1. When different fuels are burned, how much heat is released?
Heats of combustion

2. Reminder: Exothermic reactions

3. Data is called ‘heat of formation’ ∆ H You can look up the data
Energy required to break bonds (bump = activation energy) Energy released when bonds re-form (down slope) As a whole, if energy is released = Exothermic
Observe the math: activation energies for all moles of reactants + energy in bond-making for all moles of products
(-339) = -88
88 kJ is released in this reaction
Data is called ‘heat of formation’
∆ H
You can look up the data
Do the math
Predict whether a reaction will be
Exo- or endothermic

4. Burning the candle = exothermic
As candle burns, it loses mass
Overall reaction (check that it’s balanced):
C25 H O2 → 25 CO H2O + energy
Purpose of experiment: determine the heat of combustion of paraffin wax (candle)
Heat released by candle = Heat absorbed by water
Important assumption but is it true?

5. Bonds breaking and re-forming
Energy required to break up the molecules of wax C25 H52 and the oxygen gas O2 (reactants)
activation energy : from match at first, then energy released from burning activates further bond-breaking, and so on
Energy is released when products of carbon dioxide CO2 and water vapor H2O are formed
Overall reaction is exothermic
Class results were:
Per 4: 36 kJ/g of wax burned
Per 7: 24 kJ /g of wax burned

6. Is the true heat of combustion of the wax higher or lower than we calculated?
We calculated the amount of heat absorbed by water
We did not calculate the heat
released by the burning wax
Is it physically possible that the water absorbed MORE than the candle released?
If not, water should have absorbed LESS than released
Process is not 100% efficient
So the TRUE heat of combustion should be MORE than we calculated, MORE than the kJ/g

7. Looking at the data in different units
Scientists mess around with numbers and sometimes present data in different units because it might be more useful..
Example: is it more useful to think about a car’s energy efficiency as miles/gallon or miles/tankful or hours driving/gallon and so on?
Comparing the heat of combustion
in kJ/gram (how much heat per unit MASS burned)
In kJ/mole (how much heat per Avogadro’s # of molecules burned)
Which units are useful to us?
Why are the two numbers different?
Why does the ‘per mass’ data DECREASE with more carbons?
Why does the ‘per mole ‘ data INCREASE with more carbons?

8. Gram vs Mole of methane vs. octane
Methane is a small molecule, 1 Carbon
In each gram there are a certain number of molecules
Octane is a bigger molecule, 8 carbons
In each gram there are LESS molecules because each molecule has a bigger mass
In other words,
octane’s molar mass > methane’s molar mass

9. Methane vs octane So ‘per mass’ data is less for octane BUT….
It’s not ‘fair’? Because it compares different number of molecules in a gram?
Is ‘per mole’ ‘fair’? Because it compares the same number of molecules burned?
‘Per mole’ data shows that octane is higher because each octane molecule has a lot of bonds involved.
BUT which is the ‘better, fair’ way to represent the data?

10. The truth……..‘it depends’
It’s still the same basic information
It depends on how you want to use the data to communicate your specific message
If you have to carry your fuel….
Might want fuel is very high in terms of ‘per mass’
Etc.

11. Next step Using molar heats of combustion to figure out Other skills
How much heat is released if I burn MORE than 1 mole of hydrocarbon?
Other skills
Turning words into formulas and chemical equations
Balancing chemical equations
Dimensional analysis and molar heats to solve problems