Chemistry for Students of Mechanical Engineering

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Horace Austin
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1 Grudlage der hemie für Studierede des Maschiebaus, Prof. Dr. Meier hemistry for Studets of Mechaical Egieerig Friday, 4th October 2013, 15:00-18:00 No uauthorised resources (e.g. lecture otes, textbooks etc.) may be used durig the examiatio. Ay attempt to use such uauthorised resources will be cosidered as cheatig, ad will lead to immediate exclusio from the examiatio ad a mark of 5,0. Foreig studets may use a dictioary (mother togue Eglish) but this may ot cotai ay hadwritte otes. The use of a calculator is ot permitted. Numerical aswers that are give without showig ay workig or explaatio will receive o marks. I geeral, short aswers with keywords will be sufficiet; log essays are ot ecessary! To illustrate or explai a poit, a clear sketch is ofte sufficiet! The maximum umber of poits for each questio is give i paretheses. 0-49, ,0 4,0 3,7 3,3 3,0 2,7 2,3 2,0 1,7 1,3 1,0 Questio 1: (a) What is uderstood by atomic umber, ad what are isotopes? Atomic umber = umber of protos i the ucleus Isotopes = Atoms with the same atomic umber (i.e. same elemet) but with differet umbers of eutros i the ucleus (differet atomic mass) (b) Give the umbers of protos, eutros ad electros i itroge atoms with atomic weights 14 ad 15, respectively. Electros = protos = 7 14 N: 14 7 = 7 eutros 15 N: 15 7 = 8 eutros (c) Name the four quatum umbers that describe the eergy levels of electros i a atom. Briefly describe the sigificace of each quatum umber. pricipal QN (size of the orbital) l auxiliary QN (shape of the orbital) m magetic QN (orietatio of the orbital) s spi QN (orietatio of the electro s spi) (d) What do (i) the Pauli Priciple ad (ii) ud s Rules tell us? Pauli Priciple: No two electros i a atom ca have all four quatum umbers idetical. Or a orbital ca be occupied by at most two electros, which must the have atiparallel spi. ud s Rule: Orbitals will be filled with electros i order of icreasig eergy; orbitals of the same eergy will first each be filled sigly, ad oly whe each cotais oe electro will they begi to be doubly occupied. (e) Give the electroic cofiguratio of the molybdeum (Mo) atom.

2 1s²2s²2p 6 3s²3p 6 4s²3d 10 4p 6 5s 1 4d 5 or: [Kr]5s 1 4d 5 (but [Kr]5s 2 4d 4 also OK!) (f) What method is used to determie the mass of a atom? What are the three mai stages i this experimetal method? Mass Spectrometry Sample ijectio; Ioisatio ad acceleratio of the ios; deviatio of the ios by a magetic field (g) Use the VSEPR model to determie the 3-D geometries of SF 6, [Al(O) 4 ] -, O 2 ad SbF 5. SF 6 : octahedral [Al(O) 4 ] - : tetrahedral O 2 : liear SbF 5 : trigoal bipyramidal (A clear sketch is also OK) (h) State the ideal gas equatio. PV = RT Questio 2: a) What is uderstood by (i) the stadard ethalpy of formatio of a compoud, ad (ii) ethalpy of reactio? Stadard ethalpy of formatio: The chage i ethalpy (heat eergy) whe a compoud (i its stadard state) is its costituet elemets (each i their stadard states). Ethalpy of reactio: The chage i heat eergy of the system whe a chemical reactio is carried out (uder costat temperature ad pressure). b) Give the Gibbs-elmholtz equatio. G = - T S What is the criterio for a spotaeous chemical reactio? A chemical reactio is spotaeous, whe G < 0. What coditios must be fulfilled if a edothermic process is to occur spotaeously? S > 0 ud (T S) > c) What are the characteristics of a chemical equilibrium? The cocetratios (or partial pressures for a gas-phase reactio) of all the substaces ivolved (startig materials ad products) reach costat values; the rates of the forward ad reverse reactios are the equal. This fial equilibrium positio depeds oly o the iitial cocetratios (or partial pressures) ad the temperature. Give the equatio that relates the equilibrium costat K of a typical reactio to the cocetratios of the products ad startig materials, ad also the equatio that relates K to thermodyamic quatities. For a reactio aa + bb c + dd: K = [] c [D] d [A] a [B] b G = - RT lk or K = exp(- G/RT)

3 What effect does a icrease i temperature have o the equilibrium positio of a edothermic reactio, ad what is the effect of removig a product of the reactio? The equilibrium positio of a edothermic reactio shifts to the right if the temperature icreases (the system attempts to reduce the icrease i T). Removal of a product shifts the equilibrium positio to the right. What is the effect of a icrease i pressure o the equilibrium positio of a reactio i which the umber of gas-phase molecules decreases? The equilibrium is shifted to the right (fewer gas molecules = lowerig of P) Which geeral Priciple determies the effect of exteral iflueces (chages i temperature or pressure, removal of a reactio compoet etc) o a system such as a equilibrium? Le hatelier s Priciple Ethee ca be obtaied by the elimiatio of hydroge from ethae. Write dow the reactio equatio alculate the Stadard Ethalpy of Reactio ( ), the Stadard Etropy of Reactio ( S ) ad the Gibbs free Eergy of Reactio ( G ) usig the followig values: o f( 2 6 ) = kj mol -1, o f( 2 4 ) = 52.4 kj mol -1 S o ( 2 6 ) = J mol -1 K -1, S o ( 2 4 ) = J mol -1 K -1, S o ( 2 ) = J mol -1 K -1 = 52.4 kj mol ( kj mol -1 ) - 0 = kj mol -1 S = J mol -1 K J mol -1 K J mol -1 K -1 = J mol -1 K -1 G = ( ) J mol -1 = J mol-1 = kj mol -1 (3P) d) The formatio of sulphur trioxide (SO 3 ) from sulphur dioxide (SO 2 ) ad oxyge is a equilibrium reactio. Give the reactio equatio, ad decide if etropy icreases or decreases as the reactio proceeds. 2 SO 2 + O 2 2 SO 3 The etropy decreases, sice the umber of gas molecules (ad so the disorder of the system) decreases. I which directio is the equilibrium positio displaced, if the temperature is icreased a excess of oxyge is added to the iitial gas mixture? Explai your aswers, give that the reactio is exothermic ( = -99 kj mol -1 ) igher T: towards the left-had side (startig materials) Excess of O 2 : towards the right-had side (SO 3 ) Accordig to Le hatelier s Priciple: If T is icreased, the equilibrium will shift i the edothermic directio, to couteract this chage. If a excess of a startig material is added, the equilibrium will shift towards the side of the products, to cosume some of this excess. e) Determie the stoichiometric coefficiets for the followig redox reactios, ad give the oxidatio states for the species takig part:

4 [2] r 3+ + [3] 2 O 2 + [10] O - [2] ro [8] 2 O r: +3 O: -1 r: +6 O: [1] IO 3 + [3] SO 3 + [3] + [+1] I - + [+3] 2 SO 4 + [0] + I: +5 S: +4 I: -1 S:+6 f) I each of the half-cells of a Galvaic cell, a sheet of platiised platium is dipped i a 1 M 2 SO 4 solutio at 25, ad the respective gas is passed over the platium at a pressure of bar. What voltage will be measured whe 2 is passed through oe half-cell, ad O 2 through the other? What are the two half-reactios? (3P) Aode: O 2 3 O e - E 0 = 0 V Kathode: 6 2 O O O e - E 0 = 1.23 V E = (1.23-0) V = 1.23 V Questio 3: a) What is uderstood by Los Ageles Smog (also kow as Summer Smog )? Give the equatio for the chemical reactio that results i Los Ageles Smog. A O 3 -cotaiig Smog O 2 + NO 2 NO + O 3 (sulight, λ < 420 m) b) Give the equatios for the stratospheric reactios ivolvig Fs (chlorofluorocarbos, such as l 3 F) that are resposible for the formatio of the Ozoe ole. UV, λ < 220 m l 3 F l 2F + l l + O3 O 2 + Ol Ol + O O 2 + l c) What is the oxidatio state of sulphur i the followig compouds: 2 S, S 8, Na 2 SO 3 ad SO 3? 2 S: -2; S 8 (S): 0; Na 2 SO 3 : +4; SO 3 : +6 d) ow is ammoium itrate made from ammoia? Give the equatios for the four reactios. (i) 4 N O 2 4 NO O (Pt/Rh mesh catalyst, 900 ) (ii) 2 NO + O 2 2 NO 2 (iii) 3 NO O 2 NO 3 (aq) + NO The overall reactio for (ii) ad (iii) 4 NO + 3 O O 4 NO 3 (aq) is also OK (iv) NO 3 + N 3 N 4 NO 3 e) Bauxite, a mixture of Al 2 O 3 ad Fe 2 O 3, is a importat raw material i the productio of alumiium. What are the three mai steps i the productio of alumiium from bauxite? Give chemical equatios for the reactio(s) takig place i each step. (i) Reactio with aqueous NaO at 200 º:

5 Al 2 O 3(s) + Fe 2 O 3(s) O + 2NaO 2Na + + 2[Al(O) 4 ] - (aq) + Fe 2 O 3(s) (separatio of Fe 2 O 3 by filtratio) (ii) Re-precipitatio by acidificatio: [Al(O) 4 ] - (aq) + + Al(O) O (Al(O) 3 isolated by filtratio). (iii) alciatio: 2Al(O) 3 Al 2 O O (1000 º) (iv) Smeltig flux electrolysis i molte cryolite at 900 º: athode: Al e - Al (l) Aode: e.g. O 2- + O + 2e - (5P) alculate the electric curret that must be passed through the electrolysis cell, if alumiium is to be produced at a rate of 54 kg/h. Assume the Faraday costat F = /mol, ad roud the atomic mass of Al to 27 g mol kg(al)/h = 2000 mol(al)/h = 2000/3600 mol(al)/s = (2000 3)/3600 mol(e - )/s = ( )/3600 /s = ( )/600 /s = A e) Name two raw materials from which iro ca be obtaied. Two from Magetite (Fe 3 O 4 ), aematite (Fe 2 O 3 ), Limoite (Fe 2 O 3 2 O or FeO(O) 2 O ), Siderite (FeO 3 ), Pyrites (FeS 2 ) Which idustrial process is used to obtai iro? What substace is the reducig aget of the iro ore? Blast Furace. Reducig aget = O Why is lime (ao 3 ) used i iro productio? Give equatios for the reactios i which the lime (or a substace derived from it) is ivolved? Removal of P or Si as slag ao 3 ao + O 2 ao(s) + SiO 2 asio 3 3 ao + 2 P 2 O 5 a 3 (PO 4 ) 2 Durig the chemical reactios ivolved i the formatio of iro, fiely-divided carbo is formed via the Boudouard reactio, ad this carbo dissolves i the liquid iro. Give the equatio for the chemical equilibrium ivolved i the Boudouard reactio. O O What egative effect does this dissolved carbo have o the properties of the raw iro? It becomes brittle Name a process used durig steel productio, to reduce the carbo cotet of the steel! Air-refiig f) What is uderstood by corrosio? The destructio of a material by redox reactios with its eviromet g) Give equatios for the chemical reactios that take place whe iro rusts i moist air. Fe(s) Fe 2+ (aq) + 2 e -

6 O 2 (g) O + 4 e - 4 O - (aq) Fe 2+ (aq) + 2 O - (aq) Fe(O) 2 (s) (Fe + ½ O O Fe(O) 2 also OK) The 2 Fe(O) 2 (s) + ½ O 2 Fe 2 O 3 (s) + 2 O What are two importat methods that ca be used to prevet such rustig? oatigs (pait, plastic, eamel) Sacrificial electrodes (Mg or Z) Passivatio (either atural: Z, Al, or through additio of e.g. r) Electroplatig or galvaisatio h) Name three of the elemets that belog to the Platium Group of metals. Three from rutheium, rhodium, palladium, osmium, iridium ud platium (1½P) What is the mai idustrial sigificace of these metals? As catalysts Questio 4: a) There are five orgaic compouds with the sum formula 4 6. hoose ay two of these, ad draw their Lewis formulae, give their correct ames, ad state which class of orgaic compoud each of them belogs to. (3P) Alkee Buta-1,3-diee Alkee (or allee) Buta-1,2-diee Alkye 1-Butye Alkye 2-Butye ycloalkee yclobutee ½P each for Lewis formula, class ad correct ame

7 b) What hybridisatio do carbo atoms have that are ivolved i sigle, double or triple bods? What is the spatial arragemet of the bods aroud the carbo atom i each case? (3P) Sigle bods: sp 3 hybridisatio, tetrahedral geometry with bod agles ca. 109º Double bod: sp 2 hybridisatio, trigoal plaar geometry with bod agles ca. 120º Triple bods: sp hybridisatio, liear geometry with bod agle ca. 180º c) Give four characteristic structural features of the bezee molecule, ad draw the two mesomeric structures for the molecule. 3 from: plaar hexago of atoms, all - bods equal legth, bod agles 120º, 6 delocalised π-molecular orbitals, sp 2 hybridisatio. d) Draw the Lewis formulae for the characteristic fuctioal groups of carboxylic acids, esters, ketoes ad alkees. arboxylic acids Esters Ketoes Alkees e) Draw the Lewis formula for the compoud 2-hydroxy-3-amio-4-methyl-5- ethyl-cyclohexaoee

8 f) What is uderstood by the terms macromolecule ad polymerisatio? Macromolecule: A molecule of very high molecular weight, built up from either idetical or related repeatig uits. Polymerisatio: The chemical likage of small molecules (moomers) ito a macromolecule (polymer), either through radical reactios of double bods or through codesatio reactios g) Give the Lewis formulae for the repeatig uits i polyethee, polypropee, polyviylchloride, polystyree ad polyacryloitrile. From which moomers are these polymers produced? (5P) Polyethee ethee: Polypropee propee: 3 Polyviylchloride chloroethee (viyl chloride): l Polystyree styree: Polyacryloitrile acryloitrile: N h) ow much O 2 (i g/km) is emitted by a petrol motor with a fuel cosumptio of 5.7 litre/100km? Assume that the fuel is 100 % isooctae, with a desity of ρ = 0.7 g/cm³ 5.7 litre/100km = 57 ml isooctae / km: m( 8 18 ) = V ρ = (57 0.7) g/km ( 8 18 ) = m( 8 18 ) / MW( 8 18 ) = (57 0.7)/114 mol/km ½O 2 8O O: m(o 2 ) = = g/km (O 2 ) = ( )/114 mol/km = (0.7 8)/2 mol/km = 2.8 mol/km (6P)

Prüfung in Chemie für Studierende des Maschinenbaus und des Lehramts an Gymnasien Friday, 4th October 2013, 15:00-18:00

Prüfung in Chemie für Studierende des Maschinenbaus und des Lehramts an Gymnasien Friday, 4th October 2013, 15:00-18:00 No unauthorised resources (e.g. lecture notes, textbooks etc.) may be used during