Subject: Relationship between Activation Energy and Bond Energies

Is there any direct relationship between activation energy and the sum of the 
total energies required to break bonds in the reactants? If bond energies 
required to break bonds is high, can we say that the activation energy is 
high? Numerically, are they equal?

I did some simple calculation on my own and realise that Ea is in fact, NOT 
numerically equal to the sum of energy required to break bonds in the 
reactants. In fact, Ea < sum of bond energies needed to break bonds in 
reactants. 

My calculation using: CH3CH2Br + OH- ---> CH3CH2OH + Br- 

Ea for reaction = 89.5 kJ per mole or CH3CH2Br (given)
Sum of bond energy needed to break the C - Br bond in CH3CH2Br = 280 kJ/mol 
(Data booklet)

Why is this so? Since Ea is the minimum energy barrier that reactants must 
overcome before any reaction happens and I suppose it must be at least GREATER 
than the sum of bond energies required to break bonds in the reactants. I 
don't understand how if Ea is less than bond energy needed to break bonds 
(meaning not enough energy to break bonds in reactants), how can reaction 
still occur?

Please help!!! Thank you so much!

Re: Relationship between Activation Energy and Bond Energies