[neelam]

what is the difference between bond enthalpy & average bond enthalpy? why bond enthalpy of C-H bonds  is different in methane molecule?
2. how do we calculate enthalpy change and internal energy change experimentally?

[genetic1]

what is the difference between bond enthalpy & average bond enthalpy? why bond enthalpy of C-H bonds  is different in methane molecule?
2. how do we calculate enthalpy change and internal energy change experimentally?

Bond Enthalpy refers to a particular bond in a particular environment wheras mean/average is the average of values from different environments.
Ex. CH3CH2CHO has three different environments for C--H bonds which will have slightly different bond enthalpies, one in -CH3 group, one in -CH2- group and one in -CHO group.

And I'm pretty sure you would use a calorimeter to measure enthalpy but i could be wrong its been awhile.