Tangmashi
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Hi guys quick question about exo and endothermic reactions. Are these explanations okay? I was looking over my older notes and was wondering where i got these from:

"exothermic reactions are reactions that require larger amounts of energy to break the bonds in reactants, then to form bonds in the products. Unneeded energy is released as heat and temperature of surroundings increases. As energy has been released the compounds enthalpy has decreased"

"endothermic reactions are reactions that require more energy to form product bonds, than to break the bonds in reactants. Heat energy is taken in from the surroundings to supply the deficit and the temperature decreases. As energy has been taken in the compounds enthalpy has increased"

My textbook seems to explain this the opposite way "reactions are endo/exo as the energy required to break the bonds is more/less than the energy released when new bonds are formed".

Which way is the better explanation?
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lucindatownsend
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(Original post by Tangmashi)
Hi guys quick question about exo and endothermic reactions. Are these explanations okay? I was looking over my older notes and was wondering where i got these from:

"exothermic reactions are reactions that require larger amounts of energy to break the bonds in reactants, then to form bonds in the products. Unneeded energy is released as heat and temperature of surroundings increases. As energy has been released the compounds enthalpy has decreased"

"endothermic reactions are reactions that require more energy to form product bonds, than to break the bonds in reactants. Heat energy is taken in from the surroundings to supply the deficit and the temperature decreases. As energy has been taken in the compounds enthalpy has increased"

My textbook seems to explain this the opposite way "reactions are endo/exo as the energy required to break the bonds is more/less than the energy released when new bonds are formed".

Which way is the better explanation?
An exothermic reaction is where more energy is released when bonds are made and less energy is taken in by bonds breaking. This means the enthalpy of the products is lower than the enthalpy of the reactants so the surroundings are warmer.

An endothermic reaction is the opposite. More energy is taken in to break the bonds and less energy is released when making the bonds. This means the enthalpy of the products is higher than the enthalpy of the reactants so the surrounding feel cold.

enthalpy.jpg
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Tangmashi
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But where does the exothermic energy come from?

If we put in 500kj of energy to break the bonds and 1000kj energy is released, Where did the extra 500 come from and why was it released?

I thought that due to the conservation of energy, any exothermic energy was unused energy from the original input, after we had made new bonds, but that isn't correct?
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charco
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(Original post by Tangmashi)
But where does the exothermic energy come from?

If we put in 500kj of energy to break the bonds and 1000kj energy is released, Where did the extra 500 come from and why was it released?

I thought that due to the conservation of energy, any exothermic energy was unused energy from the original input, after we had made new bonds, but that isn't correct?
You haven't got the concepts...

Read through this interactive on energy (click on the buttons for explanations) ...

... and then this one on Hess's law
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Tangmashi
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I know this thread is a few weeks old but I've been away and only got back. Not sure if I still understanding this wrong or not even after using the interactive you provided. This is my understanding:

Different bond between atoms have different bond energies and require different amounts energy to break and form. The activation energy is the minimum amount of kinetic energy molecules must collide with to overcome the bonds of the atoms and separate them. To reach the activation energy required for reaction heat energy can be taken in from the surroundings and the molecules gain kinetic energy. Once separated the atoms can be recombined and depending on the bond energies of the new products there will be a chemical enthalpy change of +/-. If the bond energies of products is higher than reactants, heat energy will be taken in to become chemical energy and the reaction is endothermic. If the bond energies of products is lower than the reactants, the kinetic energy taken in by the molecules, to reach activation (?), is released as heat and reaction is exothermic. The change in heat energy is equal to the change in chemical energy due to Hess' law.
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charco
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(Original post by Tangmashi)
I know this thread is a few weeks old but I've been away and only got back. Not sure if I still understanding this wrong or not even after using the interactive you provided. This is my understanding:

Different bond between atoms have different bond energies and require different amounts energy to break and form. The activation energy is the minimum amount of kinetic energy molecules must collide with to overcome the bonds of the atoms and separate them. To reach the activation energy required for reaction heat energy can be taken in from the surroundings and the molecules gain kinetic energy. Once separated the atoms can be recombined and depending on the bond energies of the new products there will be a chemical enthalpy change of +/-. If the bond energies of products is higher than reactants, heat energy will be taken in to become chemical energy and the reaction is endothermic. If the bond energies of products is lower than the reactants, the kinetic energy taken in by the molecules, to reach activation (?), is released as heat and reaction is exothermic. The change in heat energy is equal to the change in chemical energy due to Hess' law.
This is completely wrong.

Bond breaking is an endothermic process. (kinetic energy --> chemical potential energy)
Bond making is an exothermic process (chemical potential energy --> kinetic energy)

You are also mixing up two areas of chemistry, kinetic and thermodynamics.

The (thermodynamic) energy changes of reaction are completely independent of the (kinetic) activation energy ...
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Tangmashi
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Okay I clearly need to read more about this. This is the first time i have struggled in school over something but I really want to understand it beyond the gcse definition that exo gives out heat and endo takes in heat. Do you have links of anything else I could read from the very basics of kinetics and thermodynamics?
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