Unit 1 PPA 3 ENTHALPY OF COMBUSTION. (1) C 2 H 5 OH(l) + 3O 2 (g) 2CO 2 (g) + 3H 2 O(l). (2). thermometer. known mass of water. Ethanol in spirit burner. ENTHALPY OF COMBUSTION (Unit 1 PPA3). The enthalpy of combustion is the energy released when one mole of a substance is
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ENTHALPY OF COMBUSTION
(1) C2H5OH(l) + 3O2 (g) 2CO2 (g) + 3H2O(l)
known mass of water
in spirit burner
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.
(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.
Incomplete combustion of ethanol.
(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
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