Bond energies

Bond breaking - endothermic Energy is always required to be inputted to break a bond. Bond breaking is always endothermic.

Bond making - exothermic Energy is always released when a bond is formed. Bond making is always exothermic.

Bond energies The energy released when a bond is formed or absorbed when it is broken is called the bond energy. e.g. the C-H bond in methane has a bond energy of 413 KJ/mol

Examples of bond energies Bond energy KJ/mol H-H 436 Cl-Cl 242 H-Cl 431 C-H 413 C-C 347 C-O 335 O=O 498

Energy level diagrams

Exothermic reaction The energy need to break the bonds is less than the energy released when new bonds are made C + 4H + 4O energy Energy needed to break bonds Energy released by forming bonds CH4(g) + 2O2(g) Energy released CO2(g) + 2H2O(l) “reaction path”

Endothermic reaction The energy need to break the bonds is more than the energy released when new bonds are made energy Energy released by forming bonds Energy needed to break bonds NH4NO3(l) Energy absorbed NH4NO3(s) + H2O (l) “reaction path”

ΔH – Energy change in a complete reaction If heat is given out, the reaction has lost energy so ΔH is negative

ΔH – Energy change in a complete reaction If heat is absorbed (reaction gets colder), the reaction has gained energy so ΔH is positive

Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g)

Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x 413 + 2 x 498 = 1662 + 996 = 2658 KJ/mol

Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x 413 + 2 x 498 = 1662 + 996 = 2658 KJ/mol Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x -464 + 2 x -805 = -1856 + -1610 = -3466 KJ/mol

Calculating ΔH CH4(g) + 2O2(g) 2H2O(l) + CO2(g) Bonds broken = 4 x (C-H) + 2 x (O=O) = 4 x 413 + 2 x 498 = 1662 + 996 = 2658 KJ/mol Bonds made = 4 x (O-H) + 2 x (C=O) = 4 x -464 + 2 x -805 = -1856 + -1610 = -3466 KJ/mol Overall Energy change = 2658 + -3466 = -808 KJ/mol (Exothermic)

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Bond energies?

Energy level diagrams?

Using bond energies in calculations?