HIGHER CHEMISTRY REVISION.

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HIGHER CHEMISTRY REVISION.
Unit 3 - Hesss Law
1. The Thermite Process involves the reaction
between aluminium and iron(III) oxide to produce
iron and aluminium oxide. This highly exothermic
reaction, which generates so much heat that the
temperature of the mixture rises to around
3000oC, is used for repairing cracked railway
lines as shown in the diagram below.           (a)
Suggest why this process is suitable for
repairing cracked railway lines. (b)     The
enthalpy changes for the formation of one mole of
aluminium oxide and one mole of
iron(III) oxide are shown below. 2Al(s) 1
½ O2(g) ? Al2O3(s) DH -1676 kJ
mol-1. 2Fe(s) 1 ½ O2(g) ? Fe2O3(s)
DH -825 kJ mol-1. Use the above
information to calculate the enthalpy change for
the reaction 2Al(s) Fe2O3(s)
? Al2O3(s) 2Fe(s)
(a) The iron formed is molten and it fills
the crack it then solidifies. (b)
Leave first equation and reverse
the second equation. DH (-1676)
(825) -851 kJ
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2. The enthalpy of combustion of hydrogen
sulphide is -563 kJ mol-1. Use this value
and the enthalpy of combustion values in the
data booklet to calculate the enthalpy
change for the reaction H2(g)
S(s) ? H2S(g) (rhombic)
H2 ½ O2 ? H2O DH1 -286
kJ S O2 ? SO2 DH2 -297
kJ H2S 1 ½ O2 ? H2O
SO2 DH3 -563 kJ To form the equation
shown we need DH1 DH2 - DH3 And so
DH (-286) (-297) (563) -20
kJ
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3. A calorimeter, like the one shown, can be
used to measure the enthalpy of combustion
of ethanol. The ethanol is ignited and burns
completely in the oxygen gas. The heat
energy released in the reaction is taken in
by the water as the hot product gases are
drawn through the coiled copper pipe by the
pump.
(a) Why is the copper pipe coiled as shown in
the diagram. (b) The value of enthalpy of
combustion of ethanol obtained by the calorimeter
method is higher than the value obtained
by the typical school laboratory method.
One reason for this is that more heat is lost to
the surroundings in the typical school
laboratory method. Give one other reason
for the value being higher with the calorimeter
method. (c) In one experiment the burning of
0.980 g of ethanol resulted in the temperature
of 400 cm3 of water rising from 14.2oC to
31.6oC. Use this information to calculate
the enthalpy of combustion of ethanol.

• (a) Coiling the pipe increases its surface area
  and ensures as much heat is
• transferred into the water as possible.
• (b) Using oxygen ensures complete combustion of
  ethanol.
• DH-cmDT - 4.18 x 0.4 x 17.4 - 29.1 kJ
• 0.98 g of ethanol ? -29.1 kJ
• So 1 mol, 46g, of ethanol ? -1365.9 kJ

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4. Potassium hydroxide can be used in
experiments to verify Hesss Law. The reactions
concerned can be summarised as
follows. KOH(s) KCl(aq) KOH(aq)
(a) State Hesss Law. (b) Complete the
list of measurements that would have to be made
in order to calculate DH2. (i)
Mass of potassium hydroxide (ii) (iii) (iv)
(c) What solution must be added
to the potassium hydroxide solution in order to
calculate DH3?
HCl(aq)
DH1
DH2
DH3
H2O(l)

• The enthalpy of a reaction is independent of the
  reaction pathway.
• Mass of water, initial water temperature, final
  water temperature.
• (c) Hydrochloric acid.

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5. The equation for the enthalpy of
formation of propanone is 3C(s) 3H2(g)
½ O2 (g) ? C3H6O(l) Use the following
information on enthalpies of combustion to
calculate the enthalpy of formation of
propanone. (1) C(s) O2(g) ?CO2
(g) DH1 394 kJ mol-1. (2) H2 (s)
½ O2 (g)? H2O(l) DH2 286 kJ mol -1. (3)
C3H6O(l) 4O2 (g)? 3CO2 (g) 3H2O(l) DH3
1804 kJ mol -1.
To form the equation we want we need- 3 x Eqn(1)
3 x Eqn (2) Reverse Eqn(3) DH (3 x
394) (3 x 286) (1804) -236 kJ
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6. Some rockets have a propellant system which
combines dinitrogen tetroxide with
methylhydrazine. 5N2O4 4CH3NHNH2
? xN2 yH22O zCO2 (a) State the
values of x, y and z required to balance the
above equation. (b) Draw the full structural
formula for methylhydrazine. (c)
Methylhydrazine burns according to the following
equation. CH3NHNH2 2.5O2 ? CO2
3H2O N2 DH 1305 kJ mol-1.
Use this information, together with information
from page 9 of the data booklet, t calculate
the enthalpy change for the following reaction.
C N2 3H2 ? CH3NHNH2