, one free e⁻ per C, conducts electricity, used as a lubricant and in pencils), fullerene (C₆₀ 'buckminsterfullerene', a football-shaped molecule).
Hydrocarbons
| Family | General formula | C–C bond | Examples |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | Single (saturated) | CH₄ (methane, CNG, marsh gas), C₂H₆ (ethane), C₃H₈ (propane, LPG) |
| Alkenes | CₙH₂ₙ | Double (unsaturated) | C₂H₄ (ethene), C₃H₆ (propene); add bromine water (decolourises) |
| Alkynes | CₙH₂ₙ₋₂ | Triple (unsaturated) | C₂H₂ (ethyne / acetylene, oxy-acetylene welding) |
Saturated hydrocarbons (alkanes) burn cleanly with a blue flame and do not decolourise bromine water; unsaturated hydrocarbons (alkenes, alkynes) add across the double/triple bond, decolourising bromine water — a key RRB distinction.
Functional Groups
| Group | Name | Example |
|---|---|---|
| –OH | Alcohol | Ethanol C₂H₅OH |
| –CHO | Aldehyde | Ethanal CH₃CHO |
| –C=O (–COR) | Ketone | Propanone CH₃COCH₃ |
| –COOH | Carboxylic acid | Ethanoic acid CH₃COOH (vinegar) |
| –NH₂ | Amine | Methanamine CH₃NH₂ |
| –O– (R–O–R') | Ether | Diethyl ether |
| –Cl | Haloalkane | Chloromethane CH₃Cl |
Ethanol (C₂H₅OH): colourless, pleasant smell, soluble in water, used in alcoholic drinks, antiseptic, fuel. Reaction with sodium: 2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂↑. Dehydration with conc. H₂SO₄ at 170 °C gives ethene: CH₃CH₂OH → CH₂=CH₂ + H₂O. Oxidation with acidified KMnO₄/K₂Cr₂O₇ gives ethanoic acid.
Ethanoic acid (CH₃COOH, acetic acid; 5–8% in vinegar): reacts with alcohols to form esters (sweet-smelling, used in perfumes and flavourings) — esterification: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O (catalyst conc. H₂SO₄). Saponification is the reverse: ester + base → soap + alcohol.
Soaps and detergents: soaps are sodium/potassium salts of long-chain carboxylic acids (e.g., sodium stearate). They form micelles around oily dirt with hydrophilic heads outward and hydrophobic tails inward, lifting dirt into water. Soaps do not work in hard water — Ca²⁺/Mg²⁺ form insoluble scum. Detergents are sodium salts of sulphonic acids; their calcium salts are soluble, so detergents work in hard water. The cleansing action of both depends on micelle formation.
RRB Exam Scenarios and Traps
- Trap: 'Gold is found native because it is highly reactive.' Reverse — gold is least reactive, so it does not combine with other elements.
- Trap: 'Diamond conducts electricity.' It does not; only graphite does among carbon allotropes.
- Trap: 'Both alkanes and alkenes decolourise bromine water.' Only unsaturated (alkenes/alkynes) do; alkanes do not.
- Trap: confusing rust (Fe₂O₃·nH₂O) with the formula of magnetite (Fe₃O₄) — both contain iron but rust is hydrated ferric oxide.
- Expect: 'Why does zinc coating prevent rusting even if scratched?' Zinc is more reactive than iron, so zinc oxidises first (sacrificial protection).
- Expect: 'Identify the most reactive metal.' Among given options, choose the one highest in the series (K > Na > Ca).
- Expect: 'General formula of alkynes?' CₙH₂ₙ₋₂ (e.g., ethyne C₂H₂).
- Expect: 'Soap does not work in hard water because …' calcium and magnesium ions form insoluble scum.
- Expect: 'Esterification reactants?' carboxylic acid + alcohol, with conc. H₂SO₄.
Drill the reactivity series mnemonic, the hydrocarbon formulas and the ethanol → ethanoic acid → ester pathway until they are automatic — RRB reuses these every cycle.
Which metal is most reactive and is therefore stored under kerosene?
A hydrocarbon decolourises bromine water. Which family does it belong to?
Which reaction is esterification?
Iron rusts only when which two substances are both present?