Chemistry weakness deck

How do you tell if a molecule is an alkane or alkene from its formula?

Count carbons (n). If H = 2n+2 → ALKANE (saturated, only single bonds). If H = 2n → ALKENE (unsaturated, has C=C). e.g. C₄H₈: 2(4)=8 ✓ → ALKENE. C₄H₁₀: 2(4)+2=10 ✓ → ALKANE.

Memory trick — alkanes vs alkenes formula?

AlkAnes = 2n+2 (the A stands for Add 2). AlkEnes = 2n (missing the +2 because the double bond replaces two H atoms). Alkenes are always 2 hydrogens LESS than the equivalent alkane.

Bromine water test — what happens with alkane vs alkene?

ALKANE: bromine water stays ORANGE. No reaction — no C=C bond to react with. ALKENE: bromine water DECOLOURISES (turns colourless). Bromine adds across the C=C double bond (addition reaction). This is the test for unsaturation.

Why does bromine decolourise with alkenes but not alkanes?

Alkenes have a C=C double bond with high electron density. Bromine atoms ADD ACROSS the double bond → bromine no longer free in solution → orange colour disappears. Alkanes have only single bonds — cannot undergo addition reactions → bromine remains in solution.

Required practical — disappearing cross: full method step by step?

1. Measure 50cm³ Na₂S₂O₃ into conical flask. 2. Place flask over paper cross on white paper. 3. Measure 10cm³ HCl. 4. Add HCl to flask and START STOPWATCH immediately. 5. Look DOWN through flask. 6. Stop timing when cross can no longer be seen. 7. Record time. 8. Repeat with different Na₂S₂O₃ concentrations (replace some with water to keep total volume constant at 50cm³). 9. Repeat each three times, calculate mean, exclude anomalies.

Required practical — disappearing cross: how do you change concentration without changing volume?

Replace some Na₂S₂O₃ solution with WATER. e.g. 50cm³ Na₂S₂O₃ → 40cm³ Na₂S₂O₃ + 10cm³ water → 30cm³ + 20cm³ water etc. Total always = 50cm³. Only concentration changes — everything else stays the same.

Required practical — disappearing cross: what are the control variables?

Volume of HCl (always 10cm³). Temperature of solutions. Same observer judging when cross disappears. Same paper cross. Same conical flask washed between experiments.

Required practical — gas volume (Mg + H₂SO₄): full method?

1. Measure 50cm³ H₂SO₄ into conical flask. 2. Add magnesium ribbon. 3. IMMEDIATELY connect gas syringe (before any gas escapes). 4. Start stopwatch. 5. Record volume of gas every 10 seconds. 6. Stop when no further increase (reaction complete). 7. Repeat with different acid concentration. 8. Plot both curves on same axes.

Required practical — gas volume: why must you connect gas syringe immediately?

Mg and H₂SO₄ react immediately on contact. If gas syringe not connected before adding Mg, gas escapes before collection starts → results will be inaccurate (volume recorded will be too low).

Required practical — water analysis (dissolved solids): full method?

1. Test pH with universal indicator paper. 2. Weigh empty evaporating basin to 2 d.p. 3. Add 25cm³ water sample. 4. Heat on tripod and gauze with Bunsen burner until solids start to form. 5. Allow remaining water to evaporate. 6. Cool completely. 7. Reweigh. 8. Mass of dissolved solids = final mass − empty basin mass.

Required practical — water analysis: key practical tip?

Do NOT overheat once solids start to form — risk of THERMALLY DECOMPOSING the dissolved solids → gives inaccurate (lower) mass reading.

Required practical — distillation (purifying water): full method?

1. Add impure water sample to conical flask. 2. Set up delivery tube and bung. 3. Place collection tube in ice bath. 4. Heat gently with Bunsen burner. 5. Water vapour travels through delivery tube → condenses in cooled collection tube. 6. Collect ~2cm of distillate. 7. Test purity: measure boiling point (should be exactly 100°C) and pH (should be exactly 7).

Required practical — distillation: key practical tip?

Delivery tube must sit ABOVE the water level in the collection tube. If it dips below, cold water can be sucked BACK into the hot flask when heating stops → thermal shock → glassware cracks.

Required practical — chromatography: full method?

1. Draw pencil baseline 2cm from bottom of chromatography paper. 2. Place spots of known colourings A–D and unknown U on baseline (2–3mm diameter). 3. Pour solvent into beaker to depth less than 1cm. 4. Lower paper into beaker — baseline MUST be ABOVE solvent level. 5. Allow solvent to travel ¾ up paper undisturbed. 6. Remove paper, mark solvent front immediately. 7. Measure distances from baseline to each spot and to solvent front. 8. Calculate Rf values. 9. Compare to known values to identify unknown.

Required practical — chromatography: why must baseline be above solvent?

If baseline is below solvent level, the samples dissolve directly into the solvent and wash away rather than travelling up the paper. No separation occurs.

Required practical — chromatography: how do you calculate and use Rf values?

Rf = distance travelled by spot ÷ distance travelled by solvent front. Both measured from BASELINE. No units. Always between 0 and 1. Compare unknown Rf value to reference Rf values run under SAME conditions (same solvent, paper, temperature) to identify the substance.

How to write a balanced cracking equation — step by step?

1. Reactant = alkane (CₙH₂ₙ₊₂). 2. Count carbons on RIGHT side of equation. 3. Subtract from carbons on LEFT → missing carbons. 4. Count hydrogens on RIGHT. 5. Subtract from hydrogens on LEFT → missing hydrogens. 6. Check if product is alkane (CₙH₂ₙ₊₂) or alkene (CₙH₂ₙ). Example: C₆H₁₄ → C₄H₁₀ + ? → C: 6-4=2, H: 14-10=4 → C₂H₄ → 2(2)=4 ✓ → ALKENE.

How do you check if a cracking product is an alkane or alkene?

Take the formula of the unknown product. Count carbons (n). If H = 2n+2 → ALKANE. If H = 2n → ALKENE. Always check BOTH carbons AND hydrogens balance across the whole equation before finishing.

What is natural rubber and what has replaced it?

Extracted from sap (LATEX) of rubber trees — renewable resource. Largely replaced by SYNTHETIC POLYMERS engineered to replicate and improve its properties. Synthetic versions have more consistent quality and greater scalability than natural rubber.

Describe how a reversible reaction reaches equilibrium from the start.

Only reactants present at start → forward rate at its HIGHEST. As reactants used up → forward rate DECREASES. As products build up → reverse rate INCREASES. Eventually forward rate = reverse rate → DYNAMIC EQUILIBRIUM reached. Concentrations remain constant. Only possible in a CLOSED SYSTEM.

Three difficulties in reducing global carbon footprint?

1. Alternative technologies still in early stages — not ready to fully replace fossil fuels. 2. Governments fear economic damage from rapid change — slow to act. 3. Individuals reluctant to change lifestyle. Also: lack of investment for companies to modernise facilities.

How do impurities affect melting and boiling points?

Impurities LOWER the melting point AND RAISE the boiling point. Impure substances also melt/boil over a RANGE of temperatures rather than one sharp specific temperature. Two signs of impurity: (1) melts over a range, (2) melting point lower than known value.

Human activities increasing methane levels?

More waste sent to LANDFILL (decomposing organic matter releases CH₄). Increased LIVESTOCK FARMING (cattle digestive systems produce CH₄). More intensive RICE PADDY farming (waterlogged soil releases CH₄). All increasing due to growing human population.

Four stages of sewage treatment in order?

1. SCREENING and grit removal — large solids removed. 2. SEDIMENTATION — heavy sludge sinks to bottom; lighter effluent floats to top. 3. AEROBIC DIGESTION — air pumped in; bacteria break down effluent. 4. ANAEROBIC DIGESTION — bacteria break down sludge in sealed tanks → produces methane (fuel) and digestate (fertiliser).

What is a saturated hydrocarbon?

Contains ONLY single C-C bonds — no double bonds. General formula CₙH₂ₙ₊₂. ALKANES are saturated. Less reactive — no double bond to attack. Can only undergo substitution reactions (e.g. with halogens in UV light) or combustion.

What is an unsaturated hydrocarbon?

Contains at least one C=C double bond. General formula CₙH₂ₙ. ALKENES are unsaturated. MORE reactive than alkanes — the double bond has high electron density which attracts other molecules → can undergo ADDITION reactions.

Why are alkenes more reactive than alkanes?

Alkenes have a C=C double bond with a region of HIGH ELECTRON DENSITY. This makes it susceptible to attack by other molecules → addition reactions occur easily. Alkanes only have single bonds — no weak point to attack → much less reactive.

What is an addition reaction?

A reaction where atoms ADD ACROSS the C=C double bond of an alkene → double bond becomes single bond → new atoms attached. Example: bromine adds across C=C → double bond broken → colourless product formed → bromine water decolourises.

How do you test for saturation/unsaturation?

Add BROMINE WATER (orange). Saturated (alkane): stays ORANGE — no reaction. Unsaturated (alkene): DECOLOURISES (turns colourless) — bromine adds across C=C. This is the test for the presence of a C=C double bond.

Saturated vs unsaturated — summary card?

SATURATED: only single bonds, alkanes CₙH₂ₙ₊₂, less reactive, bromine water stays orange. UNSATURATED: has C=C double bond, alkenes CₙH₂ₙ, more reactive, bromine water decolourises. Key reason for reactivity difference: C=C double bond has high electron density → attracts attacking molecules.

Homologous series — full definition?

Group of organic compounds sharing: same FUNCTIONAL GROUP; same GENERAL FORMULA; similar CHEMICAL PROPERTIES; gradually changing PHYSICAL PROPERTIES; each member differing from next by −CH₂−.

Catalytic cracking — full conditions?

Temperature: 470–550°C (to vaporise hydrocarbons). Catalyst: hot powdered ALUMINIUM OXIDE. Mechanism: thermal decomposition as molecules contact catalyst surface. Produces mix of shorter alkanes AND alkenes.

Bromine water test — exact colours?

Starts ORANGE. Alkane: stays ORANGE (no reaction, no C=C). Alkene: DECOLOURISES → turns COLOURLESS. Bromine adds across C=C double bond (addition reaction). NOT white, NOT red — orange to colourless.

Rf value formula and properties?

Rf = distance travelled by substance ÷ distance travelled by solvent front. Both measured from BASELINE. No units. Always between 0 and 1. Constant for given substance in given solvent. CHANGES if solvent changes.

Three reasons CO₂ decreased in early atmosphere?

1. Dissolved in oceans → carbonates precipitated → sedimentary rocks. 2. Absorbed by plants/algae during photosynthesis. 3. Marine organisms incorporated carbon into shells/bones → remains formed FOSSIL FUELS (coal, oil, gas) → carbon locked underground.

Desalination — two methods?

1. DISTILLATION — heat salt water, collect condensed water vapour. 2. REVERSE OSMOSIS — force water through semi-permeable membrane, only water molecules pass through. Both expensive and energy-intensive. Used in arid regions e.g. Saudi Arabia.

LCA — four stages in correct order?

1. RAW MATERIALS (mining, deforestation, resource use). 2. MANUFACTURING (energy, emissions, waste). 3. USAGE (ongoing environmental impact of using product). 4. DISPOSAL (landfill, recycling, biodegradation). NOT distribution — it's usage.

Addition polymerisation — what is it and why alkenes?

Many alkene MONOMERS join across C=C double bonds → long-chain POLYMER. e.g. nCH₂=CH₂ → (−CH₂−CH₂−)ₙ poly(ethene). Alkenes used because C=C OPENS UP to link monomers. Alkanes have no double bond → CANNOT polymerise. Product is a plastic/polymer — NOT an alkane or alkene.

Two examples of reversible reactions?

1. NH₄Cl(s) ⇌ NH₃(g) + HCl(g) — heating ammonium chloride. Forward (heating) = ENDOTHERMIC. Reverse (cooling) = EXOTHERMIC. 2. CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l) — blue hydrated → white anhydrous. Forward = ENDOTHERMIC. Reverse = EXOTHERMIC.

Energy rule for reversible reactions?

If forward reaction is EXOTHERMIC → reverse must be ENDOTHERMIC. If forward is ENDOTHERMIC → reverse must be EXOTHERMIC. The SAME amount of energy is transferred in BOTH directions. You cannot get more energy out than you put in.

Copper sulfate reversible reaction — colours and which direction is exothermic?

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l). BLUE hydrated crystals → WHITE anhydrous powder + water: ENDOTHERMIC (heat absorbed). WHITE anhydrous + water → BLUE hydrated: EXOTHERMIC (heat released, blue colour returns). Test for water: add water to white anhydrous CuSO₄ → turns BLUE.

How does equilibrium get reached from the start?

Only reactants present → forward rate at HIGHEST. As reactants used up → forward rate DECREASES. As products build up → reverse rate INCREASES. Eventually forward rate = reverse rate → DYNAMIC EQUILIBRIUM. Concentrations remain constant. Only in CLOSED SYSTEM.

Pure substance vs impure — melting point evidence?

Pure substance: melts at ONE SPECIFIC temperature (use word "specific" or "constant" — NOT "fixed"). Impure: melts over a RANGE of temperatures AND melting point is LOWER than known value. Impurities lower melting point and raise boiling point.

Components of paint as a formulation?

PIGMENT: gives colour. BINDER: forms film to hold pigment in place. SOLVENT: dissolves other components, controls viscosity. All present in exact carefully measured quantities. Each component has a specific purpose.

Four gas tests — what to do and positive result?

HYDROGEN: burning splint → squeaky POP. OXYGEN: glowing splint → relights. CARBON DIOXIDE: bubble through limewater → turns MILKY/CLOUDY. CHLORINE: damp blue litmus paper → turns RED then BLEACHED WHITE.

Fractional distillation column — fractions from bottom to top?

BOTTOM (highest boiling point, longest chains): Fuel oil (ships/power stations) → Diesel (diesel engines) → Kerosene (jet fuel) → Petrol (cars) → LPG (domestic heating/cooking) → Refinery gas at TOP (lowest boiling point, shortest chains).

Human activities increasing CO₂ specifically?

1. Burning FOSSIL FUELS (electricity generation, transport, heating) — main cause. 2. DEFORESTATION — fewer trees means less CO₂ removed by photosynthesis. NOT just "pollution" — must be specific for exam marks.

First four alkanes — names, formulae, number of carbons?

Methane CH₄ (1C). Ethane C₂H₆ (2C). Propane C₃H₈ (3C). Butane C₄H₁₀ (4C). Pattern: CₙH₂ₙ₊₂. Memory: Me Eat Peanut Butter — Methane, Ethane, Propane, Butane.

First two alkenes — names and formulae?

Ethene C₂H₄ (2C). Propene C₃H₆ (3C). Pattern: CₙH₂ₙ. Always 2 fewer hydrogens than equivalent alkane.

Trends going UP a homologous series — boiling point, viscosity, flammability?

Boiling point INCREASES (stronger intermolecular forces in longer chains). Viscosity INCREASES (thicker, less runny). Flammability DECREASES (harder to ignite). All changes gradual — key feature of homologous series.

Complete vs incomplete combustion — equations and products?

Complete (excess O₂): CH₄ + 2O₂ → CO₂ + H₂O. Always CO₂ and H₂O only. Incomplete (limited O₂): 2CH₄ + 3O₂ → 2CO + 4H₂O. Produces CO (toxic) and/or soot (carbon particles). Balance order: C → H → O.

Thermal (steam) cracking conditions?

HIGHER temperature than catalytic. HIGH pressure. Steam added. Produces MORE ALKENES and ring structures than catalytic cracking. Greater proportion of hydrogen formed at higher temperatures.

Why is cracking done? Supply vs demand?

Demand for SHORT chain hydrocarbons (petrol, kerosene, diesel) EXCEEDS supply. Supply of LONG chain hydrocarbons (fuel oil) EXCEEDS demand. Cracking converts excess long chain → more useful short chain molecules including alkenes for polymers.

What is a hydrocarbon?

Compound containing CARBON and HYDROGEN atoms ONLY. Must say "only" — without it you will NOT get the mark. e.g. methane CH₄, ethane C₂H₆.

Types of formulae — what is each one?

GENERAL formula: composition of any member of homologous series (e.g. CₙH₂ₙ₊₂). MOLECULAR formula: actual number of each atom in one molecule (e.g. C₄H₁₀). DISPLAYED formula: shows ALL atoms AND ALL bonds including their arrangement. STRUCTURAL formula: enough detail to identify structure, identical groups bracketed, most C−H bonds omitted.

Le Chatelier colour change example 1 — 2NO₂(g) ⇌ N₂O₄(g), pressure increased?

Left side: 2 moles gas. Right side: 1 mole gas. Increased pressure → shifts to side with FEWER moles → shifts RIGHT. More N₂O₄ (colourless) formed, less NO₂ (dark brown). Mixture becomes MORE COLOURLESS.

Le Chatelier colour change example 2 — ICl(l) + Cl₂(g) ⇌ ICl₃(s), forward is exothermic, temperature increased?

Temperature increases → shifts in ENDOTHERMIC direction → shifts LEFT (reverse reaction favoured). More ICl (dark brown) and Cl₂ formed. Mixture becomes increasingly BROWN.

Le Chatelier colour change example 3 — 2NO₂(g) ⇌ N₂O₄(g), temperature increased, forward is exothermic?

Temperature increases → shifts in ENDOTHERMIC direction → shifts LEFT. More NO₂ (dark brown) formed. Mixture becomes MORE BROWN/DARKER.

Why is extinguishing a burning splint NOT a valid test for CO₂?

Other gases (e.g. nitrogen) ALSO extinguish a burning splint — it is not specific to CO₂. Always use the LIMEWATER test — bubble gas through limewater, turns milky/cloudy = CO₂. Extinguishing a splint is a property but NOT an identification test.

Why are exact ratios important in pharmaceutical formulations?

Ensures the drug is delivered to the CORRECT part of the body. At the RIGHT concentration — not too weak (ineffective) or too strong (dangerous). Is SAFE to consume. Has an adequate SHELF LIFE. Even tiny variations in ratio can make a medicine ineffective or harmful.

How do you identify an unknown substance using Rf values?

1. Run chromatogram with unknown AND known reference substances under SAME conditions (same solvent, paper, temperature). 2. Calculate Rf for unknown = distance travelled by spot ÷ distance travelled by solvent front. 3. Calculate Rf for each known substance. 4. Compare — matching Rf values = same substance. 5. If unknown produces multiple spots → it is a MIXTURE — identify each component separately.

Complete vs incomplete combustion — atmosphere context?

Complete (excess O₂): produces CO₂ (greenhouse gas → global warming) + H₂O. Incomplete (limited O₂): produces CO (toxic, binds haemoglobin) + soot/particulates (lung damage, global dimming) + H₂O. Incomplete combustion MORE harmful at ground level. Both release energy but complete combustion releases more.

Photochemical smog — what is it and what causes it?

Brown haze found near ground level in cities. Caused by NOₓ gases reacting with SUNLIGHT and UNBURNED HYDROCARBONS in the atmosphere. Causes breathing difficulties — particularly harmful for asthma sufferers. Concentrated in cities with heavy traffic. NOₓ from vehicle exhausts is the main cause.

Why do NOₓ only form at HIGH temperatures?

Under NORMAL combustion conditions N₂ and O₂ do not react — the activation energy is too high. Only at EXTREME temperatures reached in vehicle engines and power stations do N₂ and O₂ react: N₂ + O₂ → 2NO; 2NO + O₂ → 2NO₂. This is why NOₓ is mainly a problem from engines, not from burning fuels in general.