Download 1. One of the isomers of C4H10O is the alcohol 2-methylpropan-2

Winter 2001 CH2
1.
One of the isomers of C4H10O is the alcohol 2-methylpropan-2-ol which has the structural
formula
CH 3
CH 3
C
CH 3
OH
(a)
There are three other structural isomers of C4H10O which are also alcohols.
(i)
Draw their structural formulae.
(3)
(ii)
One of these isomers exhibits stereoisomerism. Name the type of isomerism shown
and draw diagrams showing clearly how these stereoisomers differ from one
another.
Type of isomerism .........................................................................................
Diagrams of isomers
(3)
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(iii)
Describe a test to show that each of the isomers in (i) contains an OH group.
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(2)
(b)
Draw the structural formula of the final organic product of the reaction when each of the
three alcohols in (a)(i) is heated under reflux with a solution of potassium dichromate(VI)
in dilute sulphuric acid.
ALCOHOL
STRUCTURAL FORMULA OF PRODUCT
(3)
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(c)
2-methylpropan-2-ol can be prepared by the reaction of 2-bromo-2-methylpropane with
dilute aqueous potassium hydroxide.
(i)
Give the mechanism for this reaction.
(3)
(ii)
If a concentrated solution of potassium hydroxide in ethanol is used instead of
dilute aqueous potassium hydroxide, a different organic product is obtained. Draw
the structural formula of this product.
(1)
(Total 15 marks)
2.
(a)
Define:
(i)
the standard enthalpy of formation of benzene, C6H6(l);
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(2)
(ii)
the standard enthalpy of combustion of benzene, C6H6(l).
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(2)
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(b)
Calculate the standard enthalpy of formation of benzene, C6H6(l), using the following
enthalpy of combustion data:
Substance
❉Hc /kJ mol–1
C6H6(l)
–3273
H2(g)
–286
C(s)
–394
(3)
(c)
If the standard enthalpy of formation is calculated from average bond enthalpy data
assuming that benzene has three C==C and three C––C bonds, its value is found to be
+215 kJ mol–1.
Explain, with reference to the structure and stability of benzene, why this value differs
from that calculated in (b). Use an enthalpy level diagram to illustrate your answer.
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(4)
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(d)
Benzene reacts with bromine when gently warmed in the presence of a catalyst of
anhydrous iron(III) bromide.
(i)
The reaction is first order with respect to benzene and first order with respect to
bromine. Write the rate equation for the reaction.
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(1)
(ii)
The mechanism of this reaction involves an attack by Br+ followed by loss of H+.
H
Step 1.
Br +
+
Br
H
Step 2.
+
Br
Br
+H +
Deuterium, symbol D, is an isotope of hydrogen, and the C––D bond is slightly
stronger than the C––H bond. If step 2 were the rate-determining (slower) step,
suggest how the rate of this reaction would alter if deuterated benzene, C6D6, were
used instead of ordinary benzene, C6H6, and explain your answer.
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(2)
(Total 14 marks)
3.
Cars are fitted with catalytic converters in order to reduce the pollution caused by the
combustion of petrol. Potential pollutant gases include carbon monoxide, nitrogen monoxide
and unburnt hydrocarbons. The first two compounds are removed by passing the hot gases over
a platinum catalyst.
CO(g) + NO(g) ➤ CO2(g) +
1
2
N2(g)
❉H = –373 kJ mol–1
In the absence of a catalyst, this reaction is extremely slow.
(a)
(i)
Define the term activation energy.
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(1)
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(ii)
Comment on the relative value of the activation energy of this reaction compared
with the much faster reaction of nitrogen monoxide with oxygen.
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(1)
(b)
(i)
Draw a distribution of the molecular energies of gas molecules at two different
temperatures, T1 and a higher temperature T2. Label the curves T1 and T2 and mark
the energy corresponding to the activation energy, EA.
Fraction of molecules
with a given energy
Energy
(3)
(ii)
Use the diagram to explain why the rate of a reaction, such as that between carbon
monoxide and nitrogen monoxide, will change as the temperature increases.
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(2)
(c)
The reaction between carbon monoxide and nitrogen monoxide requires a platinum
catalyst with a large surface area. Explain the effect of a catalyst on the rate of this
reaction and why the surface area needs to be large.
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(4)
(Total 11 marks)
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4.
(a)
(i)
Calculate the concentration, in mol dm–3, of a solution of hydrochloric acid, HCl,
which has a pH of 1.13.
(1)
(ii)
Calculate the concentration, in mol dm–3 , of a solution of chloric(l) acid, HOCl,
which has a pH of 4.23.
Chloric(l) acid is a weak acid with Ka = 3.72 × 10–8 mol dm–3.
(4)
(b)
The pH of 0.100 mol dm–3 sulphuric acid is 0.98.
(i)
Calculate the concentration of hydrogen ions, H+, in this solution.
(1)
(ii)
Write equations to show the two successive ionisations of sulphuric acid, H2SO4, in
water.
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(2)
(iii)
Suggest why the concentration of hydrogen ions is not 0.20 mol dm–3 in 0.100 mol
dm–3 sulphuric acid.
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(1)
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(c)
Many industrial organic reactions produce hydrogen chloride as an additional product.
This can be oxidised to chlorine by the Deacon process:
4HCl(g) + O2(g)
2Cl2(g) + 2H2O(g)
❉H = –115 kJ mol–1.
0.800 mol of hydrogen chloride was mixed with 0.200 mol of oxygen in a vessel of
volume 10.0 dm3 in the presence of a copper(I) chloride catalyst at 400 ºC. At
equilibrium it was found that the mixture contained 0.200 mol of hydrogen chloride.
(i)
Write an expression for the equilibrium constant Kc.
(1)
(ii)
Calculate the value of Kc at 400 ºC.
(4)
(d)
State and explain the effect, if any, on the position of equilibrium in (c) of:
(i)
decreasing the temperature;
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(2)
(ii)
decreasing the volume;
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(2)
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(iii)
removing the catalyst.
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(2)
(Total 20 marks)
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