January 1999 Module 2 Exam1. Consider the esterification reaction CH3CO2H
+ CH3CH2OH (a) (i) Give the expression
for Kc.
(1)
(ii) An equilibrium mixture
from the above reaction contains 0.90 mol of ethyl ethanoate, 0.90 mol
of water, 0.10 mol of ethanoic acid, and 2.1 mol of ethanol. It has
a volume of 235 cm3. Calculate Kc to 2 significant
figures, having regard for the units.
(3)
(iii) State, giving a reason,
what would happen to the equilibrium composition if more ethanoic acid
were to be added to the equilibrium mixture.
...................................................................................................................................... ......................................................................................................................................(2) (b) The reaction employs an acid catalyst, usually a little sulphuric acid. State, with a reason, the effect on the equilibrium position if the concentration of the acid catalyst were to be increased. ...................................................................................................................................... ......................................................................................................................................(2) (c) (i) Using the concept
of mean bond enthalpies show that /\H for the esterification reaction
is zero.
(2)
(ii) State, with a reason, the
effect of an increase in temperature on the equilibrium composition of
this reaction.
...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(2)
...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(2) Total 14 marks
2. (a) Define (i) pH: ...........................................................................................................................(1) (ii) Kw: ..........................................................................................................................(1)
(2)
(c) 100 cm3 of 0.100
mol dm-3 sodium hydroxide solution was added to 100 cm3
of 0.200 mol dm-3 ethanoic acid.
(i) Find the concentration
of ethanoic acid in the mixture.
(2)
(ii) Calculate the concentration
of sodium ethanoate in the mixture.
(2)
(iii) Calculate the pH of the
mixture at 25oC; Ka for ethanoic acid at this temperature is
1.80 x 10-5 mol dm-3.
(2)
(iv) State and explain what
happens if a small amount of hydrochloric acid is added to this mixture.
...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(3) (d) When the solutions in part (e) are mixed, 560 J of heat energy is evolved. If hydrochloric acid of the same concentration is used instead of ethanoic acid, 580 J of heat energy is evolved. Suggest reasons for the difference. ...................................................................................................................................... ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(3) Total 16 marks
3. Benzene reacts with concentrated nitric acid in the presence of concentrated sulphuric acid at about 50oC in an electrophilic substitution reaction to give nitrobenzene. (a) (i) Give the equation
representing the overall reaction.
(1)
(ii) Give the equation representing
the formation of the electrophile.
......................................................................................................................................(1) (iii) Give the mechanism
for the reaction of the electrophile with benzene.
(3)
(b) In an experiment to determine
the kinetics of this reaction, it is found that the attack of the electrophile
on the benzene ring is the rate determining step. The following data
shows the effect of changing the concentrations on the rate:
(i) What is the order with respect to benzene .......................................................................................................................... the electrophile? ..............................................................................................................(2) (ii) Write a rate equation for the reaction. ......................................................................................................................................(1) (iii) In some Electrophilic substitution reactions of aromatic compounds, the rate determining step is the production of the electrophile. In such cases what would be the order of the reaction with respect to the aromatic compound? ......................................................................................................................................(1) (c) The equation representing
the hydrogenation of ethene is
(i) Assuming that benzene consists of a ring with three double bonds, predict the enthalpy change for the reaction  ......................................................................................................................................(1) (ii) The enthalpy of hydrogenation
of benzene is actually -205 kJ mol-1. What can you deduce
from this and your answer to part (i) about the stability of the benzene
ring? Use an enthalpy level diagram to illustrate your answer.
(2)
(iii) The compound cyclo-octatetraene
 has an enthalpy of hydrogenation of -480 kJ mol-1; the molecule, unlike that of benzene, is not flat. Suggest in terms of the possibility of orbital overlap why cyclo-octatetraene does not show the same type of stability as benzene, despite superficial similarities in their structures. ...................................................................................................................................... ...................................................................................................................................... ......................................................................................................................................(2) Total 14 marks
4. The pleasant smell that betrays the making of toast or of caramel comes from the compound hydroxymethylfurfural which is a product of heating sugars. The structure of the molecule is:  (a) (i) What would you see if this compound were shaken with a solution of bromine dissolved in an organic solvent? ......................................................................................................................................(1) (ii) Suggest a structure
for the product which is obtained from this reaction.
(2)
(b) In each case state a reagent
you could use, and the observations you would make, to show the presence
in hydroxymethylfurfural of
(i) the hydroxyl group; ...................................................................................................................................... ......................................................................................................................................(2)
...................................................................................................................................... ......................................................................................................................................(2) (c) If hydroxymethylfurfural is heated with potassium dichromate(VI) solution which has been acidified with dilute sulphuric acid, the compound X obtained contains 46.1% carbon, 2.56% hydrogen, and 51.3% oxygen by mass. The ring structure remains intact. (i) Calculate the empirical
formula of X.
(2)
(ii) Draw the structural formula
for X, given that its relative molecular mass is 156.
(2)
(d) Hydroxymethylfurfural can
react with hydrogen in the presence of a catalyst to give compound
Y
which has the structure:
 (i) What volume of hydrogen
would be required to convert 3.15 g of hydroxymethylfurfural into compound
Y?
(3)
(ii) Explain why compound
Y
is chiral.
...................................................................................................................................... ......................................................................................................................................(2) Total 16 marks
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CH3CO2CH2CH3 + H20