THE COLLEGE OF THE BAHAMAS

Transcription

1 TE CLLEGE F TE BAAMAS MIDSEMESTER EXAMINATIN SEMESTER FACULTY F PURE AND APPLIED SCIENCES SCL F CEMISTRY, ENVIRNMENTAL & LIFE SCIENCES X NASSAU FREEPRT EXUMA ELEUTERA DATE F EXAMINATIN: Monday 1 st March 010 DURATIN: 1½ UR CURSE NUMBER: CEM 5 CURSE TITLE: CLLEGE CEMISTRY II STUDENT NAME: STUDENT NUMBER: LECTURER S NAME INSTRUCTINS T CANDIDATES: This paper has 6 pages and sections. Please follow the instructions given with each section. PAGE 1 F 6 6 Mar 10

2 CEMISTRY 5 SEMESTER MIDSEMESTER EXAMINATIN SECTIN I: Multiple Choice Questions Select the SINGLE best alternative in each of the following cases and indicate your answer by marking the corresponding letter on the answer sheet provided. Questions 1 to 4 concern an experiment to determine the initial rate of reaction between the oxidising agent ammonium peroxodisulfate, (N 4 ) S 8 and potassium iodide. A series of experimental runs was carried out. In each case, two solutions were made up: one containing potassium iodide together with a trace of both starch and sodium thiosulfate, and the other containing ammonium peroxodisulfate. The contents of the tubes were mixed thoroughly and a stop-watch started. The time taken for the solution to darken was noted. The calculated initial concentrations of the ammonium peroxodisulfate and potassium iodide in the reaction mixture, together with the times taken for the mixture to darken, are given below. INITIAL CNCS./mol dm - AMMNIUM PERX- DISULPATE PTASSIUM IDIDE DARKENING TIME /s ) Which rate law is consistent with the above observations? A Rate k[s 8 - ] B Rate k[i - ] C Rate k[s 8 - ][I - ] D Rate k1/([s 8 - ][I - ]) E Rate k[s 8 - ] [I - ] ) What is the reason for adding the sodium thiosulphate? A To reduce iodine to iodide. B To provide sodium ions. C To catalyse the reaction. D To react with a certain definite quantity of ammonium ions. E To oxidize iodide ions to free iodine. 4) In a further experimental run the initial concentrations of ammonium peroxodisulfate and potassium iodide were 0.0 M and 0.40 M respectively. Which figure is closest to the expected darkening time? A 5 seconds. B 140 seconds. C 70 seconds. D 80 seconds. E 9 seconds. ********************* 5) During a chemical reaction the concentration of N drops from M to M in 100 s. The average rate of the reaction is closest to: A Ms -1 B Ms -1 C Ms -1 D Ms -1 E Ms -1 6) A flask is charged with 0.14 mol of a reactant A and allowed to react according to the following equation: A B After 0 seconds, 0.07 mol of A remain. ow many mol of B are present? A B 0.10 C 0.07 D E ) A reaction is found to be second order with respect to a reactant concentration. If the concentration of only this reactant is tripled at constant temperature, the rate of reaction is A tripled. B doubled. C increased six-fold. D increased five-fold. E increased nine-fold. ) The darkening of the solution was due to which of the following? A The precipitation of sulphur. B The formation of an iodine/thiosulphate complex. C The oxidation of sodium thiosulphate. D The reaction of free iodine with the starch. E The reduction of iodine to iodide by the thiosulphate ion. PAGE F 6 6 Mar 10

3 CEMISTRY 5 SEMESTER MIDSEMESTER EXAMINATIN CNTD. 8) Which letter in the diagram below corresponds 1) A reaction vessel is charged with hydrogen to the activation energy of the forward reaction? iodide. This decomposes partially according to the equation: I(g) l (g) + I (g) When equilibrium is reached at 45 C, P I atm and P PI atm. Ans: B 9) The equilibrium constant for the gas phase reaction: N (g) + (g) N (g) is 0 at 00 C. At equilibrium: A Products and reactants are present, but reactants predominate. B Equal amounts of reactants and products are present. C nly reactants are present. D nly products are present. E Products and reactants are present, but products predominate. 10) Which of the following will change the value of the equilibrium constant? A changing temperature. B adding a substance which does not react. C changing the concentration of reactants. D changing the concentration of products. E changing the pressure. 11) The equilibrium expression for the reaction: CaC (s) l Ca(s) + C (g) is given by: [ Ca][ C ] A K c [ CaC ] B C [ C] K c [ Ca][ CaC [ Ca][ C K c [ CaC ] D K c Ca][ C ] [ E K c C ]* [ ] ] K p is closest to: A 74 B 5 C D E ) Consider the following equilibrium: A(g) + B(g) l C(g) -99kJ An increase in temperature at constant volume: A Increases the partial pressure of C. B Increases the partial pressure of B. C Increases the equilibrium constant. D Decreases the equilibrium constant. E Does not affect the equilibrium constant. 14) The equilibrium constant for the reaction S Cl (g) l S (g) + Cl (g) is.9 at 7 K. If Q, the reaction quotient, is 97.0, then: A The system is at equilibrium. B The system is not at equilibrium and will proceed in the forward direction. C The system is not at equilibrium and will proceed in the backward direction. D The system is not at equilibrium but no further reaction will occur. E nly experiment can determine whether the system is at equilibrium or not. 15) ydrogen oxalate ions react with water as follows: C 4 - (aq) + (l) l + (aq) + C 4 - (aq) Which of the following is an acid/base conjugate pair? A C 4 - and + B + and C 4 - C C 4 - and D + and C 4 - E + and PAGE F 6 6 Mar 10

4 CEMISTRY 5 SEMESTER MIDSEMESTER EXAMINATIN CNTD. SECTIN B: STRUCTURED QUESTINS INSTRUCTINS: Answer ALL questions from this section in the spaces provided on the question paper. Full marks can only be obtained if complete working is shown. There is a total of 5 marks for this section. 1) This question concerns various aspects of equilibrium. a) The equilibrium constants for the reactions (g) + Cl (g) l Cl(g) (1) 4Cl(g) + (g) l (g) + Cl (g) () (g) + (g) l (g) () are K 1, K and K respectively. i) Write down the partial pressure expressions (K p ) for each of the equilibrium constants. () K p1 ( P ) Cl ( P )( P ) Cl K p ( P ) ( ) PCl 4 Cl K p ( P ) ( P ) ( P ) ii) Derive an expression relating K to K 1 and K. () 4 ( ) ( PCl ) p1 K p Cl ( P ) ( ) PCl 4 Cl ( P ) K p K iii) At room temperature K has a value of about What does this tell you about this reaction in qualitative terms? (1) The reaction essentially goes to completion at room temperature. iv) At a temperature of 600 C K has decreased to 1.5. What does this tell you about for reaction? (1) Since reaction proceeds to the left as the temperature increases, Le Chatelier s principle tells us that the reaction must be exothermic and so is negative. PAGE 4 F 6 6 Mar 10

5 CEMISTRY 5 SEMESTER MIDSEMESTER EXAMINATIN CNTD. b) When heated, sulphur trioxide shows a marked tendency to dissociate into sulphur dioxide and oxygen according to the equation: S (g) l S (g) + (g) A sample of mol of sulphur trioxide gas, kept at 57 C in a closed vessel of volume 6.56dm, yielded an equilibrium pressure of 1.4 atm. Calculate the equilibrium constant (K p ) for the reaction. (Note: PV nrt and R dm atm mol -1 K -1 ) (7) nrt (57 + 7) P atm V 6.56 Since number of moles is proportional to the partial pressure, the table can be built up in terms of atm: S l S + Initial /atm Change /mol -x +x +x Equilibrium /atm x x x âtotal pressure at equilibrium (1.001-x)+x+x (1.001+x) atm But the equilibrium pressure is given as 1.4 atm, hence x atm. But K p ( P ) ( ) S P ( ) ( ) ( ) (0.4188) x x to s.. ( P ) (1.001 x) ( ) f S ) This question concerns reaction mechanisms. a) A certain reaction goes by the following mechanism: A + B l C (Fast equilibrium step 1) C + B D (Slow step ) D + E F (Fast step ) i) Identify the rate determining step for the reaction and state its molecularity. () Step, bimolecular ii) Write down a chemical equation representing the overall reaction. (1) A + B + E F (obtained by adding equations 1 through ). iii) Identify an intermediate. (1) C or D are possible answers. iv) Write down the rate law for step (1) Rate k [C][ v) Write down the equilibrium expression for the equilibrium step. [ C] K 1 [ A][ (1) vi) Deduce the rate law for the overall reaction (in terms of the reactants for the overall reaction). () From the answer to part (v), [ C] K1[ A][ and substituting for [ C] in ( iv) Rate k ( K [ A][ )[ K k [ A][ 1 1 But since Rate is the rate of the rate-determining step, it is also the rate of the overall reaction and can simply be designated as Rate. Also K 1 k, being a product of constants, is itself a constant, and so can be replaced by k. This gives Rate k[ A][ PAGE 5 F 6 6 Mar 10

6 CEMISTRY 5 SEMESTER MIDSEMESTER EXAMINATIN CNTD. vii) State the order of the overall reaction. (1) From the above rate equation, the order of the overall reaction is 1+ PAGE 6 F 6 6 Mar 10