• Intermediate: Temporary molecular species that lasts for a short time during a reaction.
• Transition state: A quick moment during a reaction when atoms are half-way between the reactants and products.
• Reaction mechanism: A series of steps that describe how molecules change during a reaction.

Multistep reactions are like a game of snooker.

• In snooker, two balls often collide, but it's rare for three balls to hit each other at the same time.
• Similarly, reactions where two molecules collide (bimolecular) are common, but those where three molecules collide at once (termolecular) are SUPER RARE!

Imagine you're at an airport

• If there's a long queue at passport control, it delays your exit, even if your luggage is ready.
• If there's no queue but your luggage hasn't arrived, you're still stuck waiting!

Similarly, in chemical reactions

• The slowest step (like waiting for luggage or passport control) determines the overall speed of the reaction.
• This slowest step has the highest activation energy (Ea) and is called the rate-determining step.

For any given reaction, you can sketch an energy profile! Here's your guide

• Less stable species = Higher position on the graph.
• If a reaction releases heat (exothermic, ΔHr < 0), reactants are less stable than products.
• If a reaction absorbs heat (endothermic, ΔHr > 0), reactants are more stable.
• Intermediates are less stable than both reactants and products.
• The rate-determining step has the highest Ea.
• Transition states = Elementary steps.
• Intermediates = Elementary steps - 1.

Don't forget

• Y-axis = Energy
• X-axis = Progress of Reaction