How do I calculate bond enthalpy from a chemical reaction?
Example Q: See the reaction: CH4 + Br2 --> CH3Br + HBr, with bond energies as follows (in kJ/mol) :C-H = 412, Br-Br = 193,C-Br = x,H-Br = 366Overall energy = –51. Calculate x (bond energy of C-Br bond)Background'Bond energy' is the energy stored in a singular chemical bond between two atoms. Breaking these bonds requires energy. The 'overall energy' of the equation is the sum of the energy of the bonds being broken minus bonds being formed; this is also known as the 'enthalpy' of an equation. If the enthalpy is positive, the reaction takes energy IN - it's endothermic. If the enthalpy is negative, it gives energy OUT - it's exothermic. Methodidentify bonds being brokenadd up their bond energiesindentify bonds being formedadd up their bond energiesuse formula ∑(bonds broken) - ∑(bonds formed) = ∆HThis is a form of Hess' Law. In the example Q:Bonds broken = 4(412) + 193Bonds formed = 3(412) + x + 366∆H = ∑(bonds broken) - ∑(bonds formed) = -51 = 1841 - (1602 + x)x = - 1602 + 1841 + 51 = 290kJ/mol
