why are the energies below different ? both are just breaking the same type of bond: ONCl --> NO + Cl >H= 158 kj mol-1 NCl3 --> NCl2 + Cl >H= 375 kj mol-1

https://books.google.com/books?id=hsuV9JTGaP8C&pg=PA715&lpg=PA715&dq=ONCl+--%3E+NO+%2B+Cl+enthalpy+of+reaction+158+kJ/mol&source=bl&ots=789SCom4tm&sig=G1xGk_UMO-jE5_eVHf7J-HRjkHk&hl=en&sa=X&ved=0ahUKEwibxavBlZjZAhWFLmMKHWzuBCoQ6AEIKzAA#v=onepage&q&f=false

1 Answer
Truong-Son N.
Feb 9, 2018

Well, look at the Lewis structures.

The #"N"-"Cl"# bond in the second reaction is harder to break, and it makes sense... the #"ONCl"# has an electronegative #"O"# atom that pulls electron density away from the #"N"-"Cl"# bond, polarizing it and weakening it. Thus, it has a much lower #DeltaH_"bond"#.

That is seen as an #"N"-"Cl"# bond length of #"196 pm"# in #"ONCl"#, and an #"N"-"Cl"# bond length of #"176 pm"# in #"NCl"_3#.

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