PPA 3

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Unit 1 PPA 3 ENTHALPY OF COMBUSTION
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ENTHALPY OF COMBUSTION (Unit 1 PPA3)
The enthalpy of combustion is the energy released
when one mole of a substance is completely burned
in oxygen. The aim of this experiment is to
determine the enthalpy of combustion of
ethanol. A measured mass of ethanol is burned in
a spirit burner and the heat released is
transferred to a copper can containing a known
volume of ethanol. From the resulting
temperature rise, the enthalpy of combustion of
ethanol can be calculated.
(1) Write the balanced equation for the
enthalpy of combustion of ethanol.

1. Draw a labelled diagram of the apparatus used in
   this experiment.

(3) The value for the enthalpy of combustion
determined using this apparatus is much
less than the accepted value. Give two
reasons for this lower value.
3. Heat losses to the surroundings. and
Incomplete combustion of ethanol.
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(4) What measurements are taken during this
experiment?
(4) The initial water temperature. The
highest water temperature reached. The mass
of water being heated (in kg). The mass of
ethanol burned.
(5) The enthalpy change during the reaction is
calculated using DH - cmDT
(a) (i) What is c ? (ii) What is
the value of c for water? (b) If 200
cm3 of water is heated, what is the value of m?
(5) (a) (i) c is the specific heat
capacity of water. (ii) 4.18 J
kg-1 oC-1. (b) m 0.2 kg
(6) In an experiment it was found that the
temperature of 200 cm3 of water rose from
20.1oC to 32.5oC when 0.42g of ethanol
was burned. Calculate the enthalpy of
combustion of ethanol.
(6) DH - cmDT - 4.18 x 0.2 x
12.4 - 10.37 kJ When 0.42 g of
ethanol burns DH - 10.37 kJ So when 1 mole,
46g, of ethanol burns DH - 10.37 x 46/0.42
kJ - 1135 .76 kJ