What is the amount of energy needed to reach the activated complex?

Answer 1

The activated complex lies at the top of the energy curve. It is the highest energy complex in the overall reaction scheme, synonymous to the transition state.

If you look at the reaction:

A + B --> C + D

The activated complex is right in the middle of the arrow. For A + B to become C + D, we need some energy. Because you're forcing two atoms close together, bonds are being formed and broken. The activated complex is where those bonds are all half broken and formed.

To get a better feel for this, do a search for reaction coordinates. You'll see an energy curve for the reaction which will show a large bump in energy. At the very top of that bump is the activated complex.

As a side note, it can come as a bit of a shock when people start talking about activated complexes\transition states. Because often in chemistry you'll spend years just sort of assuming these things are clean and instant...which actually goes against our common sense. Just trust your instincts here, everything takes time...even if it is only a tiny fraction of a second.

Find a reaction and try to picture what the activated complex would be, you would likely be surprised how correct you are for many of them. If you are doing a chemistry degree, you should have a course or two specifically about these.

For further searching try:

activated complex, transition state, transition stage, reaction intermediate, reaction coordinate.

Answer 2

The amount of energy needed to reach the activated complex is known as the activation energy. It is the energy required to initiate a chemical reaction by breaking the bonds in the reactant molecules. The activation energy helps molecules overcome the energy barrier to form the activated complex.

Answer 3

Activation energy,

sometimes symbolised by Ea (units J/mol.K) (comes in with the 'Arrhenius equation').