Bond Energy

[Nathalia Garibay 1D]

Hello,

I'm doing the weekly achieve homework (Question 6) and the answer choices include 4xbond energy of C-H and -4xbond energy of C-H. I'm not too sure I know exactly what that means and how we would be able to know if it is equal to delta H. Any help would be appreciated!

[Nathan Chu 2G]

Hello,

The 4x bond energy of C-H is the amount of the energy required to break four C-H bonds. Remember that breaking bonds require energy (endothermic) and that forming bonds release energy (exothermic). So, for the reaction CH4(g) -> C(g) + 4 H(g), we are really just breaking four C-H bonds. Since energy is required in bond breaking, this value should be positive. On the contrary, the reaction C(g) + 4 H(g) -> CH4(g) is really just forming four C-H bonds. Since bond formation is exothermic, the energy value should be negative.

Hope this helps! Cheers.

[Sarah Wang 2G]

The bond energy of C—H is the amount of energy required to break that one single bond between the carbon and the hydrogen. Since CH4 has 4 of these bonds, the bond energy must be multiplied by 4 to find the enthalpy. Since forming bonds is exothermic, the reaction enthalpy would be negative; therefore the enthalpy of C(g) + 4H(g) —> CH4(g) would be -4x bond energy of C—H. Breaking bonds is endothermic, since heat is required for the reaction to occur, so the reaction enthalpy of CH4(g) —> C(g) + 4H(g) would be positive 4x bond energy of C—H.