Activated Complex Explained for IB Chemistry

This unstable arrangement cannot remain for more than a tiny fraction of a second.

Formation of the Activated Complex (Step-by-Step)

1. Reactant particles collide.
   They must collide with enough kinetic energy and correct orientation.
2. Energy is absorbed.
   This energy goes into stretching and weakening bonds.
3. The activated complex forms.
   Bonds are halfway broken and halfway formed.
4. 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.

Learn why the mole allows chemists to connect atomic-scale particles to measurable quantities used in real experiments.