Standard enthalpy change of combustion

Question 1

what is the standard enthalpy change of combustion?

Answer 1

is the enthalpy change measured at 100kPa and a specific temperature, usually 298K, when one mole of a substance is completely burned in oxygen. ΔH°c

Question 2

why does H2 + 1/2 O2 == H2O show ΔH°c but

2H2 + O2 == 2H2O does not?

Answer 2

the second equation shows 2x ΔH°c as ΔH°c represents the change for 1 mole

Question 3

what are the steps in calculating the enthalpy change of a reaction?

Answer 3

1) calculate the heat energy, Q in Q=mcΔT, where m= mass of substance A and c= specific heat capacity of that substance A
2) calculated the amount n, of substance B (the substance which is being tested for enthalpy change)
3) use ΔH= (-Q/n) to work out the ΔH°c

Question 4

what is the density of water?

Answer 4

1 g cm-3

Question 5

possible sources of error in enthalpy of combustion experiments are;

Answer 5

some of the heat produced in burning is transferred to the air and not the water
some of the reactant may not burn completely, producing CO as well as CO2 and H2O
some of the heat produced in burning is transferred to the copper can holding the water and not the water itself
the conditions are not standard, e.g. water vapour instead of liquid water if produced
as the experiment takes a long time, not all the heat energy transferred from the water to the surroundings is compensated for

Question 6

ΔH=

Answer 6

• Q / n

Question 7

to find the enthalpy change of combustion of a liquid, we burn a

Answer 7

known mass of the liquid and the heat energy produced is used to heat a known volume of water

Question 8

the 7 step procedure for testing the enthalpy change of combustion of a liquid is;

Answer 8

1) a spirit burner containing the liquid under test is weighed
2) a known volume of water is added to a copper can
3) the temperature of the water is measured
4) the burner is lit
5) the mixture is constantly stirred with the thermometer
6) when the temperature of the water has reached approximately 20 degrees C above its initial temperature, the flame is extinguished and the burner is immediately reweighed
7) the final temperature is measured

Question 9

it is very important that you include………..when writing a value for ΔH

Answer 9

a sign