Difficulty: Introductory | Prerequisites: General chemistry (energy, enthalpy, bond energies)
This section lays the groundwork for understanding why reactions happen and how fast they proceed. Every reaction you study in organic chemistry, from substitution to elimination to addition, will reference these energy concepts. If you can read a reaction coordinate diagram and identify the rate-determining step, you have a transferable skill that applies to nearly every mechanism in the course. You should already be comfortable with the idea that breaking bonds costs energy and forming bonds releases it.
Enthalpy tells you the net energy change of a reaction (exothermic or endothermic). Reaction coordinate diagrams map energy against reaction progress, showing transition states and intermediates. The rate-determining step is the slowest step, with the highest activation energy barrier, and it controls how fast the overall reaction goes.
Enthalpy (ΔH)
The heat energy change in a reaction, calculated as the energy of bonds broken minus the energy of bonds formed. A negative ΔH means the reaction releases heat (exothermic); a positive ΔH means it absorbs heat (endothermic). In simple terms, this tells you whether a reaction gives off energy or soaks it up.
Reaction coordinate diagram
A graph plotting energy (y-axis) against the progress of a reaction (x-axis, often labelled "reaction progress" or "time"). It shows reactants, products, transition states, and intermediates as peaks and valleys along the path. Think of it as a topographic map of the energy landscape a reaction has to traverse.
Transition state (TS)
The highest-energy point along each elementary step of a reaction. It is a fleeting arrangement of atoms that cannot be isolated. Often marked with a double-dagger symbol (‡). In simple terms, this is the energy "hilltop" the reaction must cross to proceed.
Activation energy (Eₐ)
The energy difference between the reactants (or an intermediate) and the transition state. It represents the minimum energy input required for a step to occur. Think of it as the height of the hill the molecules need to climb before the reaction can roll downhill.
Intermediate
A species that sits in an energy valley between two transition states. It is higher in energy than the reactants or products but lower than the transition states flanking it. Unlike a transition state, an intermediate has a finite (though often short) lifetime.
Rate-determining step (RDS)
The slowest elementary step in a multi-step reaction, identified by the largest activation energy barrier. It acts as a bottleneck: the overall reaction cannot proceed faster than this step. Think of it as the narrowest point in a funnel; everything else waits on it.
Exothermic reaction
A reaction where the products are lower in energy than the reactants (ΔH < 0). The reaction coordinate diagram shows the product energy level sitting below the reactant energy level.
Endothermic reaction
A reaction where the products are higher in energy than the reactants (ΔH > 0). The diagram shows the product energy level above the reactant energy level.
For a reaction X + Y → Z, the diagram plots energy on the vertical axis and reaction progress on the horizontal axis
Peaks correspond to transition states; valleys between peaks correspond to intermediates
The number of peaks equals the number of elementary steps
The overall energy difference between reactants and products gives you ΔH
Products lower than reactants → exothermic
Products higher than reactants → endothermic
Look at all the activation energy barriers (the vertical distance from a valley up to the next peak)
The step with the tallest barrier is the RDS
The RDS is the slow step; it controls the overall rate of the reaction
Example from the notes: a two-step diagram with three transition states has three elementary steps; the one with the largest Eₐ is the RDS
Bimolecular (2nd order): R = k[X][Y]
Rate depends on the concentration of both reactants
Two molecules must collide in the rate-determining step
Unimolecular (1st order): R = k[X]
Rate depends on the concentration of only one reactant
Only one molecule is involved in the rate-determining step
Enthalpy change:
ΔH = (energy of bonds broken) − (energy of bonds formed)
Rate laws:
2nd order: R = k[X][Y]
1st order: R = k[X]
Reaction kinetics is how pharmaceutical chemists optimise the speed of drug synthesis, and how materials scientists control polymer curing times. The concept of a rate-determining step applies well beyond chemistry: any multi-step process (manufacturing, logistics, cooking) is limited by its slowest stage.
Students often confuse the transition state with an intermediate. A transition state is at an energy maximum and cannot be isolated; an intermediate sits in an energy minimum between two transition states and has a (brief) real existence.
Students sometimes assume exothermic reactions are always fast. Thermodynamics (ΔH) tells you about the energy difference; kinetics (Eₐ) tells you about the speed. A reaction can be strongly exothermic but have a high activation energy, making it slow.
Students sometimes think the RDS is always the first step. It is whichever step has the highest Eₐ, regardless of where it falls in the sequence.
⚠️ You will likely be given a reaction coordinate diagram and asked to count transition states, identify the RDS, and determine whether the reaction is exothermic or endothermic.
⚠️ Be able to write a rate law given the molecularity (unimolecular vs. bimolecular) of the slow step.
⚠️ Know the difference between kinetic control (which product forms faster) and thermodynamic control (which product is more stable), even if this chapter focuses on the basics.
True or false: An intermediate appears at an energy maximum on a reaction coordinate diagram.
Fill in the blank: The rate-determining step is the step with the ________ activation energy.
True or false: A reaction with ΔH < 0 is endothermic.
Fill in the blank: A bimolecular rate law has the form R = k[][].
True or false: A reaction can be exothermic and still have a high activation energy barrier.
Answers: 1. False (intermediates sit at energy minima). 2. Largest. 3. False (ΔH < 0 is exothermic). 4. [X][Y] (both reactant concentrations). 5. True.
Q: A reaction coordinate diagram shows two transition states and one intermediate. How many elementary steps does this reaction have?
A: Two. Each transition state corresponds to one elementary step, and the intermediate sits between them.
Q: If the second step of a two-step mechanism has a larger Eₐ than the first step, which step is the rate-determining step?
A: The second step, because the RDS is whichever step has the largest activation energy barrier.
Q: Write the rate law for a reaction whose slow step involves one molecule of substrate X breaking apart.
A: R = k[X]. This is a unimolecular, first-order rate law.
Q: A reaction has ΔH = −50 kJ/mol. Is it exothermic or endothermic, and where do the products sit relative to reactants on a reaction coordinate diagram?
A: Exothermic. The products sit lower in energy than the reactants on the diagram.
This material connects directly to substitution and elimination reactions (SN1, SN2, E1, E2) later in the course, where you will use rate laws and reaction coordinate diagrams to distinguish mechanisms. It also connects to acid-base chemistry in this same chapter: the equilibrium position of an acid-base reaction is a thermodynamic question, while the speed at which it reaches equilibrium is a kinetic one.
kinetics, thermodynamics, enthalpy, ΔH, activation energy, Eₐ, transition state, intermediate, rate-determining step, RDS, rate law, first order, second order, unimolecular, bimolecular, exothermic, endothermic, reaction coordinate diagram, energy diagram, organic chemistry chapter 2