CHEM 2023-2024 S4 2nd Term Exam Revision Notes PDF

CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 1-4 Planet earth Ch 1 – Intro to Chemistry Advantage of microscale experiment 1. Reduce the amount of chemicals used. 2. Complete the experiment at a faster time. 3. Less chemical waste is produced....

CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 1-4 Planet earth Ch 1 – Intro to Chemistry Advantage of microscale experiment 1. Reduce the amount of chemicals used. 2. Complete the experiment at a faster time. 3. Less chemical waste is produced. 4. Reduce the cost of purchasing labware and chemicals. Ch 2 – The atmosphere Elements, compounds and mixtures Classification Definition Diagram for reference Substances that cannot be broken down Elements into anything simpler by chemical methods. Pure Substances composed of two or more substance elements chemically joined together. Compounds Can be broken down into something simpler by chemical methods. Two or more pure substances that are NOT Mixtures chemically joined together. Physical & chemical properties/ change Definition Example Physical Properties that can be determined without chemically - Melting point/ boiling point properties changed into another substance. - Electrical/ heat conductivity etc Physical a change without new substance formed - Changing state during heating/ cooling change (e.g.: change in state, dissolving) - Dissolving into solvent. Chemical Properties that describe the ability of that substance to - Group I Metal react in water gives properties react with other substance(s) sodium hydroxide + hydrogen Chemical Sodium reacts with chlorine to form sodium A change with new substance formed. change chloride. (Chemical reaction involved) Abundance and application of gases (just know rank is ok, NO NEED abundance) Gas Abundance (%) Applications (choose 1) Test & result Nitrogen 78% Packaging gases, refrigerant / Oxygen 21% Medical usage, welding Glowing splint, the splint relights Argon 0.9% Filling light bulb / Carbon dioxide 0.03% Fire extinguisher, refrigerant Limewater. The limewater colour changes from colourless to milky. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Fractional distillation Different gases in air can be separated by fractional distillation of liquefied air. Gas with lower b.p will condense at higher fraction. (Order of coming out: N2, Ar, O2, arranged by b.p.) Steps: Purification à Liquefaction à Fractional distillation (at fractionating column) Ch 3 – The ocean Separation methods Separation Purpose (no need to recite, just let you know) Principle (NEED TO RECITE!!!!) method But the function of set-up need to know! Filtration Separating insoluble solids (that are suspending) Small pores in filter paper block the large from a solution particles. Soluble substance still remains in solution! They Filtrate: clear solution you get are very small and able to pass through filter paper! Residue: solid left on filter paper Evaporation Extracting all the dissolved solid inside the solution The solvent (water) has a lower boiling point (e.g.: salt inside the sea water) than the solute (salts). During heating, all solvent(water) is It DOES NOT give pure NaCl! As seawater contains evaporated and the solute (salts) remains. a mixture of salt! Extracting particular pure solute the solution (e.g.: 1. Hot solvent can dissolve more solute than NaCl from sea water) cold solvent → water can no longer dissolve all common salt (NaCl) after rapid cooling. 1 Gives small crystals, 2 gives large crystals Undissolved common salt separates out as 1. crystals Crystallization 2. Water slowly evaporates in room temperature becomes concentrated 2. solution, then saturated solution. Further evaporation of water will lead to common salt being unable to dissolve in water. Undissolved common salt separates out as crystals. Extracting a solvent inside the solution 1. The solvent (water) has a lower boiling (e.g.: pure water from sea water) point than the solutes (salts) Distillation (CANNOT be used Function of anti-bumping granules: 2. At 100°C, only water boils, water vapor is to prepare pure Ensure smooth boiling formed. After the vapor meets a cold salt/solute!) surface, it condensed into distilled water. Reason of water in at lower position: (same as Fossil fuel's chapter!) Provide better cooling gradient CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Diagram to draw- distillation setup Chemical tests about sea water Testing metal ions by flame test 1. The nichrome wire is moistened with concentrated hydrochloric acid for cleaning. 2. The wire is dipped into a crushed sample of the substance being tested. 3. Wire is heated strongly in a non-luminous flame and the flame colour is being observed. Function of concentrated hydrochloric acid: 1. moisten the wire for sticking the solid being tested 2. clean the wire by removing impurities Reason of using non-luminous flame: prevent the flame from metal ions being masked Compound of metals Flame colour Potassium Lilac Sodium Golden yellow Calcium Brick red Copper(II) Bluish-green Testing chloride ions by silver nitrate test 1. Sample is dissolved in distilled water, 2. then add excess dilute nitric acid and followed by silver nitrate solution. 3. Observe the result. Formation of white precipitate suggests the presence of chloride ions. Excess nitric acid prevents formation of other write precipitates (e.g.: Ag2CO3). Testing water (or presence of hydrogen in compound) Using dry cobalt(II) chloride paper A positive result will turn the paper from blue to pink. Using anhydrous copper(II) sulphate A positive result will turn the powder from white to blue. Electrolysis of sea water (too lengthy and just ask MC, please refer to Ch.1-4 notes) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 4 – Rocks and minerals Rocks containing calcium carbonate Hardness of calcium carbonate: Chalk < Limestone < Marble List of common names Lime (calcium carbonate) cycle CaO(s) = quicklime Ca(OH)2(s) = slaked lime 1. Thermal decomposition of calcium carbonate Saturated Ca(OH)2(aq) = lime water CaCO3 !⎯# CaO + CO2 !"#$ 2 & 3. Adding water to calcium oxide CaO + H2O !⎯# Ca(OH)2 !"#$ Ca(OH)2 is slightly soluble in water. If water is limited, a milky solution is formed. If excess calcium hydroxide solid is added to water and stirred, white suspension is filtered out, limewater is formed. 4. Passing carbon dioxide into limewater Little: Ca(OH)2 + CO2 → CaCO3 + H2O CaCO3 is insoluble in water and white in colour, so CO2 turns lime water milky. When excess CO2 is passed into limewater, the milky solution becomes colourless. Insoluble CaCO3 dissolves to form soluble Ca(HCO3)2 Excess: CaCO3 + H2O + CO2 → Ca(HCO3)2 These steps repeat and forms a lime cycle. Precaution 1. Remove the tube from limewater first, otherwise sucking back of water may occur. (which further crack the test tube due to difference in T) 2. Avoid stopper another test tube, because a closed is formed, it causes pressure build up and Test of calcium carbonate breaks the test tube. Flame test can proof the existence of Ca2+, while acid followed by limewater test can proof CO32- Application of CaCO3 (too lengthy and just ask MC, please refer to Ch.1-4 notes) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 5-9 Microscopic world I Ch5: Isotopes Definition of isotopes Isotopes are atoms that have the same number of protons but different number of neutrons. Properties of isotopes same chemical properties because isotopes have the same number of electrons. different physical properties because of different number of neutrons. Definition of relative atomic mass Relative atomic mass of an element is the weighted average of the relative isotopic mass of all of its naturally occurring isotopes on the C-12 scale. Ch6: Groups and periods Gradual trend in period (from left to right) Group numbuer Group name Group I Alkali metal Decreases in metallic character Group II Alkaline earth metal Decreases in atomic size (due to stronger nuclear charge) Group VII Halogen Group 0 Noble gas (Melting pt, boiling pt is not gradual!) Gradual trend in group Increases in atomic size due to increase number of occupied electron shell Similar chemical properties in a group, due to same number of outermost shell electrons For Group I & II, reactivity increases. For Group VII, reactivity decreases. For Group 0 (noble gas), it is unreactive because it has a duplet (He) or octet structure. Application of noble gas: He (Filling floating balloons), Ne (Neon lights/advertisement sign) Ar (Filling light bulb) Exception about group 1. Hydrogen doesn’t belong to any group. (same no. outermost shell e- is NOT the same group!) For Group 0 elements, helium has 2 outermost shell electrons, while other members have 8 Starting from Period 4, elements between Group II and Group III are called transition metals CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Explanation of gradual trend in group (the bold words change with Q) (Language pattern) K/Cl has a lower position than Na/F in Group I/ VII When going down the group, the number of occupied electron shells increases Distance between the outermost shell electron and the nucleus increases Attraction between the outermost shell electron and the nucleus decreases The tendency of K/F losing/ gaining the outermost shell electron to get an octet structure increases. Ch7-8: Bondings Different types of bonding (Language pattern), seldom ask [Type] bonding a strong [direction] electrostatic attraction between oppositely charged [particle 1] and [particle 2]. Type Direction Particle 1 Particle 2 Reminder Ionic non-directional cation anion If the Q gives a specific element, Metallic non-directional metal ion delocalized electrons remember changed to it. (e.g. Al3+ for Al) Covalent directional positive nucleus bond pair electrons Formation of compounds (Language pattern), seldom ask Ionic compound (Al2O3 as example) Ionic bond is formed when 2 Al atoms each transfer 3 electrons to 3 O atoms, which forms 2 Al3+ ions and 3 O2- ions with an octet structure. Covalent compound (CO2 as example) C and O atoms are unstable , C atom has 4 outermost shell electrons, O atom has 6 outermost shell electrons. C needs 4 more electron to attain a stable octet structure, O needs 2 more electron to attain a stable octet structure. 2 O atoms each shares 2 outermost shell electrons with C atom, forms 2 double covalent bond, forming noble gas electronic arrangement in CO2 molecule. Dative covalent bond (NH4+ as example), MAY ASK Structural formula for dative covalent bond Use arrow (→) to indicate the dative covalent bond! N atom in NH3 contains lone pair electrons. While H+ ion does not have electrons in its outermost shell/ the outermost electron shell is not fully filled, which has a vacant site to accept electron pair. H+ ion accepts the lone pair electron from the N atom in NH3 to form dative covalent bond by electron sharing, so that N atom in NH4+ attains a stable octet structure. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Migration of ions (Language pattern) Function of the gel Function of moistened filter paper 1. Slow down the mixing of hydrochloric Increase the electrical conductivity acid and the copper(II) dichromate by providing mobile ions 2. Slow down the migration of Cu2+ and (Any water soluble ionic compound that Cr2O72- ions to make the result more is in liquid / solid state is conductive!) easily observed v Take the U-shape tube as an example, Positive electrode: Orange colour migrates towards the positive electrode, as the orange Cr2O72- ion is negatively charged, which is attracted towards the positive electrode. Negative electrode: Blue colour migrates towards the positive electrode, as the blue Cu2+ ion is positively charged, which is attracted towards the negative electrode. Colour of ions Ion Chemical formula Colour Cobalt(II) Co2+ pink Nickel (II) Ni2+ green Chromium (III) Cr3+ green Copper (II) Cu 2+ blue Iron (II) Fe2+ (pale) green Iron (III) Fe3+ Yellow/ brown Chromate CrO42- yellow Dichromate Cr2O72- orange Manganese (II) Mn2+ Colourless/ very pale pink Permanganate MnO 4 - purple CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ion table Common cation Common anion Charge Formula Name Charge Formula Name 1+ Li+ Lithium ion 1- H - Hydride ion Na+ Sodium ion F - Fluoride ion K+ Potassium ion Cl - Chloride ion Cu+ Copper(I) ion Br - Bromide ion Ag+ Silver ion I- Iodide ion Hg+ Mercury(I) ion NO 2 - Nitrite ion H+ Hydrogen ion NO 3 - Nitrate ion NH4+ Ammonium ion OH - Hydroxide ion 2+ Mg 2+ Magnesium ion HCO 3 - Hydrogencarbonate ion Ca 2+ Calcium ion HSO 4 - Hydrogensulphate ion Ba 2+ Barium ion CN - Cyanide ion Zn 2+ Zinc ion MnO 4 - Permanganate Ion Fe 2+ Iron(II) ion ClO- Hypochlorite ion Cu2+ Copper(II) ion ClO3- Chlorate ion Co2+ Cobalt(II) ion 2- O2- Oxide ion Ni2+ Nickel(II) ion S2- Sulphide ion Mn2+ Manganese(II) ion SO 3 2- Sulphite ion Hg2+ Mercury(II) ion SO 4 2- Sulphate ion Pb2+ Lead(II) ion S2O 3 2- Thiosulphate ion 3+ Al3+ Aluminum ion CO 3 2- Carbonate ion Fe3+ Iron(III) ion SiO 3 2- Silicate ion Cr3+ Chromium(III) ion CrO 4 2- Chromate ion Cr2O 7 2- Dichromate ion 3- N 3- Nitride ion P 3- Phosphide ion PO 4 3- Phosphate ion Ch9: Structures (Strongly suggest reference to Ch9. notes, everything is key point) Language pattern of describing physical properties (use long form in exam, i.e.: ban GCS) Physical properties Terms to use Example (need use long form like giant covalent structure) Electrical Existence of mobile ions/ Fe is an electrical conductor due to the presence of delocalized conductivity delocalized electrons electrons in its GCS. Cl2 is an electrical insulator, because there is no mobile ions and delocalized electrons present in its SMS. Hardness Strength of bond/ NaCl is hard, because its Na+ and Cl- ions are held by strong attraction ionic bonds in its GIS. I2 is soft, because its I2 molecules are held by weak vdw forces in its SMS. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Physical properties Terms to use Example Melting/ boiling Strength of bond/ SiO2 > Na2O > H2O point attraction (Usually SiO2 has a GCS, Na2O has a GIS, H2O has an SMS. GCS > GIS GMS > H2O molecules are held by weak vdw force only, while SiO2 SMS), particles involved Si and O atoms are held by strong covalent bond, Na2O Na+ and O2- ions are held by strong ionic bond. Just small amount of heat is required to break the weak vdw force. H2O has the lowest m.p. Melting Na2O requires breaking some ionic bonds only, but melting SiO2 requires breaking most covalent bond. Large amount of heat is required to break many bonds. SiO2 has the highest m.p. The order follows. Deformation Recite pattern from ionic Ionic compound: When a force is applied, relative movements behavior compound/ metals, of ions bring the opposite charge close together and result in particles involved repulsion, which breaks the crystal. Metals: [Metal] ions are packed in layers. When a force is applied, the layer of [metal] ions can slide over each other to gives a new shape. The [metal] doesn’t break because the [metal] ions are still hold by delocalized electrons by the non- directional metallic bond. Solubility of solute Whether new attraction 1. (Strong) attractive force between water molecules and in water/ organic of solute and solvent > ions of [the ionic compound] can/cannot overcome the strong solvent solvent-solvent and ionic bond between ions within [the ionic compound]. solute-solute attraction, particles involved 2. Vdw's forces between [SMS] molecules and [SMS solvent] molecules are comparable to the vdw's force within [SMS] e.g: molecules and [SMS solvent] themselves. 1. Dissolve GIS in water 2. Dissolve SMS in SMS Types of attraction: solvent Two diff. structure: Attractive force Two SMS, with water: Intermolecular force Two SMS, no water: Van der Waals force Special language pattern of graphite Electrical conductivity Graphite only forms 3 covalent bonds, the remaining unpaired electrons become explain by e--structure delocalized at the graphene layer, which can conduct electricity. Applications Lubricant: Graphite has a GCS, weak vdw force presents between C (graphene) layers, C layers can slide over each other. Pencil: Graphite has a GCS, weak vdw force presents between C (graphene) layers, C layer flakes off easily, so the layer can fall, which leave marking on the paper CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Sum up of different properties Giant metallic Giant ionic Simple molecular Giant covalent structure (GMS) structure (GIS) structure (SMS) structure (GCS) Examples All metals NaCl, KOH etc H2, CO2, NH3 etc C (diamond, graphite), quartz(SiO2) Constituent Metal ions and Cations and anions Discrete molecules Atoms particles delocalized e - Bonds holding Strong metallic Strong ionic bond Within molecule: Strong covalent constituent bond throughout throughout the Strong covalent bond throughout the particles the whole whole structure bond whole structure structure Between molecule: Weak intermolecular forces (e.g: van der Waals' force) Melting point Moderately high High Low Very high (except mercury) Break some Break some ionic Break vdw forces Break most covalent metallic bonds bonds bonds Hardness Moderately hard Hard Soft Very hard (except graphite) Deformation behavior Malleable & brittle brittle brittle ductile Solubility In water Insoluble Usually soluble Usually insoluble Insoluble (except those in (except acid, NH3) solubility table) Non-aq Insoluble Insoluble Soluble Insoluble solvent Electrical Good conductor Insulator in solid Insulator Insulator conductivity state, conductor in (except acid & NH3 (except graphite) liquid (molten) and that ionize in water) aqueous state Diagrams to draw Diamond Graphite Quartz NaCl CsCl Na+ Cl- CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 10-11 General Reactivity series: Metal Reaction with Reaction Reaction with Reaction with Extraction General cold water with steam dilute acid oxygen in air methods reactivity K Electrolysis Most Na of molten Ca ore Mg Al Zn Heating Fe with Pb carbon Cu Hg Heating Ag alone Pt Panning Least Au Extraction method: Electrolysis of molten ore Word equation: Metal compound → metal + other elements (beware of molecular gas!) Observable change: 1. Silvery liquid is formed at the negative electrode (label it!) 2. (some colour) gases are formed at the positive electrode. [Remarks: chlorine is green, fluorine is yellowish while most are colourless (NOT observable for NO COLOUR) ] Cryolite is added to reduce the melting point of Al2O3, to save energy and cost. Heat with carbon (sulphide & oxide) Word equation: 1. Metal sulphide + oxygen → metal oxide + sulphur dioxide 2. Metal oxide → metal + oxygen (beware of molecular gas!) Observable change: Silvery solid forms. (For Hg, silvery liquid; for Cu, reddish-brown solid) Heat alone Word equation: Metal oxide → metal + oxygen (beware of molecular gas!) Observable change: Silvery solid forms. (For Hg, silvery liquid; for Cu, reddish-brown solid) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Reaction: General knowledge Flame colour: Oxide colour K Lilac flame ZnO Yellow (hot) → white (cold) Na Golden-yellow flame Fe3O4 Black Ca Brick-red flame PbO Orange (hot) → yellow (cold) Mg Bright white flame CuO Black If not shown, NO flame colour & white oxide HgO Red With oxygen Word equation: Metal + oxygen → metal oxide (Fe forms Fe3O4! Not following its charge!) Observable change: (Metal oxide colour) powder forms. [Remarks: for those have two colours at diff. T, write down also. While iron will burn with yellow spark.] With steam Word equation: Metal + water→ metal oxide + hydrogen Observable change: (Metal oxide colour) powder forms. [Remarks: for those have two colours at diff. T, write down also] Way to test hydrogen: Using burning splint, a pop sound would be produced. With cold water Word equation: Metal + water→ metal hydroxide + hydrogen Observable change(For Na, K): 1. (Metal) melts into a silvery ball. 2. (metal) burns with a (flame colour) flame 3. Metals moving on water surface with a hissing sound. Observable change(For Ca): 1. (Metal) sinks and dissolves. 2. Colourless gas bubbles are evolved. 3. White suspensions are formed (if water is limited) With acid Word equation: Metal + acid → salt + hydrogen (beware of insoluble salt!) Observable change: 1. Metal dissolves. 2. Colourless gas bubbles are evolved. 3. The solution colour changes from colourless to (metal ion colour) [Remarks: 3 works on metal ions with colour ONLY] For special cases, refer to the table below. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) At first, the rate is slow but Calcium + water/ The rate is slow at first because of the CaO layer on becomes faster after some time Aluminium + acid the surface of Ca. The rate becomes faster after the CaO layer is dissolved in water. After the reaction starts, it stops Ca/Pb + dilute An insoluble layer of (salt) forms on the surface of very soon suddenly. H2SO4 metal, which prevent further reaction between Pb + dilute HCl (metal) and (acid). Displacement reaction (Assume A is more reactive than B) Word equation: Metal A + metal B compound → metal A compound + metal B Observable change: 1. (metal B colour) solids forms on (metal A) surface 2. The solution colour changes from (metal A ion colour) to (metal B ion colour) Ch 13 Rusting mechanism Full equation: 4Fe(s) + 3O2(g) + 2n H2O(l) → 2Fe2O3‧nH2O(s) Chemical nature of rust: Hydrate iron(III) oxide [Fe2O3‧nH2O] Equations Fe → Fe2+ + 2e- ½ O2 + H2O + 2e-→ 2OH- Indicator to Potassium hexacyanoferrate Phenolphthalein test Fe2+ ion turns the colour of OH- ion turns the colour of phenolphthalein K3Fe(CN)6 from yellow to blue from colourless to pink [Remarks: for corrosion of other metals, phenolphthalein still turns pink due to OH-] Rusting is a destructive process: The rust is loosely attached on the iron surface, it will fall off easily and cause the fresh iron underneath is exposed to air and water again until the iron rusts completely. Factors speed-up rusting Factor speeds up rusting Reason Presence of acidic compounds 1. Increase electrical conductivity 2. Reaction between iron and acid speeds up formation of Fe2+ Presence of soluble ionic 1. Increase electrical conductivity compounds 2. Reaction between iron and less reactive metal ion speeds up formation of Fe2+ (e.g.: Pb2+, Ag+) Higher temperature Increase reaction rate Scratching, bending or sharp Loses electrons more readily surface of iron (e.g.: tip of nail) Attachment of a more reactive Cause iron loses electrons more readily metal to iron (Connect to more reactive metals speeds up rusting) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Rusting prevention method Method Principle Protective layer The [substance] layer prevent Fe from contacting with water and oxygen (e.g.: galvanization, painting, electroplating) Sacrificial protection (metal) is more reactive than Fe, (metal) lose electrons more readily and provide sacrificial protection to Fe by preventing formation of Fe2+ Cathodic protection Electrons are provided continuously from negative electrode to iron and prevent formation of Fe2+ Corrosion resistance of aluminium Al2O3 is coated evenly on Al surface firmly, which is impermeable to water and oxygen Anodization How to conduct Al to be anodized is connected to positive terminal of battery Surface of Al is oxidized to Al2O3 , protective layer thickens Precaution Overall chemical equation: 2 Al(s) + 3 H2O(l) Al2O3(s) + 3 H2(g) Sometimes the tube may slant to the right to avoid water flowing back and crack the Set-up to draw: test tube due to temperature difference. Ch 12 Reason of percentage yield ≠100% Impurities exist at the (limiting) reactant Incomplete reaction. Side reaction (this needs example support, not suggested) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 14 Common acids Mineral acid: H2SO4, HCl, HNO3 Organic acid: CH3COOH Properties of acid Physical: Electrical conductive due to the presence of mobile ions. Sour. Indicators Colour litmus solution/ blue litmus paper Red methyl orange Red phenolphthalein Colourless Chemical reactions of acid With metals (except those less reactive than Pb) Word equation: Metal + acid → salt + hydrogen Observable change: Colourless gas bubbles are evolved. (due to H2, buy you CANNOT say H2 is formed) [Metal] dissolves. Solution colour may change (depends on type of metal) With metal oxide/ hydroxide (no limitation on metals) Word equation: Metal oxide + acid → salt + water Observable change: Temperature of the test tube increases. (The reaction is exothermic) [Solid base] dissolves. Solution colour may change (depends on type of metal ion) **Remarks: reaction may stop due to solubility table (e.g.: CaO + H2SO4 forms insoluble CaSO4) With metal carbonate/ hydrogencarbonate (no limitation on metals) Word equation: Metal carbonate + acid → salt + carbon dioxide + water Observable change: Colourless gas bubbles evolved. (due to CO2) [Carbonate/hydrogencarbonate] dissolves. Solution colour may change (depends on metal ion) **Remarks: reaction may stop due to solubility table (e.g.: CaCO3 + H2SO4 forms insoluble CaSO4) Exhibit acidic properties in water When acid dissolves in water, acid molecules ionize and form H+ ions, which are responsible for acidic property e.g.: HCl → H+ + Cl-, H2SO4 → 2H+ + SO42- Acid dissolves in non-aq solvent but no H+ ion is formed as acid cannot ionize, it fails to demonstrate acidic property. Definition of acid: Substance that dissolves in water and produces H+ as the only cation. Ionization of organic acid Only hydrogen on COOH will be ionized, arrow (⇋/→) doesn't matter THIS TIME ONLY. e.g.: CH3COOH ⇋ CH3COO- + H+ CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Common alkali Ionic compound: NaOH, KOH, Ca(OH)2 Covalent compound: NH3 Properties of alkali Physical: Electrical conductive due to the presence of mobile ions. Slippery & bitter. Indicators Colour litmus solution/ red litmus paper Blue methyl orange Yellow phenolphthalein Pink Exhibit alkali properties in water When alkali dissolves in water, it produces OH- ions, which is responsible for alkali property Definition of alkali: Substance that dissolves in water and produces OH- as the only anion. Ionization of ammonia in water e.g.: NH3 + H2O ⇋ NH4+ + OH- Chemical reactions of alkali With carbon dioxide Limited Word equation: Alkali + carbon dioxide → metal carbonate + water Observable change: No observable change, except for Ca(OH)2 case (refer to Planet Earth) Excess Word equation: metal carbonate + water + carbon dioxide → metal hydrogencarbonate Observable change: Depends on solubility. For insoluble carbonate, the precipitate redissolves. With ammonium compounds Word equation: Alkali + ammonium compound $⎯& salt + water + ammonia !"#$ Observable change: A gas with pungent smell is produced, which turns moistened red litmus paper to blue. With metal ions Word equation: salt1 (soluble) + alkali (soluble) → metal hydroxide (insoluble) + salt2(soluble) Observable change: Depends the colour of the ions, coloured/ white precipitate is formed The colour of the solution fades. Special case that redissolves: OH-: APZ, NH3: ACZ Details of redissolve: refer to next page CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Solubility table (usually used for flow chart/ distinguish) Soluble salts Insoluble salts All sodium, potassium and ammonium salts All nitrates All hydrogencarbonates All halides (Cl-, Br-, I-), with exceptions PbX2, AgX All sulphates, with exceptions BaSO4, PbSO4, CaSO4 Na2CO3, K2CO3, (NH4)2CO3 All carbonates, with exceptions NaOH, KOH, Na2O, K2O, All oxides and hydroxides, with exceptions Ca(OH)2: only slightly soluble Special case that dont dissolve: ABCPP Handling conc. acid/ alkali They need to handle with protective gloves and safety goggles. To dilute concentrated acid or alkali, Add small amount of concentrated acid/alkali into a large amount of water with constant stirring Otherwise, large amount of heat will be produced, which will boil the water and cause the acid/alkali splashing out Concentrated mineral acids Concentrated Concentrated Concentrated hydrochloric acid sulphuric acid nitric acid Volatile acid ? Yes No Yes (give out acid mist) (due to high boiling point) Reacts similary as Show usual acidic properties for bases and Acidic property its dilute form, but carbonates/hydrogencarbonates, faster due to EXCEPT metals (not producing H2, oxidize to other compounds) higher conc. Oxidizing No Yes Yes (for both dilute and concentrated, property very dilute is NOT oxidizing) Dehydrating No Yes No property (e.g. turns sugar black, (can remove H2O C6H12O6 →6 C + 6 H2O) from a molecule) Burns skin most readily Turns yellow on storage (due to large amount of (decomposed by light to give nitrogen Remarks heat produced when acid dioxide(NO2), which dissolves in water dehydrates the skin) to form a yellow solution) 4HNO3(aq) → 4 NO2(g) + 2 H2O(l) + O2(g) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Drying acidic/alkaline gas Common drying agents Includes hygroscopic and deliquescent substances Hygroscopic substance Deliquescent substance Definition substances that can absorb water vapour substances that can absorb water vapour from air from air and they dissolve in the absorbed water Example concentrated H2SO4 sodium hydroxide/ potassium anhydrous CuSO4 hydroxide silica gel anhydrous calcium chloride calcium oxide ( ) Chooisng suitable drying agents Criteria for choosing a suitable drying agent: NO reaction with the substance to be dried Otherwise, the gas and drying agent will undergoes neutralization. Drying agent Acid/alkaline/neutral Drying alkaline Drying acidic Drying neutral gases (NH3) Gases (HCl, CO2) gases CaCl2 Neutral CaO Alkaline ✗ H2SO4 Acid ✗ Common collection set-up Upward delivery Downward delivery Water displacement For light gases (e.g.: NH3) For densed gases (e.g.: CO2) For gas not soluble in water (e.g.: H2) Density: estimate by relative molecular mass ***Remarks: Setup produce NH3 need a funnel at the back to prevent sucking back, which may crack the test tube.**** CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 16-17 Definition of neutral: [H+] = [OH-], not pH 7! (As pH will change with temperature, pH = 7 only at 25℃) Common indicator colour (MUST remember methyl orange & phenolphthalein): Strong acid Weak acid Definition Acid that completely ionize in water to give H+ Acid that partially ionize in water to give H+ Example Aqueous HCl, HNO3 and H2SO4 Aqueous H2SO3, H2CO3, H3PO4 & organic acids Equation For HCl, HCl(aq) → H+(aq) + Cl-(aq) For CH3COOH, CH3COOH(aq) ⇋ CH3COO-(aq) + H+(aq) Species exist H+ ion and Cl+ ion CH3COO- ion, H+ ion and CH3COOH molecule in the NO HCl molecule due to complete ionization! solution (Also have OH- and H2O) (Also have OH- and H2O) Strong alkalis Weak alkalis Definition Alkali that completely dissociate in water to give OH - Alkali that partially ionize in water to give OH- Example Aqueous Ca(OH)2 and Group I hydroxides Aqueous NH3 (Although Ca(OH)2 is slightly soluble, it is strong!) Equation For NaOH: NaOH → Na+ + OH- For NaOH: NH3 + H2O → NH4+ + OH- Species Na+ ion and OH- ion NH4+ ion, OH- ion and NH3 molecule exist (NO NaOH due to complete dissociation!) in the solution (Also have H+ and H2O) (Also have H+ and H2O) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Results of test with different acid pH meter Electrical conductivity Reaction rate with metals/ metal carbonate Factors When comparing 1 variable only (in the 3 listed below, 2 of them should be the same): affecting Strength: Acid X is a strong acid, Acid Y is a weak acid. Acid X completely ionize in water to produce H+, while the [H+] Acid Y partially ionize to produce H+. Molarity: Acid X has a higher molarity than Acid Y. Basicity: Acid X is dibasic, Acid Y is monobasic. When comparing 2 variables (both molarity and basicity, e.g.: 0.1 M H2SO4(A) vs 0.15 M HCl(B)): Both A and B are strong acids. As acid A is dibasic, [H+] of A is 0.2M. While acid B is monobasic, [H+] of B is 0.15M. Summarize Acid X has a higher [H+]/ mobile ions concentration than acid Y. on [H+] Conclusion pH decreases for higher Electrical conductivity increases for higher Reaction rate increases for higher [H+]. [H+]. Therefore, X has a mobile ions concentration. Therefore, X Therefore, X has higher reaction rate lower pH than Y. has a lower electrical conductivity than Y. than Y. The words can be interchange: Acid à alkali, [H+] à [OH-]. But alkali CANNOT use metals to test rate! And the pH increases with higher [OH-]. (As alkali have no reaction with metals!) Reading a rate graph Strong alkalis Rate Steeper slope stands for a higher rate. (Explanation of slope based on [reaction rate] above) Final value If amount of metal added is same, if curve X & Y stays at same level, The nH+ is same. If curve X has a higher level nH+ of X is higher. (Explanation of slope based on calculation of nH+) Ch 18 Reading a temperature graph Mention 1. Increases → 2. Maximum → 3. Decreases in T 1. From 0 cm3 to 20.0 cm3 of NaOH added: Temperature increase, because neutralization is an exothermic reaction. 2. When 20.0 cm3 of NaOH added: Maximum temperature is reached, because neutralization is just completed. 3. After 20.0 cm3 of NaOH added: Temperature decreases. Neutralization stops, addition of alkali in room temperature cools the solution mixture. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Temperature change of different acid alkali (usually MC, look at Ch18 for detail explanation) Experiment Acids and alkalis reacted Temp. rise 1 25 cm3 of 1.0 M HNO3(aq) + 25 cm3 of 1.0 M KOH(aq) T 25 cm3 of 1.0 M HNO3(aq) +25 cm3 of 1.0 M NH3(aq) 2 < T (or 25 cm3 of 1.0 M CH3COOH(aq) +25 cm3 of 1.0 M KOH(aq)) 3 25 cm3 of 2.0 M HNO3(aq) + 25 cm3 of 2.0 M KOH(aq) 2 T 4 50 cm3 of 1.0 M HNO3(aq) + 50 cm3 of 1.0 M KOH(aq) T 25 cm3 of 1.0 M HNO3(aq) + 25 cm3 of 2.0 M KOH(aq) 5 T (or 25 cm3 of 2.0 M HNO3(aq) + 25 cm3 of 1.0 M KOH(aq)) Guideline: based on strength (weak acid/ alkali uses some T for compete ionization), (may need calculate sometimes)/ %' $ /)' % % !" # & Application of neutralization (usually MC) 1. Adjusting soil pH with quicklime, slaked lime 2. Neutralizing excess acid in stomach with antacid 3. Making fertilizer with ammonia and nitric acid/sulphuric acid 4. Neutralize industrial waste Wrong application of acid/alkali neutralization 1. Using strong alkali to neutralize excess acid in stomach/ farmland Reason: strong alkali may corrode the stomach/ farmland 2. Using ammonia to neutralize excess acid in stomach Reason: ammonia is pungent Type of salt Type How to produce pH of solution produced at 25℃ Acid Strong acid + weak alkali/ insoluble base 7, alkaline (Reminder: place the anion at the front for organic acid! E.g.: CH3COONa) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Preparation of salt Soluble salt Soluble salt (NH4+, K+, Na+) Insoluble salt (not NH , K , Na ) 4 + + + Preparation 1. Add excess metal/ Perform a titration first: 1. Mixing between two method & insoluble base/ 1.Transfer a fixed volume of dilute [alkali] in a conical solutions insoluble carbonate to flask, add methyl orange as indicator, which gives a (If solid is given, dissolve the required acid. yellow colour. the solid in distilled 2. Filter off the excess 2. Fill the burette with [acid], run the acid from burette water first, then mixing) solid and collect the into conical flask, calculate the volume of acid needed 2. Filter off the residue filtrate. when the solution in flask turns to a persistent orange. and collect it. 3. Jot down the volume of acid required for complete neutralization. 4. Repeat step 1-2 without adding methyl orange Rationale Excess solid metal/ NH4+, K+, Na+ are all soluble, which cannot be prepared Adding excess solid into (No need to insoluble base/ by dissolving excess solid into acid as excess soluble acid will produce write in insoluble carbonate can base will mix with the soluble salt formed. insoluble layer that procedure) be filtered form the hinder the reaction, limited acid solution won't. Washing Heat the filtrate until the solution becomes saturated. Rinse the reside with and drying Let the hot saturated solution to cool at room temperature slowly with a filter large amount of distilled paper covered. Wait for crystal formation water. Filter off the crystals, wash with little amount of cold distilled water Dry the crystal with a (to wash away water soluble impurities without dissolving the whole crystal) dessicator. Dry the crystal with a desiccator. Reminder: If an incorrect substance is given you at first (e.g.: give you CaCO3 solid to prepare insoluble CaSO4), convert to the correct substance first. (e.g.: dissolve excess CaCO3 into HNO3 to give pure Ca(NO3)2(aq) first. Then do precipitation) Ch 19 Standard solution and primary standards Definition of standard solution: A solution with accurately known concentration. How to choose primary standard (only Na2CO3 and oxalic acid crystals are used usually): 1. Readily in high purity 2. Chemically inert (doesn't react with O2/ CO2 / H2O/ decompose) 3. Dissolve readily in water 4. Not hygroscopic (not a drying agent) 5. High molar mass (minimize weighing area) 6. Non-toxic and non-volatile CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Procedure of preparing standard solution Prepare from primary standard Prepare from existing standard solution May require calculation in mass first. May require calculation in volume/ molarity first. 1. Dissolve solid acid A with little distilled water in beaker. 1. Transfer xx cm3 solution to a yy cm3 volumetric flask 2. Transfer dissolved acid A solution to a 250.0 cm3 with a xx cm3 pipette. volumetric flask. 2. Add distilled water to the flask until the bottom of 3. Transfer all washings from the filter funnel, glass rod and meniscus reach the graduation mark. beaker to the flask. 3. Stopper the flask and invert it for several times to mix 4. Add distilled water to the flask until the bottom of the solution thoroughly. meniscus reach the graduation mark. 5. Stopper the flask and invert it for several times to mix the solution thoroughly. Washing method Apparatus Washing method Rationale Burette Wash with distilled water, then wash Distilled water: removes impurities Pipette with the solution to be contained Solution to be contained: prevent dilution by water adhered Volumetric flask Wash with distilled water only Water: removes soluble impurites Conical flask Water remained DOES NOT affect nacid/alkali contained If wash with solution contained, nacid/alkali increases Apparatus Preparation of equipment Rationale/ consequence Burette Ensure no air bubble is trapped at the burette Some titrants would be used to fill up the air bubble/ Ensure the space between tap and tip of the space between tap and tip of burette. Amount of burette is filled with the solution. titrant will be overestimated. Remove the filter funnel on the top of burette after Solution clinging on the funnel may fall into burette, filling the burette with solution amount of titrant will be underestimated. Pipette Touch the liquid surface for last tip / E.g.:Procedure of preparing a burette with 0.1M HCl(aq) for titration. 1. Wash the burette with distilled water, then wash with 0.1M HCl. 2. Add 0.1M HCl(aq) into the burette with a filter funnel, then take away the filter funnel. 3. Allow the solution to run, ensure the space between the tap and tip of the burette is filled and no gas bubbles are trapped inside. E.g.: Procedure of conducting a titration between 0.1M HCl and 0.2 M NH3 (continue from Step 1-3) 4. Wash the conical flask with distilled water, transfer 0.2 M 25.0 cm3 NH3 into the flask. 5. Add [indicator] into the flask, which gives a [colour] colour. 6. Jot down the initial V of burette. Run the 0.1M HCl until the flask gives a persistent [colour] colour. Jot down the final V.  CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Equivalence point and endpoint Equivalence point Endpoint Definition Point that acid and alkali are Point that the indicator change the colour sharply completely neutralized with each other. (it only use to ESTIMATE equivalence point, they are NOT the same thing!) Ways to estimate equivalence point Method How Change in temperature Maximum temperature (refer to Ch18 part) Change in electrical conductivity Minimum value as the concentration of ions is lowest at equivalent point due to cancel out of H+ and OH- Change in pH The midpoint of sharp change pH range End-point Using a suitable indicator, where it changes the colour sharply Information from a titration curve Need to draw: titration set-up pH of alkali Equivalent point pH of acid Choosing suitable indicator (methyl orange/ phenolphthalein) Acid Alkali pH at equivalence point Indicator strong acid strong alkali 7 BOTH are suitable strong acid weak alkali/ soluble carbonates 7 Phenolphthalein weak acid weak alkali/ soluble carbonates ~7 no suitable indicator E.g.: Is phenolphthalein a suitable indicator in the titration between HCl and NH3? No. Equivalent point of neutralization between HCl and NH3 is 7. Conducting titration 1. Titration is repeated several times to improve the precision, all titrant volume should be in 2 d.p. 2. To deduce the final result is overestimate/ underestimate, you need to state how the titrant volume is affected. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 20 How fossil fuel is formed Fossil fuel Coal Petroleum Natural gas Major Over 50% carbon, with HC (hydrocarbon), Liquid mixture of HC Gaseous mixture of HC: mainly composition compounds of S, N and O CH4 Origin Remains of ancient plants Remains of ancient marine animals Formation Covered with sand and mud, under the effect of high pressure, temperature and bacterial actions on dead bodies Separating petroleum in laboratory Need to draw: fractional distillation set-up Principle of separation Petroleum is a mixture of hydrocarbons with different boiling points During heating, [smaller hydrocarbons] with lower boiling points will vaporize first, while [larger hydrocarbons] with higher boiling points will remain in liquid The vapour then condensed back to liquid upon cooling by iced water and is collected in the test tube Fractions with different boiling point ranges are collected in test tubes at different temperature ranges. Difference of boiling point in a molecular view larger molecular size, strength of vdw forces increases, boiling point increases. State of Hydrocarbons: Gradual change of petroleum fraction 1-4 C atoms: gases For heavier fractions, as the molecular size is larger, vdw forces increases: 5-17 C atoms: liquids Boiling point increases (State changes from gas liquid solid) > 18 C atoms: solids Colour intensity increases Viscosity (resistance of flowing) increases, volatility/ ease of evaporation decreases Flammability / ease of burning decreases, burns with sootier flame CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Refining of petroleum in oil refinery Step 1: Petroleum is vaporized in a furnace to about 400oC most hydrocarbons are vaporized, very long hydrocarbons remain in liquid/ solid. Step 2: Pump into fractionating tower ⟹ liquid/solid part falls to the bottom and becomes residue while vapour part rises in the tower Step 3: [Larger fractions] with higher boiling point: condense at lower, hotter level [Smaller fractions] with lower boiling point: condense at lower , cooler levels (Lightest fraction (very short hydrocarbons) with the lowest boiling point rise to the top. It will NOT condense and come out as gas.) Demand of petroleum fraction (too length and usually just MC, plz refer to the notes) Combustion of hydrocarbons Complete combustion Incomplete combustion Condition Enough oxygen Limited supply of oxygen, chance increases with 1. carbon content (e.g.: C3H8 vs C7H16) 2. C:H ratio (e.g.: C3H6 vs C3H8) Product CO2 (carbon dioxide), H2O (water) CO2 (carbon dioxide), H2O (water), CO (carbon monoxide), C (carbon/ soot) Flame Blue, clearer, higher temperature Yellow/ orange, sooty, lower temperature (known as non-luminous flame, produced by (known as luminous flame, produced by closing the opening the airhole of bunsen burner) airhole of bunsen burner) Equation CH4 (g)+ 2O2(g)→ CO2(g) + 2H2O(l) 2CH4 (g)+ 3O2(g)→ 2CO(g) + 4H2O(l) example General case: 2C9H20(l) + 45 O2(g) → 14 CO2 (g)+ 2CO(g) + 2C(s) + 10 H2O(l) ( ( #& $' + &' + * +( → ' #+( + $( + ) , E.g.: Combustion of C3H6 will produce more sooty flame than C3H8. The C3H6 has a higher C:H ratio, therefore it undergoes incomplete combustion easier. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Air pollution arising from burning fossil fuel Air pollutant Major Source Formation Harmful effects Human health Environment Carbon toxic (binding to monoxide haemoglobin, reduce O2 carrying capacity) Car exhaust irritate respiratory cause photochemical smog Incomplete combustin Unburnted tract (decreases visibility, of fossil fuels hydrocarbons carcinogenic irritates eyes and cause respiratory system) Suspended Car exhaust/ cardiovascular darken building walls particulates power plants diseases decreases visibility 1. At high temperature of car engine, nitrogen and oxygen in cause photochemical Nitrogen Car exhaust air combines to form NO, smog (produced when oxides UV react with NOx and 2. which rapidly oxidizes to (NO and NO2) irritate eyes and HC) NO2 in air respiratory tract cause acid rain N2(g) + O2(g) ⟶ 2 NO(g) (CO2 wont!) 2 NO(g) + O2(g) ⟶ 2 NO2(g) Sulphur Powerplant/ Burning fuel with sulphur cause acid rain dioxide incinerator / its compounds (e.g: coal S(s) + O2(g) → SO2(g) Reducing the emission of air pollutants from burning of fossil fuels Air pollutant Measures of reducing emission Nitrogen oxides Installing catalytic converter in cars (using Pt/ Rh as catalyst) Carbon monoxide - 2 CO(g)+ O2(g) ⟶ 2 CO2(g) Unburnt - CxHy + ( x + * ) O2(g) ⟶ x CO2(g) + + H2O(l) ) ) hydrocarbon - 2 NO + 2CO à CO2 + N2 The converter just works on unlead petrol, because Pb will poison the catalyst Honeycomb structure can increase the surface area for faster reaction AND decrease the amount of catalyst used Sulphur dioxide Installing scrubber (Passing through limewater [wet] or quicklime[dry] ) Ca(OH)2(aq) + SO2(g) ⟶ CaSO3(s) + H2O(l) / CaO(s) + SO2(g)à CaSO3 (s) Using fuel with low sulphur content Suspended Installing electrostatic precipitator particulates Mechanical filtering (for large particulates) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Ch 21 (for naming, please refer to the notes yourself) Homologous series Def.: a series of carbon compounds that have the same general formula and each member differs from next by a -CH2- unit (14 in molar mass). Homologous series Functional group General formula Example Alkanes CnH2n+2 ( n starts from 1 ) CH4, CH3CH3, CH3CH2CH3, Alkenes CnH2n ( n starts from 2 ) CH2= CH2, CH3CH= CH2, CH3CH2CH=CH2, C=C double bond Alkanols CnH2n+1OH ( n starts from 1 ) CH3OH, CH3CH2OH, CH3CH2CH2OH, Hydroxyl group Alkanoic acid CnH2n+1COOH ( n starts from 0) HCOOH, CH3COOH, CH3CH2COOH, Carboxyl group Reminder: even showing the same structural formula, they need to have the similar chemical properties for being in a homologous series! (e.g.: cyclobutene and propene both are C3H6, but only propene do addition reaction!) Trend in homologous series They have the similar chemical properties because they have the same functional group. They show gradual trend in physical properties (due to ↑ of molecular size, v.d.w. force ↑) Melting point and boiling point increases Density increases Volatility decreases Viscosity increases Ch 22 Chemical reactions of alkanes They are unreactive as the C-H and C-C bonds are strong. (e.g.: paraffin oil) Combustion (refer to Ch.20) Substitution reaction with halogens Condition: Alkane will react with will halogen in the presence of UV light (sunlight) or heat for some time. Explanation: Time and energy is needed to break the X-X bond in halogen for forming radicals. Amount of halogen: If excess, gives fully-substituted alkane as major product. If limit, give monosubstituted alkane as major product. CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Step name Simplified electron diagrams Equation(s) Reminder Step 1 Initiation Cl2 → 2Cl Energy from UV light/heat break the UV light Cl-Cl bond and produce two radicals , a species with unpaired electrons, (not an octet electronic arrangement), which is very reactive Step 2 Propagation CH4 +Cl → A radical combine with a molecule, CH3 + HCl which produces a new radical and a new molecule (chain reaction) CH3 + Cl2 → Key steps of forming a mixture CH3Cl + Cl Step 3 Termination 2Cl → Cl2 Any two of the radical combine and form a stable molecule CH3 + Cl → The chain reaction will stop until all CH3Cl radicals are consumed. 2 CH3 → C2H6 Substitution gives a mixture of product, as the [haloalkane] can be further substituted by the halogen. Observable change: The [halogen] solution colour changes from [xxxx] to colourless under light after some time. (If aqueous halogen is used) Two immiscible layer of solution is formed. (aqueous cannot mix with organic the haloalkane formed.) Halogen Colour in aqueous Colour in organic solvent Chlorine Pale green Pale green Bromine Brown Orange Iodine Dark brown Purple Chemical reactions of alkenes Cracking Cracking is the breaking down of large molecules into smaller ones. (chemical change) General equation: Larger alkane → 1 smaller alkane + smaller alkene(s) The reaction is endothermic as energy is required to break the C-C bond. Condition required: 1. Absence of air (oxygen) [If air is present, it undergoes combustion instead] 2. Heated to high temperature 3. Involve catalysts Site of reaction: On the surface of catalyst Collection method: Alkanes and alkenes are insoluble in water, so they can be collected by displacement of water (Ch14) CHEM 2023-2024 S4 2nd term exam – Revision Notes (Only include things to recite) Precaution 1. The catalyst is heated instead of the wool because heating paraffin directly will vaporize the paraffin quickly without being cracked by the catalyst. 2. The first few cm3 of gas obtained is discarded because it is mainly composed by atmospheric air. 3. After cracking, the delivery tube should be removed before stop heating. It prevents sucking back of water, otherwise the boiling tube will be cracked. Importance of cracking 1. Producing extra petrol from heavy fractions (to meet the increasing demand) 2. To produce alkene for making useful chemicals (e.g.: plastics) Addition reaction (can test for unsaturation/ double bond!) Benzene cannot undergoes addition reaction!! (due to extra stability) Addition with halogen General equation (please change the alkene & halogen accordingly, depends on the Q) C C + X2 C C X X Reminder: If there is more than double bond in the molecule, the amount of halogen required doubles. Observable change: The [halogen] solution colour changes from [xxxx] to colourless. (If aqueous halogen is used) Two immiscible layer of solution is formed. (aqueous cannot mix with organic the dihaloalkane formed.) Addition with cold, dilute KMnO4/H+ General equation (please change the alkene accordingly, depends on the Q) + H2O + [O] C C OH OH Reminder: If there is more than double bond in the molecule, the amount of water and [O] required doubles. Observable change: The solution colour changes from purple to colourless. Two immiscible layer of solution is formed. Test of unsaturation: Procedure: Add cold, dilute KMnO4/H+(aq) (or KMnO4/OH-) to alkane and alkene(or alkynes) respectively. Only the alkene can change the solution colour from purple to colourless. (for OH-, it forms brown ppt) Cold, dilute KMnO4/H+ CANNOT be used to synthesize diol, because it may further oxidize the diol. Cold dilute KMnO4/OH- is used to synthesize the diol instead. Ch23 Refer to the main notes, you need to study more as there is no assessment on it!