Activation energy lower for exothermic reactions-why?

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legallyblind
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Can someone offer an explanation at the molecular level as to why exothermic reactions have lower activation energies than endothermic reactions? Thank you!
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charco
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(Original post by legallyblind)
Can someone offer an explanation at the molecular level as to why exothermic reactions have lower activation energies? Thank you!
1. Lower than what?
2. Who says that they do?
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legallyblind
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(Original post by charco)
1. Lower than what?
2. Who says that they do?
1. I edited it. Lower than endothermic reactions'.
2. they do- i just don't know how to explain why
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username1973239
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(Original post by legallyblind)
1. I edited it. Lower than endothermic reactions'.
2. they do- i just don't know how to explain why
Exothermic reaction means more bonds are made than broken. This means less energy is needed to break bonds (making bonds don't require much energy to happen)
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charco
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(Original post by TSlayerr)
Exothermic reaction means more bonds are made than broken. This means less energy is needed to break bonds (making bonds don't require much energy to happen)
This is incorrect. You cannot change bond energy.
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charco
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(Original post by legallyblind)
1. I edited it. Lower than endothermic reactions'.
2. they do- i just don't know how to explain why
I think that you are confusing two issues.

You are probably thinking about reversible processes in which case the exothermic activation backward (say) must be lower than the endothermic activation forwards.

To understand why just look at the energy profile.

Image

The forward activation energy is shown. The reverse activation energy would start from the energy level of the products and go to the top of the activated complex.
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username1973239
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(Original post by charco)
This is incorrect. You cannot change bond energy.
No less energy is needed to break bonds as there's less bonds to be broken.
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charco
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(Original post by TSlayerr)
No less energy is needed to break bonds as there's less bonds to be broken.
This is not necessarily correct.

What you are trying to say is that more energy is released forming bonds than is used breaking bonds. This is just a description of an exothermic reaction and has nothing whatsoever to do with activation energy.
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username1973239
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(Original post by charco)
This is not necessarily correct.

What you are trying to say is that more energy is released forming bonds than is used breaking bonds. This is just a description of an exothermic reaction and has nothing whatsoever to do with activation energy.
In a reversible reaction, less bonds are broken in the exothermic reaction than in the endothermic reaction.
This is why more energy is needed to be put into the system (higher activation energy) for the endothermic reaction to occur (because more bonds are broken). You need to put energy into the system to break bonds. Energy is released if you make bonds.
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charco
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(Original post by TSlayerr)
In a reversible reaction, less bonds are broken in the exothermic reaction than in the endothermic reaction.
This is why more energy is needed to be put into the system (higher activation energy) for the endothermic reaction to occur (because more bonds are broken). You need to put energy into the system to break bonds. Energy is released if you make bonds.
As I am trying to explain, it is not fewer bonds, it is bonds with lower bond energy.

AND this assumes that the particles are completely broken apart during reaction, which is usually not the case.
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