C3 3.3
Comparing the Energy Released by
fuels
How much energy comes from
burning fuels?
At the end of the lesson, you
should be able to:
1. Describe how we can
measure the energy
produced by different fuels
2. Be able to state the unit of
energy
3. Be able to calculate the
energy given off when a
fuel is combusted (in
kJ/mole) from given data
Method
1. Put 100 cm3 of water in the calorimeter.
2. Record the temperature of the water.
3. Clamp the calorimeter in position about 10 cm above the protective
mat.
4. Find the mass of a spirit burner containing an alcohol.
5. Place the burner under the calorimeter and light the wick.
6. Stir the water with the thermometer.
7. Stop heating when the temperature has risen by 20 °C.
8. Replace the cap on the burner.
9. Reweigh the burner as soon as possible.
10.Keep noting the temperature – record the highest value reached.
11.If you can, repeat the experiment with other alcohols.
12.Try to keep all the factors the same
Calculation
1. Find out the amount of energy transferred to
the water:
Energy (J) = m(water)xSHC(water)xtemp change
Energy (J) = 100 x 4.2 x 20 = 8 400J
Energy (kJ) = 8.4 kJ
2. Work out the mass of fuel burnt:
Mass of burner (before) – mass of burner (after)
For example: if the burner and ethanol fuel had a
mass of 196.5g before and 195.4 g after.
Mass of fuel burned = 196.5 – 195.4 = 1.1g
Calculation
3. Number of moles of fuel burned:
You need to know the mass of the fuel burned
and the molar mass.
For example:
mass of ethanol = 1.1 g
Mr of ethanol =
Ethanol (CH3CH2OH):
(12x2) + 16 + 6 = 46 g/mole
Moles of ethanol (CH3CH2OH) = mass / molar mass
1.1 / 46 = 0.024 moles
Calculation
4. Divide the energy transferred to the water by
the number of moles of alcohol to find the
enthalpy change of combustion (ΔH).
For example:
Energy transferred to water
= 8.4 kJ
Moles of ethanol burned
= 0.024 moles
Enthalpy change of combustion (ΔH) = 8.4 / 0.024
Enthalpy change of combustion (ΔH) = 350 kJ/mole
Calculation 2
1. Find out the amount of energy transferred to
the water:
Energy (J) = m(water)xSHC(water)xtemp change
Energy (J) = 100 x 4.2 x 20 = 8 400J
Energy (kJ) = 8.4 kJ
2. Work out the mass of fuel burnt:
Mass of burner (before) – mass of burner (after)
For example: if the burner and methanol fuel had a
mass of 196.5g before and 195.4 g after.
Mass of fuel burned = 196.5 – 194.5 = 2.0g
Calculation 2
3. Number of moles of fuel burned:
You need to know the mass of the fuel burned
and the molar mass.
For example:
mass of methanol = 2 g
Mr of methanol =
Methanol (CH3OH):
(12+ 3+ 16 + 1 = 32 g/mole
Moles of methanol = mass / molar mass
2 / 32 = 0.0625 moles
Calculation 2
4. Divide the energy transferred to the water by
the number of moles of alcohol to find the
enthalpy change of combustion (ΔH).
For example:
Energy transferred to water
= 8.4 kJ
Moles of methanol burned
= 0.0625 moles
Enthalpy change of combustion (ΔH) = 8.4 / 0.0625
Enthalpy change of combustion (ΔH) = 134.4kJ/mole
What is Really Happening?
1. Write a word equation for the reaction
between the alcohol and oxygen (the first has
been done for you)
methanol + oxygen  carbon dioxide + water
CO2 (g) + 2 H2O(l)
CH3OH (l) + 1½ O2 (g)
2. Where did the energy come from that was
transferred to the water?
All of the bonds in the reactant molecules are
broken and all of the bonds in the product
molecules are formed.
The Energy in our Food…
• We can find the energy in our food in a
very similar way, if we can make it burn:
Food
Energy per 100g 1. What nutrient type is
Bread
1,200 kJ present in the three
most ‘high energy’
Chocolate biscuit
2,240 kJ foods?
Butter
3,310 kJ 2. What sort of problems
Apple
160 kJ do scientists face,
when trying to
Celery
32 kJ measure these values
Sausages (fried)
1,450 kJ accurately?
Example
Susie burned some propanol
(C3H7OH). She set up her
equipment. She added 100g of
water to a copper calorimeter
(100ml).
She weighed her mass of fuel and
the burner before the experiment;
it had a mass of 193.45g.
She allowed the fuel to raise the
temperature of the water by 30.C.
She weighed her mass of fuel and
the burner after the experiment; it
had a mass of 192.55g.
How can she calculate a ΔH for the
fuel (in kJ/mole)?
Step 1:
Heat change = m x C x ΔT
= 100 x 4.2 x 30
= 12600 J
= 12.6 kJ
Step 2:
Mass of fuel burned
= 193.45 – 192.55
= 0.90 g
Example
Jezebel burned some propanol
(C3H7OH). She set up her
equipment. She added 100g of
water to a copper calorimeter
(100ml).
She weighed her mass of fuel and
the burner before the experiment;
it had a mass of 193.45g.
She allowed the fuel to raise the
temperature of the water by 30.C.
She weighed her mass of fuel and
the burner after the experiment; it
had a mass of 192.55g.
How can she calculate a ΔH for the
fuel (in kJ/mole)?
Step 3: Moles of fuel burned
Mr (propanol)
= 60 g/mole
n = m/M = 0.90/60
= 0.015 moles
Step 4: Heat change (kJ/mole)
= 12.6/0.015
= 840 kJ/mole
YOUR TURN…
These are the techniques that you can use to
work out the heat changes (in kJ/mole) for
chemical reactions. It is a classic Higher Tier
CHEM3 skill.
Have a go at a few examples yourself now.
Prove these answers
are correct!
Q1:
2565 kJ/mole
Q2:
2800 kJ/mole
Q3:
458.2 kJ/mole