The activated complex—also known as the transition state—is one of the most important concepts in IB Chemistry Topic 6 (Kinetics). It explains how reactants turn into products and why activation energy exists. Although the activated complex cannot be isolated or observed directly, understanding it helps you interpret energy profiles, explain reaction rates, and analyze the effect of catalysts.
What Is an Activated Complex?
The activated complex is a short-lived, high-energy arrangement of atoms that forms at the top of the activation energy barrier during a reaction.
It is:
- Unstable
- Temporary
- Formed only when reactants have enough energy
- Present for an extremely short time
- The highest-energy point on the reaction pathway
The activated complex appears at the transition state, which is the point of maximum potential energy.
Where the Activated Complex Appears on an Energy Profile
On a reaction coordinate diagram:
- Reactants start at one energy level
- Products end at a different energy level
- The peak between them represents the activated complex
This peak corresponds to the activation energy (Ea).
If the system reaches the peak, it can proceed to products.
If not, the reactants fall back without reacting.
Why the Activated Complex Has High Energy
Because:
- Bonds are partially breaking
- New bonds are partially forming
- Atoms are in a strained, unstable configuration
- Electron distribution is in flux
- Molecular collisions have distorted geometry
This unstable arrangement cannot remain for more than a tiny fraction of a second.
Formation of the Activated Complex (Step-by-Step)
- Reactant particles collide.
They must collide with enough kinetic energy and correct orientation. - Energy is absorbed.
This energy goes into stretching and weakening bonds. - The activated complex forms.
Bonds are halfway broken and halfway formed. - The complex collapses.
It can either:- Form products
- Fall back to reactants
Whether it proceeds depends on the energy of the system and the arrangement of atoms.
Activated Complex vs Transition State
These terms are often used interchangeably, but IB students should know the difference:
- Transition state refers to the point on the energy diagram (a peak).
- Activated complex refers to the actual molecular arrangement that exists at that point.
The concepts refer to the same instant in the reaction.
How Activation Energy Relates to the Activated Complex
Activation energy is the energy required to form the activated complex.
A reaction with:
- High Ea → hard to reach the activated complex → slow reaction
- Low Ea → easier to reach the complex → fast reaction
Temperature and catalysts both influence the ability to form the activated complex.
How Catalysts Affect the Activated Complex
Catalysts work by:
- Providing an alternative pathway
- Lowering the activation energy
- Creating a different activated complex with lower energy
Lowering Ea increases the number of particles that can reach the transition state at a given temperature.
Catalysts do NOT change:
- ΔH
- ΔG
- The total energy of reactants or products
Only the activation energy barrier changes.
Activated Complex in Endothermic vs Exothermic Reactions
Exothermic reactions
Products are lower in energy than reactants.
The activated complex is still the highest point.
Endothermic reactions
Products are higher in energy than reactants.
The activated complex still represents the peak.
Regardless of reaction type, the activated complex always sits at the maximum point of potential energy.
Why the Activated Complex Is Important in IB Chemistry
It explains:
- Why reactions require energy to start
- How catalysts speed up reactions
- Why temperature dramatically affects reaction rate
- The shape of reaction coordinate diagrams
- The role of collision theory
Many Paper 2 questions ask you to describe changes at the activated complex.
Common IB Misunderstandings
- “The activated complex is the same as an intermediate.”
False. Intermediates are stable enough to be isolated; activated complexes are not. - “Activated complexes last for a long time.”
No. They exist for only 10⁻¹³ seconds or less. - “Every collision forms an activated complex.”
Only collisions with enough energy and correct orientation do.
FAQs
Can you isolate an activated complex in the lab?
No. It is too unstable and exists for an extremely short time.
Does every reaction have an activated complex?
Yes. All reactions involving bond breaking/forming go through a transition state.
Why is the activated complex at the highest energy point?
Because bonds are distorted, partly broken, and partly formed, which is energetically unfavorable.
Conclusion
The activated complex is the high-energy, unstable structure formed at the transition state of a reaction. It represents the dividing point between reactants and products and determines the activation energy barrier. Understanding this concept allows IB Chemistry students to explain reaction rates, catalyst behavior, and energy profile diagrams with clarity and confidence.
