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

DE
Answered by Darcy E. Chemistry tutor

4608 Views

See similar Chemistry GCSE tutors

Related Chemistry GCSE answers

All answers ▸

What are the components of an atom and what are their properties?


How does bonding effect the melting point of a substance?


What are the properties of ionic compounds?


Balance the equation for the reaction of sodium hydroxide with sulphuric acid.


We're here to help

contact us iconContact ustelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo

MyTutor is part of the IXL family of brands:

© 2025 by IXL Learning