Each type of bond (for example O-O or C-H) has a particular bond energy... we will be given these in the exam so don't worry about learning them. These energies can be used to calculate enthalpy change, for example...
Using bond energies, calculate the enthalpy change for the following reaction
H2 + Cl2 ---> 2HCl
Bond energies...
H-H: +436 kJ/mol
Cl-Cl: +242 kJ/mol
H-Cl: +431kJ/mol
1. Work out what bonds are broken & the energy made...
1 mole of H-H is broken and 1 mole of Cl-Cl is broken. Therefore, +436 + +242 = +678 kJ/mol is required to break the bonds in this reaction
2. Work out what bonds are being made & the energy released...
Forming 2 moles of H-Cl bonds. This released 2 x +431 = 862kJ/mol
3. Use the formula 'ΔH = total energy absorbed to break bonds - total energy released in making bonds' to find out the enthalpy change
ΔH = 678 - 862 = -184 kJ/mol
4. ΔH is negative which means the reaction must be exothermic
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