June 1998 Module 2 ExamTwo types of reaction in organic chemistry are electrophilic addition and nucleophilic substitution. (a) Define the terms: (i) Nucleophile: ...................................................................................................................................... ......................................................................................................................................(1) (ii) Electrophile: ...................................................................................................................................... ......................................................................................................................................(1) (iii) Substitution: ...................................................................................................................................... ......................................................................................................................................(1) (iv) Addition. ...................................................................................................................................... ......................................................................................................................................(1) (b) Give the mechanism of
a reaction of your choice which proceeds by electrophilic addition.
......................................................................................................................................(3) (c) (1) Describe, briefly, a reaction you could carry out in the laboratory which occurs by nucleophilic substitution. ...................................................................................................................................... ......................................................................................................................................(1) (ii) Write a mechanism for
this reaction.
(3)
(iii) What type of nucleophilic
substitution is occurring in this reaction? Justify your answer.
......................................................................................................................................(1) (d) When a solution of bromine
is shaken with cyclohexene, (i) Write an equation for
the reaction of cyclohexene with bromine.
(3)
(ii) Explain, in terms of the
bonding why no reaction occurs when a solution of bromine is shaken with
benzene.
...................................................................................................................................... ......................................................................................................................................(2) Total 15 marks
2. Citric acid is used in
foodstuffs as an antioxidant and, together with its sodium salt, as an
acidity regulator. It occurs naturally in fruit juices.
(a) (i) Assuming citric acid behaves in aqueous solution as a monoprotic acid: RCO2H + H2O write an expression for Ka
for this acid.
(1)
(ii) Calculate the pH of lemon
juice which contains citric acid at a concentration of 0.200 mol dm-3.
(Ka for citric acid= 7.4 x 10-4 mol dm-3).
(3)
(b) The use of citric acid together
with its salt, sodium citrate, as an acidity regulator depends on the ability
of this mixture to act as a buffer.
(i) What is the function of a buffer solution? ...................................................................................................................................... ......................................................................................................................................(2) (ii) Describe how the mixture of citric acid and sodium citrate achieves this buffering action. Give equations for the TWO reactions you describe. ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(3) (iii) Calculate the pH of a buffer solution containing 0.200 mol dm-3 of citric acid and 0.400 mol dm-3 of sodium citrate. (2) (c) Citric acid forms
a liquid ester which has the structural formula
(i) Describe a test you could use to show that the ester contains an -OH group. ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(2) (ii) What reagent would you use to hydrolyse the ester? ...................................................................................................................................... ......................................................................................................................................(1) (iii) Treatment of the products of the reaction in (c)(ii) leads to the production of a pure sample of citric acid. How would you show the presence of the -CO2H group in the citric acid other than by the use of an indicator? ...................................................................................................................................... ......................................................................................................................................(2) Total 16 marks
3. This question concerns the efficiency and economics of the Haber synthesis of ammonia represented by the reaction: N2(g) + 3H2(g) Le Chatelier's Principle predicts that the equilibrium concentration of ammonia will be greater at high pressure and low temperature. Typical values used industrially are 500oC and 200 atm which gives about 15% conversion to ammonia at equilibrium. (a) Explain in molecular terms, why a temperature lower than the one quoted is not used. ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(3) (b) Explain why a pressure higher than the one quoted is not often used. ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(2) (c) The gases are passed through a conversion chamber containing beds of granulated iron which acts as a catalyst. Describe and explain the effect of the iron on: (i) the rate of the production of ammonia, ...................................................................................................................................... ......................................................................................................................................(2) (i i) the amount of ammonia in the equilibrium mixture. ...................................................................................................................................... ......................................................................................................................................(2) (d) The equilibrium mixture formed is passed into a refrigeration plant. Explain why this is done and what follows this process? ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(3) Total 12 marks
4. This question concerns the reaction H2(g) + I2(g) which, even at high temperatures, is slow. Data: Bond energies in kJ
mol-1
(a) (i) Calculate /\H
for the reaction between hydrogen and iodine.
(2)
(ii) Sketch an energy level
diagram for this reaction.
(2) (b) Indicate on your
sketch in (a)(ii)
(ii) the activation energy for the forward reaction (Ea(F)) (iii) the activation energy for the reverse reaction (Ea(R)) (3)
(c) For the analogous reaction
for the formation of hydrogen chloride
H2(g) + Cl2(g) --> 2HCI(g) suggest how you would expect the activation energy of the forward reaction to compare with that shown for the formation of HI. Give a reason for your answer. ...................................................................................................................................... ......................................................................................................................................(2) (d) The reaction for the formation of hydrogen iodide does not go to completion but reaches an equilibrium. (i) Write an expression for
the equilibrium constant, Kc, for this reaction.
(1)
(ii) A mixture of 1.9 mol of
H2 and 1.9 mol of I2 was prepared and allowed to
reach equilibrium in a closed vessel of 250 cm3 capacity at
700oC. The resulting equilibrium mixture was found to
contain 3.0 mol of HI.
Calculate the value of Kc
at this temperature.
(3)
(e) The rate expression for
the forward reaction between hydrogen and iodine is
Rate = k[H2][I2] (i) What is the order of the reaction with respect to iodine? ......................................................................................................................................(1) (ii) When 0.10 mol of each of H2 and I2 were mixed at 700oC in a vessel of 500 cm3 capacity, the initial rate of formation of HI was found to be 1.5 x mol dm-3 s-1. Calculate a value for k at 700oC, stating the units. (3)
Total 17 marks |
, the bromine is
decolourised. However, when bromine is added to benzene there is
no decolourisation.
RCO2- + H3O+