The energy barrier that must be overcome for a reaction to occur — the energy difference between the reactants and the activated complex — is called what?

• A. Catalyst
• B. Activated complex
• C. Activation energy
• D. Half-life

Answer: (C)

Activation energy is the energy barrier a reaction must overcome. It represents the difference in energy between the reactants and the activated complex—the high-energy, unstable arrangement of atoms at the top of the energy profile. This energy is the minimum amount needed for bonds to break and new bonds to form as the reaction proceeds. Temperature or catalysts can lower this barrier, speeding up the reaction by making it easier to reach the transition state. The activated complex itself is a fleeting structure at the peak, not the barrier. A catalyst changes the pathway to require less energy, while half-life just measures how long it takes for half the reactant to disappear and doesn’t describe the energy barrier.