Signs for enthalpy

[Miriam Velez 1I]

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

[Alex Nguyen 3I]

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]

Thanks!

[Cesar Martinez 2A]

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.