دورةالعام 2015 العادية الثالثاء 16 حسيراى 2015

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1 وزارة التربية والتعلين العالي الوديرية العاهة للتربية دائرة االهتحانات اهتحانات الشهادة الثانىية العاهة فرع العلىم العاهة هسابقة في هادة الكيوياء الودة: ساعتاى االسن: الرقن: دورةالعام 5 العادية الثالثاء 6 حسيراى 5 This Exam Includes Three Exercises. It Is Inscribed on Four Pages Numbered from to 4. The Use of Non-programmable Calculator Is Allowed. Answer The Three Following Exercises: First Exercise (6 points) Kinetic of Dimerization of Butadiene At high temperature, butadiene dimerizes in gaseous phase in a complete reaction of which the equation is: C 4 H 6(g) C 8 H (g) In an evacuated container of constant volume V, maintained at a temperature T = 69K, n mol ofbutadiene gas is introduced. A manometer, attached to this container, measures the total pressure P at different instants of time of the evolution of the reacting system. The concentration of C 8 H gasis determined at those instants. The results are listed in the table below: t (min) [C 8 H ] mol.l Gases of the reacting mixture are supposed to be ideal. Given: - Ideal gas constant: R =.8 atm.l.mol -.K Preliminary Study.- Draw up a table representing the composition of the reacting mixture at the initial state and at the instant of time t of its evolution as a function of n and x. (x is the number of moles of C 8 H formed at t)..- Establish the relation among: the concentration ofc 8 H gas at the instant of time t, the total pressure P at this instant, and the initial pressure P in the container..- Show that the concentration of C 8 H at the end of the chemical transformation is equal to. - mol.l - knowing that the initial pressure inside the container is P =. atm. - Kinetic Study.- Plot the curve representing the change in the concentration of C 8 H as a function of time: [C 8 H ] = f (t), in the interval of time: [ 8 min]. Take the following scales : cm for 5 min in abscissa and cm for. mol.l - in ordinate..- Deduce, graphically, the change in the rate of formation of C 8 H with time..- Determine the half-life time of the reaction..4- Consider each one of the three following curvesand specify whether it represents the change in the concentration of C 4 H 6 as a function of time.

2 5 [C 4 H 6 ] mmol.l - [C [C 4 H 6 ] mmol.l - 4 H 6 ] mmol.l Curve (a) 5 Curve (b) 5 Curve (c) t (min) t (min) t (min) Second Exercise(7 points) The pheromones Pheromones are organic substances (esters, ketones, alcohols ) very fragrant emitted by most animals (ants, bees ) and act as messengers among the individuals of the same species. Given, in the table below, the chemical formulas and the names of some pheromones: Pheromone Chemical formula or systematic name A CH C O CH B O -methylbutyl ethanoate C CH CH CH CH CH CH CH D CH OH -Hexanone -hexanone is a liquid compound at room temperature. - preliminarystudy.- Name the common functional group of pheromones A and B..- Write the possible condensed structural formulas of the organic compounds that can react with methanol to produce pheromone A..- The compound (pheromone) C is subjected to mild oxidation by the reaction with an oxidizing agent. Write the condensed structural formula of the organic compound obtained and give its name..4- Describe two chemical tests that allow identifying the chemical function of compound (pheromone) D, by listing the reagents and the materials used in those tests.

3 - Preparation of the pheromone B Given: - Molar mass in g.mol : M (acetic acid) = 6 and M (pheromone B) = - Density of acetic acid : d =.4 g / ml. - Starting from initial equimolar mixture of carboxylic acid and alcohol, the percentage yield at equilibrium is:67% if the alcohol is primary and 6% if the alcohol is In order to prepare the pheromone B, one proceeds as follows: - A volume V of isoamyl alcohol (equivalent to. mol of this alcohol) is mixed with a volume V = ml of acetic acid. - This mixture is heated for 45 min. - The pheromone B is extracted from the reacting medium and it is purified and weighed. The mass is found to be m = 7.4 g..- Write, using the condensed structural formulas of the organic compounds, the equation of the reaction that took place..- Deduce from the experimental procedure, described above, one of the characteristics of this reaction..- Calculatethe initial number of moles of acetic acid..4- Determine the yield of this reaction..5- Verify, whether the heating duration (45 min)was sufficient to reach the equilibrium..6- Suggest two experimental ways that help to increase the yield of this reaction (without changing the chemical nature of reactants). Third Exercise(7 points) Sodium Bicarbonate Sodium bicarbonate or sodium hydrogen carbonate, of formula NaHCO, is used in the production of beverages and carbonated water. Also it is used to reduce the excess stomach acid. This exercise aims to study the behavior of hydrogen carbonate ions HCO in water, as well as to determine the percentage by mass (degree of purity) of NaHCO in a sample of medicinal sodium bicarbonate. Given: - Ionic compounds NaHCO and Na CO are solids highly soluble in water. [CO ] %(bymol)co -. [HCO ] %(by mol) HCO - M (NaHCO ) = 84 g.mol -. - Study of Hydrogen Carbonate Ion Hydrogencarbonate ion, HCO, can react with water according to two chemical reactions of equations: HCO + H O CO + H O + () or HCO + H O HO + CO, H O ().- Show that hydrogencarbonate ion has an amphoteric character (ampholyte)..- The followinggraph represents the change in the percentages by moles of the chemical species HCO and CO in an aqueous solution (A) as a function of ph:

4 Percentage by mol: % %HCO ) % CO ph [CO ]..- Calculate, based on the above graph, the ratio when the ph of solution (A) is equal [HCO ]..- It is required to prepare a buffer solution of ph = using. - mol of Na CO and a mass m of NaHCO. Determine the mass m needed for this preparation. - Degree of Purity of a Sample of Sodium Bicarbonate In order to determine the degree of purity of a sample of powder medicinal sodium bicarbonate one proceeds as follows: - A mass m =.5 g, of the powder of this sodium bicarbonate, is weighed and dissolved in distilled water in such a way to obtain a volume V =. ml of a solution noted (S). - A volume V =. ml of the solution (S) is removed and introduced into a beaker then few drops of an appropriate colored indicator are added. - A hydrochloric acid solution of concentration C = 5. - mol.l - is added progressively to reach equivalence. The equation of this titration reaction is: H O + + HCO CO,H O + H O.- Choose, from the list below, the materials needed for the preparation of the solution (S) : Precision balance, 5 ml graduated buret, ml volumetric pipet, ml volumetric flask, magnetic stirrer, funnel, watch glass and stand..- Determine the molar concentration of HCO ionsin the solution (S), knowing that the volume of the acid added to reach equivalence is V E = 6.8 ml..- Deduce the percentage by mass of NaHCO in the sample of the medicinal powder..4- The above titration is carried out again but with one change: the volume V is introduced into a beaker containing ml distilled water and the appropriate colored indicator. By consulting the table below, compare V E and V E on one hand and ph E and ph E on the other hand. Justify. Volume of the acid added at equivalence ph of the mixture at equivalence Titration without addition of distilled water V E ph E Titration with addition of distilled water V E ph E 4

5 وزارة التربية والتعلين العالي الوديرية العاهة للتربية دائرة االهتحانات هشروع هعيار التصحيح اهتحانات الشهادة الثانىية العاهة فرع العلىم العاهة هسابقة في هادة الكيوياء دورةالعام 5 العادية First Exercise (6 points) Kinetic of Dimerization of Butadiene Question Answer mark C 4 H 6(g) C 8 H (g). Initial state n -.5 At t n xx. Ideal gas law: P V = (n x) R T = n R T x R T but [C 8 H ] = x = P P V RT = P P 8H ] in mol.l At the end of the transformation, P = P /. P [C 8 H ] = =. mol.l The curve is : 9 [C 8 H ] mol.l t (min) 5 5 t /. Graphically, the rate of formation of C 8 H is equal to the slope of the tangent at the curve at the point of abscissa t. At t =, this slope is maximal and decreases with time.. The half-life time of a reaction is the time required for C 8 H to reach half its maximum concentration. [ C. At t /, [C 8 H ] = ]max 8H = 5. - mol.l

6 Graphically : t / = 6 min..4 The curve that represents the change in the [C 4 H 6 ] as a function of time should pass through two specific points: (At t = ) [C 4 H 6 ] initial = [C 8 H ] at the end of transformation =. mol.l -. At t /, [C 4 H 6 ] should have the value of. / =. mol.l -. The curve (a) is not the convenient one since its initial concentration [C 8 H ] =6.8mmol.L - The curve (b) is not the convenient one since t =min Therefore, the graph (c) represents this change since [C 4 H 6 ] =. mol.l - and t =6 min.5 Second Exercise (7 points) The pheromones Question Answer mark. The common functional group of pheromones Aand B is the ester group..5. the possible formulas are:.75 CH COOH ; CH CO Cl and CH CO O CO CH.. the formula of this product is: CH CH CH CH C CH CH.75 CH O Its name: 4 methyl--heptanone.4 To identify the chemical function of the compound D, the two following tests are carried out: - Few drops of.4-d.n.p.h are added into a test tube containing few milliliters of the compound D a yellow-orange precipitate is obtained. - Few drops of Fehling reagent are added into a test tube containing few milliliters of the compound D. After gentle heating, the medium remains blue.. The equation of this reaction is: CH CH CH CH OH + HO C CH CH CH CH CH O C CH + H O.75 CH O CH O. This reaction is slow; from the sentence of the experimental procedure. «This mixture is heated for 45 min».. m( acid) initial d V.4 n(acid) initial = =.5 mol M ( acid) M ( acid) 6.5.5

7 .4 The yield of this reaction: yield = n = m( ester) actual M ( ester) 7.4 n( ester) actual n( ester) theoretical =.4 mol. n n n = n(alcohol) initial =. mol. So yield =.67, or 67%. with.5 The initial mixture is not equimolar, the yield at equilibrium must exceed 67%..5 So, the duration of min is not sufficient to reach the equilibrium of this reaction..6 The two ways are: - removing the water from the reacting medium as long as it is formed. - using an initial mixture where the difference between the two reactants exceeds the value of. (.5.). Third Exercise(7 points) Sodium Bicarbonate Question Answer mark. Hydrogen carbonate ion has an amphoteric character since it belongs to two acid/base pairs: HCO / CO and CO, H O / HCO..5.. From the graph, the % by molof HCO is 67 ; that of CO is [ CO ] = [ HCO ].75 %by molco %by molhco = / 67 =.5.. The introduced quantity of HCO ions (produced by NaHCO ) and that of CO ions (produced by Na CO ) are conserved in the obtained solution. n [ CO ] V ( solution) n = [ HCO ] n n V ( solution) With n = n(co ) formed in the solution = n(na CO ) initial =. - mol and n = n( HCO ) formed in the solution= n(nahco ) initial = m ( NaHCO M ( NaHCO ) initial By calculation m= 5.4 g.. The materials needed for the preparation of the solution (S) is : Precision balance, watch glass, funnel and ml volumetric flask.. n (NaHCO ) dissolved in V = n (HCO ) initial in V = n (H O + ) added to reach equivalence. Therefore [ HCO ] V =C V E ; [ HCO ]= C VE V = =8.4 - mol.l - ) m = mol. 84. n (NaHCO ) in the sample = n (HCO ) dissolved in V = =.68 - mol..5

8 .4 m (NaHCO ) in the sample = n (NaHCO ) M(NaHCO ) = =.4 g. m( NaHCO.4 Degree of purity = ) pure = 94 %. m( sample).5 n( HCO ) initialinv V E =. the dilution of solution does not change the initial quantity of C HCO and C is the concentration of the acid solution in the buret, therefore V E = V E The solution obtained at equivalence is acidic, the dilution of an acidic solution increases its ph, therefore ph E >ph E

This Exam Includes Three Exercises. It Is Inscribed on 3 Pages Numbered From 1 to 3. The Use of A Non-programmable Calculator is Allowed.

This Exam Includes Three Exercises. It Is Inscribed on 3 Pages Numbered From 1 to 3. The Use of A Non-programmable Calculator is Allowed. وزارة التربية والتعلين العالي الوديرية العاهة للتربية دائرة االهتحانات اهتحانات الشهادة الثانىية العاهة الفرع : علىم عاهة هسابقة في هادة الكيوياء الودة ساعتاى االسن: الرقن: دورة العام 203 االستثنائية الثالثاء