Why is there a minus in the answer? The answer should be -102 kJ/mol, but I get 102 kJ/mol
Using the set of enthalpy bonds calculate the standard reaction enthalpy of methane with chlorine and write the thermochemical equation.
# ΔH_b^°# of #Cl-Cl# #=# 243 kJ/mol
# ΔH_b^°# of #CH_3# =438 kJ/mol
# ΔH_b^°# of #H-Cl# = 432 kJ/mol
# ΔH_b^°# of #CH_3-Cl# = 351 kJ/mol
Using the set of enthalpy bonds calculate the standard reaction enthalpy of methane with chlorine and write the thermochemical equation.
1 Answer
Well, there is a sign convention associated with bond energy....
Explanation:
Bond-breaking is always considered an ENDOTHERMIC process. Bond-making is conceived to be an EXOTHERMIC process...and so the reported bond enthalpies are always quoted as endothermic, i.e. bond-breaking, quantities....
We want....
And thus we want to interrogate the thermochemical process...
In the given equation.... we BREAK
And so we make the summation on that basis...
And so.......
...which gives you the required answer with units of kilojoules per mole of REACTION as written....
If this does not address your question I am willing to try again. Certainly you should be mindful of the given conventions.