[aficesk8ingal]

 ???Hey every1!  I was wondering if anyone could please help me with reaction mechanisms and understanding bond dissociations.  For the enthalpy or delta H changes, I have written in my notes that it is bonds formed-bonds broken, but i always thought it was bonds broken-bonds formed.  Also, we are doing reactions where methane is reacted with bromine, iodine, flourine, and clorine.  I found the different delta H's for the initiation and propagation steps for each of these.  I was wondering why it would be that flourine tends to explode, chlorine reacts in sunlight, bromine requires heating up to 200 degrees, and iodine does not react.  Would it have some to do with polarity and how big the molecule is with how tightly the electrons are being held to determine if its harder or easier to break a bond?  How does delta H relate to this?  Any help would be amazing!!!!  Thank you sooo much!  

[Hello12]

Bond dissociation energy is the energy needed to break a particular covalent bond. For the enthalpy change your right it is bonds broken-bonds formed. Flourine bonds are easily broken so will react violently, the chlorine bonds are broken by the UV light to produce free radicals. I thought the propagation steps are more to do with organic chemistry than foundation chemistry. Thats all i know on that hope it helps