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

Reminder: Exothermic reactions

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 347 + 243 + 2 (-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

Burning the candle = exothermic As candle burns, it loses mass Overall reaction (check that it’s balanced): C25 H52 + 38 O2 → 25 CO2 + 26 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?

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

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 24-36 kJ/g

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?

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

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?

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.

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