Signs for enthalpy

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Miriam Velez 1I
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Signs for enthalpy

Postby Miriam Velez 1I » Fri Jan 15, 2016 6:24 pm

Can someone please explain why enthalpy is negative when a bond is formed? And positive when a bond is broken?

Alex Nguyen 3I
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Joined: Fri Sep 25, 2015 3:00 am

Re: Signs for enthalpy

Postby Alex Nguyen 3I » Fri Jan 15, 2016 7:37 pm

Enthalpy is positive when a bond is broken because an energy input is required to break the bond (i.e. heat energy is added to the system from the surrounding-endothermic). The same logic but opposite process is the reason enthalpy is negative when a bond is formed (exothermic).

Miriam Velez 1I
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Signs for enthalpy

Postby Miriam Velez 1I » Fri Jan 15, 2016 10:34 pm

Thanks!

Cesar Martinez 2A
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Joined: Fri Sep 25, 2015 3:00 am

Re: Signs for enthalpy

Postby Cesar Martinez 2A » Sat Jan 16, 2016 1:18 pm

Alex Nguyen 3I wrote:Enthalpy is positive when a bond is broken because an energy input is required to break the bond (i.e. heat energy is added to the system from the surrounding-endothermic). The same logic but opposite process is the reason enthalpy is negative when a bond is formed (exothermic).


Just going to expand on this a bit. Think of molecular orbital theory from last quarter. When two atoms bond, they become more stable and their overall energy decreases. The excess energy is released in the form of heat. Creating a bond creates stability and releases energy, which is why the enthalpy is negative.


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