Combustion of alkanes

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Sophie.cerys
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Why do larger alkanes release more energy than shorter alkanes? As energy is required to break a bond, so as there are more bonds to break in a longer alkane, why does it release more energy?
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Pigster
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(Original post by Sophie.cerys)
Why do larger alkanes release more energy than shorter alkanes? As energy is required to break a bond, so as there are more bonds to break in a longer alkane, why does it release more energy?
Aha, but after you've broken the [more] bonds, you can then form more bonds.

More energy is released breaking bonds than forming them (hence overall it is exothermic).
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charco
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(Original post by Pigster)
Aha, but after you've broken the [more] bonds, you can then form more bonds.

More energy is released breaking bonds than forming them (hence overall it is exothermic).
Having difficulty believing that you mean to to say this!
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CuriosityYay
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(Original post by charco)
Having difficulty believing that you mean to to say this!
He said it before: Humans make mistakes!
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liam.ro
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(Original post by Sophie.cerys)
Why do larger alkanes release more energy than shorter alkanes? As energy is required to break a bond, so as there are more bonds to break in a longer alkane, why does it release more energy?
During the complete combustion of alkanes, the C-C and C-H bonds in the alkane are broken, then O-H and C=O bonds are formed to produce water and carbon dioxide. As O-H and C=O bonds have a higher enthalpy than C-C and C-H bonds, the amount of energy released by forming bonds exceeds the amount of energy required to break bonds, making the reaction exothermic.

Hence, the larger your alkane, more potential energy can be released when the products are formed.
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Pigster
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(Original post by charco)
Having difficulty believing that you mean to to say this!
It was my daughter's fourth birthday party and I was being rushed out the door and didn't have time to check what the heck I seemed have written.

-1 marks for me.
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charco
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(Original post by Pigster)
It was my daughter's fourth birthday party and I was being rushed out the door and didn't have time to check what the heck I seemed have written.

-1 marks for me.
I have a five year old girl - I understand perfectly!
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Sophie.cerys
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(Original post by liam.ro)
During the complete combustion of alkanes, the C-C and C-H bonds in the alkane are broken, then O-H and C=O bonds are formed to produce water and carbon dioxide. As O-H and C=O bonds have a higher enthalpy than C-C and C-H bonds, the amount of energy released by forming bonds exceeds the amount of energy required to break bonds, making the reaction exothermic.

Hence, the larger your alkane, more potential energy can be released when the products are formed.
But wouldn't the net energy released be the same? As in, larger alkanes release more energy as they break more bonds, but wouldn't they have to use more energy in bond breaking as there are more bonds to break?
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liam.ro
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(Original post by Sophie.cerys)
But wouldn't the net energy released be the same? As in, larger alkanes release more energy as they break more bonds, but wouldn't they have to use more energy in bond breaking as there are more bonds to break?
No, it cannot be constant.

Think of it this way: In a reaction where no C-C and C-H bonds are broken, and no O-H and C=O bonds are formed, the net energy released will obviously be zero. Now, when a certain amount of C-C and C-H bonds are broken, and the proportional amount of O-H and C=O bonds are formed, the net energy released will become exothermic by some constant value. If the number of C-C and C-H bonds are doubled, the proportional amount of O-H and C=O formed will be doubled, hence the energy released will also be doubled.

Simply put, since the amount of energy released for a given number of these bonds is constant, it will change linearly depending on the amount you have.
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Sophie.cerys
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(Original post by liam.ro)
No, it cannot be constant.

Think of it this way: In a reaction where no C-C and C-H bonds are broken, and no O-H and C=O bonds are formed, the net energy released will obviously be zero. Now, when a certain amount of C-C and C-H bonds are broken, and the proportional amount of O-H and C=O bonds are formed, the net energy released will become exothermic by some constant value. If the number of C-C and C-H bonds are doubled, the proportional amount of O-H and C=O formed will be doubled, hence the energy released will also be doubled.

Simply put, since the amount of energy released for a given number of these bonds is constant, it will change linearly depending on the amount you have.
Thank you!!
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