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) C2H5OH(l) + 3O2 (g)
(2)
2CO2 (g) + 3H2O(l)
thermometer
(2)
Draw a labelled diagram of the apparatus used in this experiment.
known mass of water
Ethanol
in spirit
burner
3. Heat losses to the surroundings.
and
Incomplete combustion of ethanol.
(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.
(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
(5) (a) (i) c is the specific heat
DH = - cmDT
capacity of water.
(a) (i) What is c ?
(ii) 4.18 J kg-1 oC-1.
(ii) What is the value of c for water?
(b) m = 0.2 kg
(b) If 200 cm3 of water is heated, what is the value of m?
(6) In an experiment it was found that the temperature of 200 cm3
(6) DH = - cmDT
of water rose from 20.1oC to 32.5oC when 0.42g of ethanol
= - 4.18 x 0.2 x 12.4
was burned.
= - 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
= - 1135 .76 kJ
kJ
Calculate the enthalpy of combustion of ethanol.