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Unit 1 PPA 3 ENTHALPY OF COMBUSTION PowerPoint Presentation

Unit 1 PPA 3 ENTHALPY OF COMBUSTION

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(1) C2H5OH(l) + 3O2 (g) 2CO2 (g) + 3H2O(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

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.

- 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.

(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

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