Download IAL Edexcel chemistry unit 5 Questions with accurate answers and more Exams Chemistry in PDF only on Docsity!
IAL Edexcel chemistry unit 5 Questions
with accurate answers
Question correct answer Content What is Oxidation? correct answer - Gain of oxygen
- Loss of electrons/Hydrogen / What is Reduction? correct answer - Loss of Oxygen - Gain of electrons/hydrogen / What happens in a redox reaction? correct answer - OA loses x electrons - RA gains x electrons / Why is a high resistance voltmeter used in standard measurements? correct answer So that no current flows and maximum potential difference is achieved / What is Standard Electrode Potential? correct answer Potential difference/voltage produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions / Standard Hydrogen Electrode correct answer - H2 in equilibrium with H+ ions - Inert Pt electrode in contact with H2 gas and H+ ions / Describe the standard conditions of SHE correct answer - Pt at 298K - 1 mol dm-3 wrt each metal ion - Acidified H+ (e.g HCl) - H2 at 1atm - Increasing conc/pressure/temp depends on reaction, as same rules as equlibria / Why is Pt used and why is it 'coated in black? correct answer - Chemically inert and does not interfere in reaction - Increases SA, making catalyst more efficient (for H2 ⇒ 2H+ + 2e-) / Why is a reference electrode needed? correct answer A potential difference requires a complete circuit to be measured /
Types of half cells correct answer - Metal/metal ion solution (If positive EMF, at cathode and is reduced)
- Non-Metal/NM ion solution; Pt wire used to make electrical contact with solution; redox equilibrium established on Pt surface - Ion/Ion (same element, different oxidation numbers). Both ions in solution, and Pt used / Standard Emf Calculation correct answer Ecell= E(reduced) - E(Oxidised) Redox cells Representation correct answer - ROOR - X(s) l X+ (aq) ll Y+(aq) l Y(s) - Oxidised ll Reduced - Order of electron flow - One solid line= phase boundary Iron half cell correct answer Pt(s) l Fe2+(aq), Fe3+(aq) Feasibility and Ecell values correct answer - More positive= easier to reduce reactant species= reaction goes in forward direction - More negative= easier to oxidise product= backward reaction - Positive Ecell= reaction is feasible Electrode potentials correct answer - Negative at anode, is oxidised - Positive at cathode, is reduced - Combine half equations Half cell of MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O correct answer Pt | Mn2+ , MnO4- | | Why is a salt bridge used? correct answer - Filter paper soaked in salt solution - salt cannot interfere with reaction - Completes circuit - Maintains electrolyte concentration - Carries flow of charge Why does the reaction not occur, even with a positive Ecell? correct answer - Not standard conditions - Very high Ea/kinetically stable - Slow rate of reaction Suggest why reaction between Cu2+ and I- takes place correct answer - Ecell is negative so reaction is not feasible - However, CuI is a solid - Cu+ (product) decreases to Ecell increases and becomes positive Why is MnO4- not used to oxidise alcohols? correct answer - Higher E value - very strong oxidising agent, may oxidise further to CO2 and H2O
Percentage Purity correct answer Mass of sample/ mass of impure sample x 100 Iodine-Thiosulfate Titrations correct answer - S2O32- + I2 ⇒ 2I- + S4O6- - Yellow of iodine becomes paler as converts to iodide ions (colourless) - When solution is pale yellow, starch is added to clarify endpoint - Solution turns blue black, then colourless when all iodine reacts - Used to determine concentration of oxidising agent used to oxidise iodide ions into iodine - Iodine: thiosulfate = 1: Percentage Uncertainty correct answer - uncertainty/measured value x 100 - Used twice, multiply by 2 - Burette: used twice - Pipette & volumetric flask used once Reducing uncertainty correct answer - Dilute chemical concentration in burette - More required for titration - Overall uncertainty reduced Why does using four tablets give a more accurate result than using two? correct answer Using a larger amount reduces % error in weighing Role of CO2 being bubbled through solution correct answer Lowers pH of solution State the meaning of disproportionation and whether or not [CuCl2]- formation is such reaction. (Given CuCl2+Cu+2HCl⇒ 2[CuCl2]- +2H+) correct answer - Where a single species is simultaneously reduced and oxidised - No, as two different species are reduced and oxidised Concentration of KMnO4 can be found by titration with As2O3. In this reaction, As2O3 is oxidised to As2O5, and the mole ratio of As2O3 to MnO4 is 5:4. Deduce the final oxidation number of manganese. correct answer - +2 - Each As loses 2 electrons - As2 so each molecule loses 4 - 5 moles so loses 20 electrons in total - Each Mn(+7) gains five electrons Rusting in two steps correct answer - 2Fe+O2+2H2O⇒2Fe(OH)2 - 4Fe(OH)2+O2+2H2O⇒4Fe(OH) Why does adding salt speed up reaction? correct answer Dissolved salt makes water a better conductor Corrosion of steel prevented by connecting Mg blocks. How? correct answer Mg has more negative ECell so reduces Fe2+
Why does titration give lower value? correct answer Ti3+ oxidised by oxygen Why is titrating with EDTA not a suitable method of measuring amount of copper in such coins with alloy of Cu and Ni correct answer Ni ions also form complexes with EDTA Photochromic glasses contain silver(l) and copper(l) chlorides. Explain why these glasses go dark in sunlight. correct answer - Cu+ + Ag+ ⇒ Cu2+ + Ag - Silver is formed How can an experiment be improved to give a more suitable titre? correct answer - Experiment inaccurate as it requires refilling burette, which increases burette error - Use more concentrated K2MnO Outline how to prepare 250cm3 solution for titration correct answer - Transfer reaction mixture to volumetric flask - Add washing - Make solution up to line, then mix If two mols/atoms of element in molecule, divide by 2 correct answer Why does the reaction favour tertiary? correct answer - carbocations have different stabilities - Alkyl group stabilise carbocation by charge through positive inductive effect - Raecimic mixture formed as carbocation planar, so OH- can attack from either side Purpose of H2SO4 correct answer Convert salt to product Explain stereoisomerism of product of SN2 correct answer - Optically active - OH- only attacks on one side - Product rotates plane of polarised light in opposite direction of reactant What are transition metals? correct answer - Elements with incomplete d-orbitals that can form at least one stable ion with an incomplete d-orbital - Titanium to Copper What are exceptions to transition metals? correct answer - Sc- only forms Sc3+; [Ar] 3d0 - Zn- only forms Zn2+, [Ar]3d
Ni oxidation states correct answer 2+,3+,4+ Cu oxidation states correct answer 1+,2+ Why does Cr have its electronic configuration? correct answer - Expected: [Ar] 4s2 3d4 - Actual: [Ar] 4s 3d5 - 3d5/half filled subshell is more stable - Paired electrons repulse - All six electrons are in separate orbitals, minimising repulsion Why is the metallic radius of copper smaller than potassium? correct answer It has a higher nuclear charge/more protons Why does Cu change to Cu2+ by shaking? correct answer Oxidised by air Why does Fe form stable compounds with more than one oxidation number correct answer Increasing Ionisation energy balanced by increase in lattice energy Why are the values of ionisation energy similar? correct answer - Electrons removed from 4s - Shielding by 3d electrons same in each element Why is the second ionisation energy significantly different between Cu and Ni? correct answer - 1 electron removed from 4s1 - Second electron in Cu in 3d, so harder to remove as closer to nuclei Why does the third ionisation energy increase from Ni to Zn? correct answer Attraction on 3d electrons increases due to increase in number of protons Why are the sums of the first two ionisation energies of Ca, Sc and Ti all similar, but less than that of K? correct answer - First two electrons for the three removed from 4s - Second electron removed from 3p for K Suggest to reasons why CrO3 is unlikely to contain Cr6+ correct answer - Ionisation energy is greater than lattice energy - Sum of first six IE is very high - Highly charged ion and small positive ion is highly polarising
Why is the second ionisation energy of Cr much higher than V and Mn? correct answer - In Cr+ electron (2) lost from 3d, in Vn+ and Mn+ lost from 4s subshell - 3d is closer to nucleus than 4s, so harder to remove - 3d is half full, so relatively stable and harder to remove Explain why the atomic radius decreases in K and Ca, Sc and Ti, and why decrease in K and Ca greater? correct answer - Second member of each pair has one more proton - Outer electrons in Ti shielded by 3d electrons, while Ca is not shielded by 3d What is a complex? correct answer Central metal ion surrounded by ligands What is a ligand? correct answer - Molecule or ion that can donate a pair of electrons to central metal ion, to form one dative covalent bond - One lone pair= one coordinate bond What is meant by Coordination Number? correct answer Total number of dative/coordinate bonds central ion forms with ligands Ligand: Water correct answer - H2O - 0 charge - Name: Aqua - e.g: [Fe(H2O)6]2+ ⇒ hexaaquairon (ll) Ligand: Ammonia correct answer - NH3 - 0 charge - ammine - e.g [Cu(NH3)4(H2O)2]2+ ⇒ tetraamminediaquacopper (ll) Ligand: Hydroxide correct answer - OH- - -1 charge - hydroxo Ligand: Chloride correct answer - Cl- - -1 charge - chloro - e.g [FeCl4]- ⇒ tetrachloroferrate (lll) (ferrate as negatively charged) Naming ligands correct answer - number of ligands - name of ligands - metal ion name - oxidation number of metal ion Negative ion names correct answer - Copper- cuprate - nickelate- nickel
What does change in energy indicate? correct answer - Difference between energy levels - Larger energy difference means electrons absorb more energy - Change in energy is proportional to frequency of absorbed light - Inversely proportional to wavelength Why are some ions colourless? correct answer - Cu+, Sc3+, Zn2+ - Cu+ and Zn2+ have completely filled d orbitals, so there are no spaces where electron can be excited, and electron transitions not possible - Sc3+ has empty d orbitals, so no available electrons to excite - therefore, they cannot absorb light, as no energy change Describe ligand exchange correct answer - One ligand in complex replaced by another - New complex is more stable - Ligands can be fully or partially substituted - If ligands are of similar size, coordination number and shape remain the same - If ligands different, it changes Describe the chelate effect correct answer - Replacement of monodentate ligands with bi/polydentate ligands - Small enthalpy change - Always (large) positive entropy change - This is because there is an increase in the number of moles on the product side Addition of concentrated HCl/NaCl to aqueous ion correct answer - Cl- ligand bigger than H2O /NH3 - Coordination number changes from 6 to 4 - Colour changes from blue to green to yellow - Charge changes from 2+ to 2- - Entropy of system increases Why can H2NCH2CH2NH2 act as a bidentate ligand but not H2NNH2? correct answer - Both have nitrogen atoms with lone pairs (2) - Dative bonds are too close and bond angles are too acute Why does one of the water ligands lose a proton? correct answer - Metal ion is highly charged and polarises OH- ion - Water acts as base and and gains a proton from water ligand Complexes with six-fold coordination correct answer - Central metal ion forms six coordination bonds - octahedral shape with 90 degrees bond angle - Formed by six small monodentate ligands 9H2O, NH3, OH-) - Three bidentate ligands - 1 hexadentate ligand (EDTA4-) Electron repulsion theory correct answer Shapes formed so minimum repulsion and maximum distance/angle between electrons
Tetrahedral complexes correct answer - Four coordinate bonds - e.g 4Cl- ions - bond angles: 109. Square planar complexes correct answer - Also four coordinate bonds - CN- ions are an example - 90 bond angle Cisplatin correct answer - 2Cl-, 2NH3 - Used as cancer treatment - Pt as central metal ion - Has geometric isomer - As passes through cell membrane, Cl- exchanged with water - Nitrogen in DNA bases exchanges with water, and DNA distorted and prevented from replicating - Can bind to healthy cells too - Only cis used, not trans isomer Haemoglobin and Ligand exchange correct answer - Fe2+ centre - Polydentate ligand with four haem groups (4 coordinate bonds with Nitrogen) - Fifth covalent bond with protein globin - Oxygen as sixth coordinate bond - Oxygen molecules bonded weakly as not strong ligands, so break off easily nd transport into cells - CO better ligand than O2/CO2, and binds strongly and irreversibly to Fe2+ - Creates darker red colour Vanadium colours and oxidation states correct answer - V+2= purple - V+3= green - VO2+ (+4)= blue [oxovanadium] - VO2+ (+5)= yellow [dioxo] - Addition of zinc to +5 in acidic solution reduces V down each successive oxidation state Vanadium at +5 correct answer - Exists as solid compound in the form of VO3- - Usually as NH4VO (ammonium vanadate [V]) - Strong oxidising agent - Adding acid turns it into yellow solution containing VO2+ Reduction of V correct answer - Usually using zinc in acidic solution - ECell decreases/becomes more negative with each reduction - V+2 ⇒ V+3 (oxidation) more favourable than reduction to V (s) Reduction of dicrhomate (VI) ion, Cr2O72- correct answer - Use zinc in acidic conditions - Zinc is oxidised
- Reduced to Cr3+ and Cr2+ ions - Cr3+ then reduced to Cr2+ by zinc Oxidation of Cr3+ correct answer - oxidised to CrO42- then acidified to Cr2O7 2- - H2O2 is reduced - H2O2 IN ALKALINE CONDITIONS IS OXIDISING AGENT (followed by acidification) - H2O2 + 2e- ⇒ 2OH- What does coloured precipitate indicate? correct answer - Complex has 0 charge - With limited NH3/OH-
[Cu(H2O)6]2+ ⇒ [CuCl4]2- correct answer - React with HCl - Green to yellow [Co(H2O)6]2+ ⇒ [CoCl4]2# correct answer Pink to Blue solution Adding concentrated HCl slowly to [Cu(H2O)6] correct answer - Blue coloured ppt [Cu(OH)2(H2O)4] - Green (CuCl3-) - Yellow (CuCl4 2-) Adding H2SO4 to CrO4 2- correct answer - Yellow to orange - Forms Cr2O7 2- Adding dilute H2SO4 to aq Cu2+ until solution is pale blue, then adding KI correct answer - White ppt= CuI - Brown solution: I Describe the surface adsorbtion theory correct answer - Adsorption: one or more reactants attaches to surface of catalyst - Reaction: Reactants held in reactive position and made to react with each other, and bonds weakened - Desorption: Product molecules leave surface of catalyst - Adsorption at active sites may lead to catalytic action - Higher concentration of reactants at solid surface, leading to higher collision frequency Strength of Adsorption correct answer - Too strong: products cannot be released - Too weak: not high enough concentration on surface - Ni and Pt are the best Effect of surface area on catalysts correct answer - Increasing SA of solid catalyst improves effectiveness
- Support medium often used to maximise SA and minimise cost Advantages of Heterogenous catalysts correct answer - Better in industrial processes, as can be filtered off and easily separated from products - Suitable in continuous process Catalysts mechanism correct answer - Speeds up rate of reaction without being used up - Provides alternative pathway with a lower activation energy - Allows equilibrium to be reached faster - TM make good catalysts due to variable oxidation states - Electrons transferred to produce reactive intermediate - Catalyst always reformed to original oxidation state
How does a heterogenous catalyst act as a catalyst? correct answer - Reactants adsorbed onto active site of catalyst - Ea of reaction lowered - Bonds weakened in reactant molecules - Products desorbed - Overall oxidation state of metal does not change Why do catalysts speed up rate of reaction? correct answer - Lower Ea - Bonds in reactants weakened - Increase likelihood of molecules colliding - Adsorption means more reactant molecules in given space Catalysis of the Contact Process correct answer - Forming sulfur trioxide and uses Vanadium as a heterogenous catalyst - S(s)+O2(g)⇒SO2(g) (roasting sulfur in air) - 2SO2(g)+O2(g)⇔ 2SO3(g) - INTERMEDIATE: SO2(g)+V2O5(s)⇒ V2O4(s)+SO3(g) [Vanadium reduced from+5 to +4) - O2(g) +2V2O4(s)⇒ 2V2O5 (s) Explain the term 'chiral'? correct answer - Carbon atom attached to four different groups - Non- superimposable on its mirror image - Optical Isomers: 2^n Physical properties of enantiomers correct answer One rotates plane of polarised light in a clockwise direction, one in anti-clockwise What is a Racemic Mixture? correct answer - Contains equal amounts of each enantiomer - Optically inactive- no rotation of plane polarised light SN1 Mechanism and Optical Activity correct answer - Tertiary carbocation is trigonal planar structure - Nucleophile can attack from either side of plane - Racemic Mixture is formed SN2 Mechanism and Optical Activity correct answer - X atom on halogenoalkane causes steric hindrance
- Nucleophile only attacks from opposite side of C-X bond - Molecule undergoes inversion of configuration - Enantiomer formed Why is there a difference in boiling points? correct answer 1. All have similar london forces due to similar number of electrons 2. X has a permanent dipole-dipole interactions which is stronger than London force 3. Y has dipole-dipole AND hydrogen bonds which are the strongest intermolecular forces
Describe the Silver Mirror test correct answer - Tollen's reagent contains silver complex ion - Ag+ is oxidising agent - Colourless solution - Gently warm with solution - Aldehyde becomes oxidised - Silver complex ion reduced to Ag (S) - Silver precipitate - Negative with ketone as it cannot be oxidised Describe Fehlings Test correct answer - Deep blue solution of Copper (II) ions dissolved in sodium hydroxide - Aldehyde oxidised to carboxylic acid - Cu 2+ reduced to Cu+ - Brick red precipitate (Cu2O) formed Nucleophillic Addition correct answer - C=O Bond polarised - Carbonyl carbon slightly positive, susceptible to attack from nucleophile Nucleophillic addition of HCN in the presence of KCN correct answer - HCN in presence of KCN - KCN and sulfuric acid = HCN and CN- - CN- attacks carbon to form negatively charged intermediate - Negatively charged oxygen atom reacts with aq H+ (from HCN, water or dilute acid) to form 2-hydroxinitrile compound e.g 2-hydroxipropanenitrile - Adds carbon atom to carbon chain - Addition to ketone results in racemic mixture Explain why HCN addition will not take place in very low pH? correct answer Very few CN- Explain why HCN addition will not take place in very high pH? correct answer Very few H+ ions How to cheapen catalytic converters correct answer - Some TM are precious metals and expensive - Honeycomb structure used - Increase SA of catalyst - Coating inert surface medium with catalyst to avoid using large amounts Preparation of Carboxylic Acids correct answer - Oxidation of aldehyde - Hydrolysis of Nitriles Oxidation of aldehyde correct answer - Potassium Dichromate solution with dilute sulfuric acid - heated under reflux Hydrolysis of Nitriles correct answer - Heated under reflux - Acid hydrolysis - Alkaline hydrolysis followed by acidification - CN- Group converted to COOH
Reactions of Carboxylic Acids correct answer - C.A + Metal Carbonate→Metal Ethanoate Ion + CO2 + H2O (CO2 bubbles) - C.A + Metal→Metal Salt + Hydrogen - C.A + Metal Oxide/ hydroxide→Metal Salt + Water - C.A + Alkali→Salt + Water - Reduction with LiAlH4 with dry ether at rtp to primary alcohol. Addition of water will destroy excess LiAlH4 - C.A + PCl5→Acyl Chloride + POCl5 + HCl (White, misty fumes) - C.A+ Alcohol (in concentrated sulfuric acid)→Ester + Water (Esterification; condensation) Preparation of Acyl Chlorides correct answer - C.A + PCl5→Acyl Chloride + POCl5 + HCl (pass through NH3- NH4Cl, white solid) - OH- group removed, replaced by Cl- Describe the HABER PROCESS correct answer - Iron forms interstitial hydride with Hydrogen Molecules - H Mols held in spaces between metal ions - Once surface of iron covered with molecules, no further increase in rate Mechanism of Homogenous catalysts correct answer Form an intermediate species Reactions of Acyl Chlorides correct answer - A.C + Water→Carboxylic Acid + HCl (Hydrolysis- Water molecule added, HCl removed. OH- replaces Cl) - A.C + Alcohols→Ester + HCl - A.C + Concentrated Ammonia→Amides + HCl (Ammonia taken in excess so HCl does not react with amide) - A.C + Primary Amine→N-substituted Amides + HCl Amides correct answer - -CONH2 group - Amine/ Ammonia replaces Cl - N takes place of Cl - N- (carbon chain after N) (Carbon chain before N) Amide N-ethylpropanamide Hydrolysis of Esters correct answer - heated under reflux - Ester + Water (dilute acidic medium) ⇆ Carboxylic Acid + alcohol - Ester + Water (alkaline medium)→Carboxylate salt and alcohol (Irreversible- ester fully hydrolysed. Carboxylate ion acidified to form carboxylic acid). Preparation of esters correct answer - C.A + alcohol (presence of acid) ⇆ Ester + water (Reversible, less yield) - A.C + alcohol→Ester + HCl (more polar bond) Why can TM be used to catalyse redox reactions? correct answer - Variable oxidation states - Can act as both OA and RA - e.g, if Fe(s) used as catalyst: - Act as RA: Fe2+⇒Fe3++e- - Act as OA: Fe3++e- ⇒ Fe2+
Two reasons why chosen reagent is faster correct answer 1. propanoyl chloride 2. C-Cl bond weaker than C-O bond 3. Carbonyl carbon more positive Suggest why PLA is biodegradable correct answer Ester links can be hydrolysed Two measures to maximise yield of carboxylic acid from alcohol correct answer 1. Reflux 2. Excess oxidising agent Ways acid hydrolysis of ester is different correct answer 1. Acid is a catalyst 2. reversible 3. produce COOH, not salt 4. H+ electrophile NaHCO3 test of carboxylic acid correct answer effervescence, turns limewater milky How to use 2,4-DNPH to find type of aldehydes correct answer 1. add reagent and filter 2.recrystallise 3. measure melting temperatures and compare with data book How to recognise ammonia correct answer pungent smell BaCl2 identifier correct answer white paste/liquid What happens to H nuclei in NMR? correct answer 1. Proton has property called spin 2. flips and changes 3. align against external magnetic field Describe GLC correct answer - stationary phase: liquid/ inert solid - Mobile phase: inert carrier gas, e.g CO2, N2, Ar - Seperation achieved due to components having different interactions with mobile and stationary phase Properties determining speed thrugh column in GLC correct answer 1.Solubility in stationary phase 2. interaction with stationary phase 3. Adsorbtion on stationary phase 4. Attraction to stationary phase Describe HPLC correct answer - Stationary phase: liquid or inert solid 2. Mobile phase: liquid 3. Adsorbtion onto stationary phase
Which compound takes the longest time to travel through common? 1-bromo, 1-iodo, 1-chloro, pentane correct answer 1-chloropentane Factors affecting time taken for component to pass through column (HPLC) correct answer 1. Material of stationary phase 2. Particle of stationary phase 3. temperature of column 4. NOT type of detector Explain the ring structure of benzene and how it arises correct answer - Electrons delocalised around ring - Due to overlap of p-orbitals Properties of Arenes correct answer - High MP due to stability of delocalised benzene ring - Low solubility as non-polar - Planar compound with bond angle 120 Evidence for this structure of benzene (not cyclohexatrein) correct answer - Thermochemical - X-Ray Diffraction - Infrared - Bromine water Thermochemical evidence for benzene structure correct answer - Expected enthalpy of hydrogenation: - 360 kJ mol (3X cyclohexene) - C6H6+3H2⇒C6H12 - Actual: -208 kJmol - Less exothermic than expected - Benzene more thermodynamically stable and has lower enthalpy content X-Ray diffraction evidence for benzene structure correct answer - Expected: bond lengths of C-C differs (3 single, 3 double) - Actual: all bond lengths are the same - Show intermediate length between C-C and C=C Infrared evidence for benzene structure correct answer - Infrared data does not match - Expected: aromatic C=C peak at 1650 (like cyclohexene) - Peaks at different wavelength Bromine water evidence for benzene structure correct answer - Expected: has 3 C=C bonds, so should easily decolourise bromine water - No decolourisation, so no C=C - Benzene has no localised area of high electron density, so cannot polarise Br2 - Needs halogen carrier to react with bromine Why does benzene have a higher BP than ethanol? correct answer London forces between benzene molecules stronger than H-bonds between ethanol molecules
