6.2 Energy Diagrams

An energy diagram, also called a potential energy diagram or reaction coordinate diagram, shows how the potential energy of a system changes during a chemical reaction or phase change. If the products sit lower than the reactants, the process is exothermic and $\Delta H$ is negative. For AP Chemistry, read reactant and product energy levels before deciding whether the process absorbs or releases energy.

Chemical Energy Diagrams in AP Chem

A chemical energy diagram shows potential energy on the y-axis and reaction progress on the x-axis. In AP Chem 6.2, you use the graph to represent a physical or chemical transformation and decide whether the process is endothermic or exothermic.

The most important comparison is products versus reactants. Products below reactants means ΔH is negative and the process releases energy. Products above reactants means ΔH is positive and the process absorbs energy. The peak of the curve is the transition state, and the rise from reactants to that peak is the activation energy.

Why This Matters for the AP Chemistry Exam

Energy diagrams turn an abstract idea (energy changes you cannot see) into a picture you can read and explain. On the AP Chemistry exam, you may need to build or interpret one of these diagrams using correct axis labels, scale, and units, and use it to identify whether a process is endothermic or exothermic. This skill connects directly to enthalpy work later in the unit, where you calculate ΔH from bond energies, formation enthalpies, and Hess's law. Reading these diagrams accurately also sets you up for kinetics topics, where the same type of diagram is used to show activation energy and reaction progress.

Key Takeaways

• The y-axis shows potential energy and the x-axis shows the progress of the reaction (the reaction coordinate). Always label both with correct units.
• Products lower than reactants means exothermic (ΔH < 0); products higher than reactants means endothermic (ΔH > 0).
• ΔH = (potential energy of products) - (potential energy of reactants).
• Activation energy is the gap from the reactants up to the highest point (the activated complex or transition state).
• Phase changes can be shown on energy diagrams too: melting, boiling, and sublimation are endothermic, while freezing, condensation, and deposition are exothermic.
• The same diagram works for chemical reactions and physical changes, so the shape tells you the direction of energy transfer.

How to Read and Build an Energy Diagram

An energy diagram is a graph of potential energy versus reaction progress. The vertical axis is energy, and the horizontal axis tracks how far the process has moved from reactants to products.

Key parts to identify:

• PE of reactants: the energy of the starting materials, read off the y-axis on the left.
• PE of products: the energy of the final materials, read off the y-axis on the right.
• Activation energy: the minimum energy needed to start the reaction, measured from the reactants up to the peak.
• Activated complex (transition state): the highest point on the curve, the least stable arrangement before products form.

The shape tells the story. A curve that ends lower than it started is exothermic, and a curve that ends higher than it started is endothermic.

Exothermic Reactions

In an exothermic diagram, the products sit lower than the reactants. Energy is released to the surroundings, usually as heat. The change in enthalpy is negative because the products hold less potential energy than the reactants.

Endothermic Reactions

In an endothermic diagram, the products sit higher than the reactants. Energy is absorbed from the surroundings. The change in enthalpy is positive because the products hold more potential energy than the reactants.

Phase Changes on Energy Diagrams

The same diagram approach works for physical changes. During a phase change, the energy absorbed or released happens without a temperature change while the phase is converting.

• Melting: H2O(s) → H2O(l) is endothermic. The system absorbs energy to go from solid to liquid.
• Condensation: H2O(g) → H2O(l) is exothermic. The system releases energy as gas becomes liquid. Condensation is the reverse of boiling, so since boiling absorbs energy, condensation releases it.

Here is a quick reference for all six phase changes:

How to Use This on the AP Chemistry Exam

Free Response

When a question asks you to draw an energy diagram, label both axes (potential energy on the y-axis, reaction progress on the x-axis) and use a consistent scale. Show the reactant level, the product level, and the peak clearly. If the question gives ΔH, make the product level match the sign of ΔH: lower for negative, higher for positive.

Problem Solving

To read values off a diagram:

• ΔH = (PE of products) - (PE of reactants). A negative answer means exothermic; a positive answer means endothermic.
• Activation energy = (energy of the peak) - (PE of reactants).

Worked example using a diagram where reactants = 40 kJ, the peak = 100 kJ, and products = 20 kJ:

• PE of reactants: 40 kJ
• PE of products: 20 kJ
• ΔH = (20 kJ) - (40 kJ) = -20 kJ, so 20 kJ is released.
• Activation energy = (100 kJ) - (40 kJ) = 60 kJ.
• Because ΔH is negative and the products are lower than the reactants, this is an exothermic reaction.

Common Trap

Do not confuse activation energy with ΔH. Activation energy is the climb from reactants to the peak, while ΔH is the difference between products and reactants. A reaction can have a large activation energy and still be exothermic overall.

Common Misconceptions

• "Higher activation energy means more positive ΔH." Not true. Activation energy is the barrier height, and ΔH is the start-to-finish energy difference. They are independent of each other.
• "Exothermic reactions do not need any energy to start." Most reactions still need activation energy to get going, even when they release energy overall.
• "The peak of the diagram is the products." The peak is the activated complex (transition state), not the products. Products are at the final level on the right.
• "Temperature must change during a phase change." During a phase change of a pure substance, the temperature stays constant even though energy is being absorbed or released.
• "A steeper or taller curve means a more exothermic reaction." The height of the peak shows activation energy, not how exothermic the reaction is. Only the difference between reactant and product levels tells you ΔH.

Related AP Chemistry Guides

• Unit 6 Overview: Thermochemistry
• 6.1 Endothermic and Exothermic Processes
• 6.4 Heat Capacity and Calorimetry
• 6.3 Heat Transfer and Thermal Equilibrium
• 6.6 Introduction to Enthalpy of Reaction
• 6.7 Bond Enthalpies