Standard enthalpy change of combustion (8.1 3)

Question 1

standard enthalpy change of combustion (see symbol pg.235)

Answer 1

enthalpy change @ 100 kPa at specified temperature usually 298K when one mole of substance is completely burned in oxygen

Question 2

experimental determination of enthalpy change of a combustion of a liquid

Answer 2

known mass of liquid burned and heat produced to heat a known volume of water

Question 3

procedure

Answer 3

1.spirit burner containing liquid being tested weighed
2.known volume water added to copper can
3. temperature of water measured
4.burner is lit
5.mixture constantly stirred with the thermometer
6.when temp of water has reached 20 degrress aprrox above inital flame extingished and burner immediately reweighed
7.final temperature is measure
see fig a pg235

Question 4

how to calculate enthalpy change of combustion

Answer 4

1. Q=mc change in temp m= mass of water c=heat capacity of water Q=heat energy transferred to water
2. calculate moles of liquid burnt
3. Q/mols (should be a minus number)

Question 5

how to see if value worked out is reasonable

Answer 5

look and compare with data booklet

Question 6

possible source of error

Answer 6

1. heat energy produced in burning transferred to air not water
2. some heat energy used for incomplete combustion -soot on bottom of can)
3. some heat energy is produced transferred to copper
4. conditions are not standard e.g. water vapor not liquid water is produced
5. experiment takes a long time not all of heat energy transferred from the water to surroundings is compensated for