In A-Level Chemistry, why do longer chain-alkanes have a more negative enthalpy of combustion?

Question

(release more energy?)

Answer 1

Heats of Combustion are quoted in kJ/mole. However, the amount of Carbon and Hydrogen that you have in bigger molecules is larger for 1 mole.

Heat is released because Carbon - Oxygen and Hydrogen - Oxygen bonds are more stable than Carbon - Carbon or Carbon - Hydrogen bonds. When you burn an alkane, you break the Carbon - Carbon and Carbon - Hydrogen bonds and form Carbon - Oxygen and Hydrogen - Oxygen bonds. A way of thinking of this is that because these Carbon - Oxygen and Hydrogen - Oxygen bonds are more stable, less energy is required to hold the pieces together. The energy you no longer need is released as heat.

So why release more when you have longer chains? 1 mole is a set number of molecules, but the longer the carbon chain is, the more Carbon and Hydrogen you have.

As a result, more Carbon - Carbon and Carbon - Hydrogen bonds are being converted to Carbon - Oxygen and Hydrogen - Oxygen bonds. Result, you have to get rid of more heat.

So the Heat of Combustion becomes more negative, that is to say you release more heat to the surroundings.

Any help?

Answer 2

When complete combustion of any alkane occurs, the hydrogen and carbon atoms present in the chains are oxidised to water and carbon dioxide respectively. Every carbon oxidation process (combustion) releases the same amount of energy, similarly with the hydrogen oxidation.

It follows that the more carbon and hydrogen molecules there are present in the molecule, the more energy is released during complete combustion/oxidation of that molecule.

For 1 mole of any substance the number of molecules is the same (Avogadros law.) So, the longer the alkane chain is, the more energy will be released per mole.

Because the energy is released, the convention is that the delta H is given a negative sign.

Hopefully this helps

Answer 3

Contradictory to what most people have already stated.
BREAKING BONDS REQUIRES ENERGY- IT DOES NOT RELEASE ENERGY!!!! This is why there is a hill in the energy/reaction progress graph

The longer the chain the more carbon atoms present per mole. C-C bonds are not particularly strong, whereas C=O bonds are. It is at this part of the reaction where energy is released A Hexane will make 6 times more C=O bonds per mole than methane, Dodecane 12 times etc.

Answer 4

negative enthalpy means greater energy released. So, you have more carbon-carbon bonds, which store alot of energy, and they're getting broken. Releasing that.
Enthalpy is just energy not used for expansion. And since it's a measure of energy released, it's negative.

Answer 5

Simple because more bonds are broked when combustion occures... With a proportional release of energies.

Answer 6

Hit them with a hammer boy. There aint gonna be, a say boy, there aint gonna be no more fancy chicken noises after that.

Lol, and this brain passes a lot of College-classes top of class.