14.51 The gas-phase reaction
Cl(g) + HBr(g) ----> HCl(g) + Br(g)
has an overall enthalpy change of -66 kJ. The activation energy for the reaction is 7 kJ.
First, we look at the overall DH for this reaction (-66 kJ); it is negative, which means that the products are lower in energy than the reactants (this is analogous to a reaction being exothermic). The activation energy Ea is the minimum energy required to initiate a chemical reaction. It is the energy required to "excite" the reactants so they reach the top of the energy barrier which separates products from reactants. We sketch the energy profile as follows:
you can see that the Cl and HBr must gain 7 kJ of energy to get to the top of the 'hill' and fall apart into products. this energy comes from collisions.
(b) looking at the energy diagram in part (a), we see that if we run the reaction in reverse (from HCl(g) + Br(g) and over the hill to Cl(g) + HBr(g)) we must provide