Planet Earth Microscopic world I Metal Acid and base Fossil fuels and carbon compound Microscopic world II Redox Planet Earth What is the purpose of a condenser? - To cool down the vapourised solvent and condense it into distillate What is the purpose of anti-bumping granules? - To prevent bumping due to overheating of seawater (ensure even boiling) Where should the thermometer be placed? - It’s near the side arm of the still head As neutralization is exothermic, the temperature of the solution rose when sulphuric acid was added to the sodium hydroxide solution - When sodium hydroxide was completely reacted, the temperature reached a maximum - After that, the excess cold sulphuric acid will lower the temperature of the reaction mixture. Why is gunpowder explosive? - Thermal expansion - A lot of gaseous products produced What is the purpose of electrolytes? - To provide a medium for ion transfer Crystallization - Since hot solvents can dissolve more solute than cold ones, when the solution cools, the solubility decreases. - The solvent cannot hold all the dissolved solute and some solid crystals will be separated. Method to measure the percentage of solubility of a substance - Prepare a saturated solution of KCl by dissolving excess KCl into hot water and allowing it to cool down to room temperature. Filter the undissolved KCl - Measure the weight of the container - Measure the weight of the saturated solution - Evaporate the water to dryness in the oven and keep on evaporating and measuring until the mass of the solid is constant Solid minus the weight of the container/whole solution minus solid (= solid/water) Silver nitrate test - Warm the compounds with NaOH - Cool add acidified Silver nitrate Underlying principle in distillation set-up - The water boil at 100C but salts in sea water are non-volatile - THe steam formed condenses at the cool surface of the condenser to liquid distillate. Microscopic world I Relative isotopic mass - It is the relative mass of one atom of that isotope on the 12C = 12 scale Relative atomic mass - It is the average isotopic mass of all naturally occurring isotopes on the 12C = 12 scale. Relative molecular mass - It is the sum of the relative atomic masses of all the atoms present in the molecule on the 12C scale Formula mass - It is the sum of the relative atomic masses of all atoms present in a formula unit of the substance. Ionic bond - It is a strong nondirectional electrostatic attraction between a cation and anion by transferring electrons from one atom to another. Metallic bond - The strong nondirectional electrostatic attraction between a sea of delocalized electrons and the positively charged metal cations. Covalent bond - It is a strong directional electrostatic attraction between the shared electron and two positively charged nuclei of the bonded atom Dative covalent bond - It is a covalent bond between two atoms where both electrons of the shared pair are contributed by the same atom - Lone pair electron on the Oxygen atom - There is a vacant site on the H+ ion's outermost electron shell - By accepting the lone pair electrons on the N atom, a dative covalent bond is formed between N and H+ such that H+ can form a stable duplet structure Why is soluble in water - The strong attractive force between water molecules and ions can overcome the strong ionic bond between ions in the ionic compound Hard but brittle - There are strong ionic bonds between opposite-charged cations and anions. - Ions are regularly packed in a regular cubic pattern in an ionic compound so when force is applied, the layer will slip over each other and end in repulsion between the same charged ions Allotropes - Several possible forms of an element that have different structures Molecules soluble in organic solvent - The attraction between molecules is quite similar to that in organic substance Metals are malleable - They can slide over each other when an external force is applied and settle in a new position and continuously hold by metallic bonds. - Alloy is less malleable because other atoms will disturb the regular arrangement pattern of metal, preventing the smooth sliding of atom layers over each other Ion migration - Under the influence of the electric field, Cr2O72- with be attracted to the positive pole while Cu2+ will be attracted to the negative pole How atoms can attain noble gas electron arrangements? - For bonding between nonmetal atoms, they share their outermost shell electrons to form a molecule - For bonding between metal and nonmetal, the metal donates electrons to the nonmetal to form a cation while the nonmetal accepts electrons to form anion. They are held together by ionic bond Types of bonding - Group I-III forms an ionic bond with group 4-7 atoms - Group IV-VII atoms form covalent bonds within those atoms - Group I-III atoms form metallic bonds within those atoms - Group 0 atoms do not form any chemical bonds. Strength of ionic bonds - The strength of the ionic bond is affected by the charge of cations and anions - Mg2+ and O2- are doubly charged while Na+ and Cl- are singly charged - Therefore the ionic bond between Mg2+ and O2- is stronger than between Na+ and ClComparing diamond and graphite - Diamond has a giant covalent structure with strong covalent bonds between the C atoms in the network. - The strong directional character of covalent bonds restricts the relative motion between C atoms, so it is hard - Graphite is held in the layer structure, so the weak van der Waals force allows the layers to slip over each other. Why nitrogen can’t form pentachloride (NCl5) Metal Limestone in blast furnace - Remove impurities - The calcium carbonate in the limestone reacts with the silicon dioxide to form calcium silicate (also known as slag) Slag - The raw material of cement Why metal X is more reactive? - It loses electrons more readily to form stable ion Acid and base Why cannot rinse the conical flask with a solution to be contained - Some H+ will remain in the flask - Such that more alkali than actually required is used to titrate until the endpoint Why cannot leave the filter funnel on the burette? - Solution clinging on the stem of the filter funnel will drop into the burette - The volume of alkali used will be less than actual What is the strength of acids/alkalis? - The extent of ionization/dissociation Methods used to compare the strength of the acid - pH meter → check H+ concentration The brightness of light bulb → using alkali/acid as electrolyte. Higher mobile ion concentration, different electrical conductivity. Rate of gas bubbles production with Mg and Zn → Amount of gas bubbles produced is affected by the no. of a mole of limiting reactant used. Arrange 1M Ethanoic acid, 1M HCl, and 1M H2SO4’s pH in descending order - Ethanoic acid is a weak acid that only undergoes partial ionization to give out H+ so HCl and H2SO4 are strong acids that undergo complete ionization to give out H+. Therefore, [H+] of HCl and H2SO4 is higher than that of Ethanoic acid. - H2SO4 is dibasic while HCl is monobasic so when they ionize, the former ionizes more H+ such that the [H+] of H2SO4 is higher than HCl. Therefore the pH of H2SO4 is lower. Three factors lead to pH - Basicity → under the same acid concentration and strength, acid with higher basicity gives a higher concentration of H+ - Strength of acid → under the same basicity and concentration, strong acids completely ionize while weak acids only partially ionize - Concentration → under the same basicity and strength, the more concentrated an acid is, the more concentrated the H+, the lower the pH. Why wash precipitate with distilled water - To remove any water-soluble impurities Titration steps - Titrate dilute hydrochloric acid against aqueous ammonia with a known volume using methyl orange as an indicator until the color change from yellow to orange. - Repeat the titration once by adding the same volume of titrant required to reach the endpoint in the first titration without adding any indicator. Ammonium nitrate of Ammonium chloride is better as a fertilizer - Ammonium nitrate - It has a higher percentage by mass of nitrogen than ammonium chloride Conduct a physical experiment to test the extent of ionization - Method → Put a known volume of 1M HCl and 1M CH3COOH into two separate beakers and connect a battery and a light bulb in the circuit with acid as the electrolyte to compare the brightness of the light bulb. - Hypothesis → The stronger the acid, the brighter the light - Result → 1M HCl has a brighter light than 1M CH3COOH - Explanation → HCl is a strong acid that completely ionizes to give H+ when dissolved in water. However, CH3COOH is a weak acid that partially ionizes to give H+ when dissolved in water such that the concentration of mobile ions in HCl is higher than in CH3COOH. Conduct a chemical experiment to test the extent of ionization - Method → Put the known volume of 1M HCl and 1M CH3COOH into two beakers and put two identical magnesium strips in separate beakers with the acids. Comparing the initial rate of gas bubbles evolved. - Hypothesis → The stronger the acid, the higher the initial rate of gas bubble effervesce - Result → 1M HCl has a faster rate than 1M CH3COOH - HCl is a strong acid that completely ionizes to give H+ when dissolved in water. However, CH3COOH is a weak acid that partially ionizes to give H+ when dissolved in water such that the concentration of mobile ions in HCl is higher than in CH3COOH. - Fossil fuels and carbon compound Types of fossil fuels - Coal(mainly C with S and N and Hydrocarbon mixture), Petroleum(Hydrocarbon mixture), and natural gas(Small hydrocarbon mixture) → covered with sand and mud under high temperature, high pressure, and bacterial action on dead plants and animals. Petroleum - 1-4 C gas, 5-17 C liquid, more than 18 C solid - 1-4 Refinery gas, 5-10 petrol/naphtha, 10-14 Kerosene, 14-25 diesel oil, >25 fuel oil/lubricating oil/wax/bitumen - Bubble cap can lower the rising rate of vapor → more effective separation - Some very short hydrocarbons won't condense and rise to the top to come out as gas. Why smaller carbon compound better fuel than larger carbon compound - Smaller compound has a smaller relative molecular mass than larger compound so the former vapourize more easily/has a lower boiling point. - Smaller compound burns more completely than larger compound. How petroleum can be separated - They are a mixture of hydrocarbons with different boiling points - Heavier, larger hydrocarbons will be collected at the lower, hotter fraction - Lighter, smaller hydrocarbons will be collected a the higher, cooler fraction. Photochemical smog - NO2 reacts with O2 to form O3(ozone) - O3 oxidize with unburnt hydrocarbons and form photochemical smog. Reduce air pollution - Catalytic converter → CO, NO, unburnt hydrocarbons - Scrubber → SO2 (dry CaO, wet Ca(OH)2 - Electrostatic precipitators → suspended particles negatively charged so will attach to positively charged plates Substitution reaction (photochemical free radical substitution) - A chemical reaction in which an atom in a molecule is replaced by another atom. - Initiation → free radical = group of atoms with an unpaired electron - Propagation → radicals keep on consumed and regenerated - Termination → two radicals combine together to form a stable molecule, when no more radical remains, the reaction stops. Substitution - A chemical reaction in which an atom or a group of atoms in a molecule is replaced by another atom. Addition reaction - A chemical reaction in which two or more atoms react to form a single product When handling hydrocarbon gasses - Ensure sufficient ventilation What is cracking - The process of breaking down large hydrocarbon molecules into smaller ones by high temperature or catalyst. What are free radicals? - Atoms with an unpair electron Acids have a higher boiling point than alcohol - Can form a more extensive hydrogen bond between molecules (extensiveness is the strength) - (because C=O is very electron drawing) Amides have a higher boiling point than acid - They can form a greater number of hydrogen bonds than acid - So they can form a more extensive hydrogen bond. Water solubility - Maximum hydrogen bond that a molecule can form with water. - Polar molecules → They are soluble in water because both water and __ are polar molecules. Therefore, the intermolecular forces between water are of comparable strength as the intermolecular forces between __ molecules - Nonpolar molecules → They are insoluble in water because the intermolecular forces between water are much stronger than the intermolecular force between __ molecules. Free-radical Addition polymerization - A reaction in which monomer molecules are joined together and repeated to form a polymer without eliminating small molecules. - Conditions → high temperature/high pressure/under suitable catalyst Why polymer does not have a constant relative molecular mass - Mixture The boiling point of organic compound - Carbon chain length increase - VDWF stronger - So boiling point will be higher Melting point - More symmetrical - Higher packing efficiency - More heat is needed to overcome the enough VDWF between molecules Polar molecules that are soluble in water - Both water and X are polar molecules - The intermolecular force between X is of comparable strength as the intermolecular focus between water molecules. Boiling point comparison (Alkane and Ester) - Alkane is nonpolar while ester is polar - Stronger VDWF Boiling point comparison (Ester and Alcohol or all other H bonds) - Alcohol can form strong intermolecular H bonds while Ester can only form weak VDWF Boiling point comparison (Acid and alcohol) - Acids can form more extensive hydrogen bonds than alcohol - In a gaseous state, carboxylic acid exists in a form called dimers Boiling point comparison (Amides and acid) - Amide can form a greater (average) number of H bond - More extensive hydrogen bond Solubility (alkanes) - They are nonpolar molecules - The weak VDWF between alkane molecules is weaker than the intermolecular H bond between water molecules. Solubility (Haloalkane) - Insoluble or slightly soluble - Polar haloalkane has a relatively higher VDWF - However, not strong enough to overcome the intermolecular hydrogen bond between water molecules Solubility (small-sized aldehydes, ketones, esters) - Contain carbonyl group that can form hydrogen bonds with water molecules - If large sized, the larger nonpolar hydrocarbon part predominates over the H bond so the solubility decreases with size. Solubility (Alcohols, acids) - Can form hydrogen bonds with water - More extensive hydrogen bond H bond strength between branched and unbranched - More branched has higher steric hindrance on the formation of hydrogen bonds - More difficult to form hydrogen bonds - The strength of hydrogen bonds decrease - Lower boiling point and viscosity Cis trans boiling point - Cis isomers are more polar than trans isomers as the polar C-Cl bond in cis isomers cannot be canceled out each other while they can be canceled in the trans isomers - Therefore, VDWF in the cis isomer is larger than the trans isomer Cis trans melting point - The melting point of trans isomer is higher - It is more symmetrical than the cis isomer - Higher packing efficiency - More heat is needed to overcome the enough VDWF between molecules during melting Melting point of alkenes that are amine, amide, acid, and alcohol - Since butenedioic acid forms intramolecular H bonds while trans-butenedioic acid would not, trans-butenedioic acid can form more extensive intermolecular hydrogen bond Catalytic hydrogenation - Use finely divided Pt Pd Ni as a catalyst - Hydrogen gas - Room temp - Convert liquid animal oils and vegetable oil to margarine (artificial butter) but trans fat is produced Elimination reaction - A reaction in which a small molecule is removed from a reaction Markovnikov’s rule - The major product is the product where the hydrogen atom is added to the carbon atom of the C=C double bond that already carries a larger number of hydrogen atoms Zaitsev rule - The major product is the more substituted alkene Enantiomers - Two stereoisomers that are mirror image of one another but not superimposable. Microscopic world II What is electronegativity - The power of an atom attracts bonding electrons. Polarity of bonds - Based on EN Polarity of molecules - Based on the number of polar bonds and shape Deduce the shape of the molecule - The molecule has __ bond pair electrons and __ lone pair electrons in the outermost shell of the central atom - The __ electron pairs adopt a __ geometry so that the electrons can be furthest from each other to minimize the repulsion - No lone pair: Therefore the molecule adopts a __ shape - Lone pair: Since the molecular shape is only determined by the arrangement of atoms, the molecules adopt a __ shape. Is the covalent bond polar? No - 2 chlorine atoms have the same electronegativity. - The shared electrons between 2 atoms are evenly distributed Is the covalent bond polar? Yes - Hydrogen and Oxygen have different electronegativity. - The shared electrons between 2 atoms are unevenly distributed Explain the polarity of molecules (Linear-Polar) - Oxygen has a stronger electronegativity than hydrogen so the OH bond is polar - OH is linear in shape - There is one polar OH bond in the molecule Explain the polarity of molecules (Linear-Non polar) - Oxygen has a stronger electronegativity than carbon so the C=O bond is polar - CO is linear in shape The two polar bonds are arranged linearly So the polarity of bonds can cancel out each other Explain the polarity of molecules (V-shaped-Polar) - Oxygen has a stronger electronegativity than hydrogen so the OH bond is polar - H2O molecules are arranged in a V-shaped shaped - The two polar HO bonds are not arranged linearly - So the polarity of bonds in H2O cannot cancel out each other Explain the polarity of molecules (Tetrahedral-Polar) - Chlorine has a higher electronegativity than carbon and carbon has a higher electronegativity than hydrogen so the CCl bond and CH bond is a polar bond - CHCl3 is tetrahedral in shape - The polarity of CCl and CH bonds are different - So the polarity of bonds cannot cancel out each other. Explain the polarity of molecules (Trigonal planar-Non polar) - Fluorine has a higher electronegativity than Boron so the BF bond is a polar bond - BF3 is trigonal planar in shape - The three BF polar bonds are symmetrically arranged on the same plane - The polarity of bonds in BF3 can cancel out each other. Liquid deflection - When a positively charged rod is placed near the jet - The water molecules orientated themselves such that the negative ends of the molecules are attracted toward the rod Van der Waals in nonpolar molecules - Random movements of electrons in Cl2 molecules result in uneven distribution of electrons. Partial positive and negative charges are created at the ends of a nonpolar molecule - Partial charges of chlorine induce partial charges on another nonpolar molecule. - The partial positive end of a nonpolar molecule is attracted to the partial negative end of another nonpolar molecule. Formation of hydrogen bonding in HF - The fluorine atom is a highly electronegative element - HF bond is very polar - The highly partially positive charged H atom attracts the lone pair electrons on the fluorine atoms of another HF molecule to form a Hydrogen bond. Surface tension - The tendency of the surface of a liquid to resist an external force Each water molecule forms extensive hydrogen bonds in every direction with neighboring molecules Viscosity - Molecules become entangled with each other because of highly branched shape Steel ball to drop - The rate of steel ball dropping depends on the viscosity of the liquid. - The intermolecular attraction is predominately hydrogen bonds. The number of hydrogen bonds formed per molecule is 1, 2, and 3 respectively. __ is the most vicious and the ball will drop the slowest Why does ice floats on water - The attractions between water molecules are predominately hydrogen bonds - A hydrogen bond is directional. In ice, water molecules have a tetrahedral arrangement and are packed in an open structure (causing a lower density of ice) - In liquid water, water molecules have relative motion, which leads to hydrogen bonds between water molecules forming and breaking continuously. Open structure collapses and the water molecules become more closely packed. (causing the higher density of water) Why is water a good cooling agent? - It has strong hydrogen bonds between water molecules - So lots of energy is absorbed from the hot object to break these strong bonds to raise the temperature of water. Formation of hydrogen bonds - NOF is highly electronegative elements - HF OH NH bonds are very polar - The highly partial positive charged H atom attracts the lone pair electrons on NOF Hydrogen bonds - N is a highly electronegative atom - The lone pair on N attracts the H atom of an adjacent NH3 molecule What is a hydrogen bond? - It is the electrostatic force between an H atom bonded with a very electronegative atom in a molecule and a lone pair of electrons forming a very electronegative atom in another molecule. Soluble in water? (Organic substances) - Yes as it can form a hydrogen bond with water - It has a small molecular size Soluble in water? (Organic substance) - No, although it can form a hydrogen bond with water - There is a large nonpolar part Van der Waals’ force predominates over hydrogen bond Not strong enough to overcome the intermolecular hydrogen bond between water molecule Redox Disproportionation - An element in one substance that is simultaneously reduced and oxidized. Is it cathode or anode, using ON - Cathode - Because the ON of Br decreased from +3 to +1 - Therefore it is a reduction → cathode Preparing salt bridge - Soak filter paper in concentrated NaNO3 Cu-Mg cell replaced with Cu-Al cell - Voltage will increase - Because Al has a stronger reducing power than Mg - So the distance between Cu-Al are further apart than Cu-Mg Porus device/salt bridge removed in a chemical cell - Mixing of electrolyte - No electron flow at the external circuit so the voltage drops The function of salt bridge - Complete the circuit by allowing ions to pass through - Balancing the two half cells by providing ions. The function of porous device - Complete the circuit by allowing anions to pass through Why platinum can’t be used is Cl2 present - Cl2 will corrode or attack the electrode The theoretical volume of gas collected - 2H2O → 2H2 + O2 - H2:O2 = 2:1 - Under the same temperature and pressure, an equal number of moles of gas occupies the same volume. pH at the cathode when H+ is discharged - Increase Water ionizes to replace the discharged H+ such that OH- builds up around the anode such that [OH-] is higher than [H+], therefore alkaline. Universal indicator when Cl- discharged - Cl2 + H2O → HCl + HOCl - HCl is acidic such that it is red - OCl- has bleaching properties such that red becomes colorless Cl- discharged for a long time Purify impure copper - Cu at the anode will become Cu2+ - Cu2+ at a solution will become Cu and deposit on the cathode - Silver and gold will remain because their reducing power is weaker than Cu Will [Cu2+] change if Fe2+ and Zn2+ are present in the electrolyte? - Yes - At the anode, Zn, Fe, and Cu would dissolve to form ions - Zn and Fe will preferentially discharge because they have stronger reducing power than Cu - At cathode, Cu2+ will preferentially because it has stronger oxidizing power than Zn2+ and Fe2+ - [Cu2+] decreases How sodium amalgam forms - Na+ preferentially discharged to form Na - Because mercury cathode favors discharge of Na+ more than H+ - Na dissolves in a mercury cathode to give sodium amalgam. How sodium amalgam forms NaOH - Sodium amalgam migrates toward the water - It reacts with water to form NaOH Why does plastic cover the electrode? - To prevent electrolysis of water when the wire is exposed - No longer mercury cathode Cu anode and CuSO4 electrolyte, will there be a color change - No - For every Cu discharged at the cathode - 1 Cu2+ will form at anode from Cu - [Cu2+] remains unchanged so blue remains unchanged Electroplating avoids using too high current - H+ will also discharge at the same time with metal ions - H2 forms between metal plating and the object - Loosely plated and peel off easily Low pH - [H+] is high so it will also discharge - H2 forms between metal plating and the object - Loosely plated and peel off easily High pH - [OH-] is high so metal precipitate may form Solution - Use weak acid buffer solution Treating effluents of electroplating - Add NaOH or NaCO3 to precipitate and filter out the metal precipitate - Add Na2SO3 to chromate ions such that redox happens to form Cr3+. Add NaOH to precipitate the Cr(OH)3 Electrolysis - Chemical decomposition of a compound by passing an electric current through a liquid or a solution containing ions Porus electrode in fuel cells - Act as an electrical conductor and catalyst - Allow O2 H2 Steam to pass through in and out of the compartment - Electrode and electrolytes in fuel cells remain unchanged Voltage remains constant if the fule and oxidants are continuously supplied Rate of reaction If the gas formed is very soluble in water, the measurement is not accurate because the volume change is inaccurate. The volume recorded is lower than the actual value. Why does measuring pressure the slope is shallow initially - In the beginning, the pressure does not increase rapidly as carbon dioxide would dissolve in hydrochloric acid - To obtain a constant slope, hydrochloric acid should be first saturated by carbon dioxide. Data logger - Sensitive to small pressure to provide more accurate readings - Measure continuously Purpose of cotton wool - To allow gas to pass through but prevent splashing of solution H2 formed not accurate - H2 has a small molar mass - Even though hydrogen gas escapes, the change of mass of the reaction mixture is too small to be detected and the measurement is inaccurate Pressure graph initially unequal to 0 - It is because there was air inside the flask before the start of the experiment Measuring absorbance - Plot a calibration curve with known concentrations to find out the absorption - Put the reaction mixture and find the absorption Refer to the calibration curve to determine the concentration Clock reaction - Rate of sodium thiosulphate solution and dilute acid Quenching - Slow down or stop the reaction to prevent changes in the concentration of the reactant or product to be analyzed Purpose of ice water in quenching - It can provide a very low temperature and also lower the concentration of reactants which greatly reduces the rate of reaction Way to monitor the changes in the concentration of Br2 - Measure the absorbance of the reaction over a regular period of time using a colorimeter - During the reaction, the concentration of Br2 decrease so the brown color intensity decrease. Measure the rate of NO2 - Measure the change in pressure of the reaction over a regular period of time interval using a pressure sensor - As the reaction proceeds, 3 moles of gas reacts to form 2 moles of gas so the gas pressure decrease when the reaction proceeds. Chemical method to determine the rate - Mix NaOH with ethyl ethanoate - At regular time intervals, withdraw a small and fixed portion of the mixture using a pipette. - Quench the mixture by pouring it into ice-cold distilled water - Titrate the portions against standard HCl using phenolphthalein as an indicator until the endpoint is reached Why is the reaction ok to use a colorimeter to find the rate - In reaction, the colors reactant is converted to a yellow product - The absorbance measured by the colorimeter is directly proportional to yellow color intensity as well as the concentration of the yellow product in the reaction mixture.
