Energy diagrams show how the potential energy of a chemical system changes as reactants become products. They are useful because they connect the shape of a reaction pathway to reaction speed, heat flow, and the effect of catalysts. The highest point on the curve represents an unstable transition state that must be reached before products can form.
By reading the diagram, you can compare activation energies and decide whether a reaction is exothermic or endothermic.
The vertical gap from reactants to the peak is the forward activation energy, Ea forward, and it strongly affects the reaction rate. The vertical gap from products to the peak is the reverse activation energy, Ea reverse. The energy difference between products and reactants is the reaction enthalpy, ΔH, which is negative for exothermic reactions and positive for endothermic reactions.
A catalyst provides a different pathway with a lower activation energy, but it does not change the energies of the reactants or products.
Understanding Chemistry: Energy Diagrams and Transition States
The horizontal axis is called the reaction coordinate. It does not show time, distance, or the number of molecules present. It shows the sequence of bond changes needed for a reaction to occur.
A steep curve does not mean a fast reaction, and a wide curve does not mean a slow one. The important feature for rate is the energy barrier. At the top, old bonds may be partly broken while new bonds are partly formed.
This arrangement exists for an extremely short time. It cannot usually be collected in a container or measured as an ordinary substance.
Particles need more than enough energy to react. They must collide in a suitable orientation. For example, if one end of a molecule needs to meet a particular atom on another molecule, a collision at the wrong angle may fail even when the particles move quickly.
Temperature changes the picture because warmer particles have a wider spread of kinetic energies. A larger fraction can reach the barrier, so successful collisions happen more often.
This is why heating often speeds up reactions. It does not necessarily make the products more stable or change the total energy released.
Some reactions have several steps. Their energy diagram has several peaks, with lower valleys between them. Each peak represents a transition state for one step.
A valley between peaks can represent an intermediate. Unlike a transition state, an intermediate may sometimes survive long enough to be detected. The tallest barrier, measured from the energy level just before it, usually controls the overall reaction rate.
Chemists call this the rate determining step. When reading a multistep diagram, measure each barrier from its own starting valley. Do not simply choose the highest point above the original reactants.
Catalysts matter in living cells, industry, and daily life. Enzymes are biological catalysts that hold reacting molecules in positions that make bond changes easier. A car catalytic converter uses solid surfaces to help harmful exhaust gases react.
A catalyst speeds both the forward and reverse reactions because it lowers the barrier in both directions. It does not force a reaction to make more product at equilibrium. When studying diagrams, label the reactants, products, peaks, and any intermediates before making conclusions.
Then compare vertical energy differences carefully. This prevents a common mistake of confusing reaction speed with the overall energy change.
Key Facts
- Activation energy forward: Ea forward = Etransition state - Ereactants
- Activation energy reverse: Ea reverse = Etransition state - Eproducts
- Reaction enthalpy: ΔH = Eproducts - Ereactants
- Exothermic reaction: ΔH < 0, so products are lower in energy than reactants
- Endothermic reaction: ΔH > 0, so products are higher in energy than reactants
- Catalysts lower Ea by changing the reaction pathway, but ΔH stays the same
Vocabulary
- Potential energy
- The stored energy of the reacting particles due to their positions and chemical bonds.
- Reaction progress
- A measure of how far a reaction has moved from reactants toward products along a reaction pathway.
- Transition state
- The highest-energy, most unstable arrangement of atoms during a reaction.
- Activation energy
- The minimum energy needed for reactants to reach the transition state.
- Reaction enthalpy
- The energy difference between products and reactants, written as ΔH.
Common Mistakes to Avoid
- Confusing activation energy with ΔH, which is wrong because activation energy is measured from a starting level to the peak, while ΔH is measured from reactants to products.
- Thinking the transition state is a stable intermediate, which is wrong because it is the maximum-energy arrangement and usually cannot be isolated.
- Assuming a catalyst changes the product energy, which is wrong because a catalyst lowers the activation energy pathway but leaves ΔH unchanged.
- Reading an exothermic diagram backward, which is wrong because the products must be lower than the reactants for ΔH to be negative.
Practice Questions
- 1 A reaction energy diagram has reactants at 40 kJ/mol, a transition state at 115 kJ/mol, and products at 10 kJ/mol. Calculate Ea forward, Ea reverse, and ΔH.
- 2 For a reaction, Ereactants = 25 kJ/mol, Eproducts = 70 kJ/mol, and Etransition state = 130 kJ/mol. Is the reaction exothermic or endothermic, and what are Ea forward and ΔH?
- 3 A catalyst is added to a reaction and the peak of the energy diagram becomes lower, but the reactant and product energy levels stay the same. Explain what changes and what does not change on the diagram.