An enthalpy diagram is a graph of the energy changes in a chemical reaction. In General Chemistry II, it shows reactant and product enthalpy, the activation energy peak, and whether the reaction is exothermic or endothermic.
An enthalpy diagram is a reaction-energy graph used in General Chemistry II to show how enthalpy changes as reactants turn into products. It plots the energy level of the system on the vertical axis and tracks the reaction path from left to right, so you can see whether the products end up higher or lower than the reactants.
The first thing to read is the relative height of the reactants and products. If the products are lower, the reaction releases heat to the surroundings and has a negative ΔH. If the products are higher, the reaction absorbs heat and has a positive ΔH. That vertical difference is the enthalpy change, not the total energy of the molecules in every possible sense, but the heat change at constant pressure that this course focuses on.
Most enthalpy diagrams also show a hump in the middle. That hump is the activation energy, the minimum energy barrier the reactants must overcome before bonds can rearrange. A reaction can be exothermic and still need a large activation energy, which is why some reactions do not start on their own even when they give off heat overall.
The diagram is useful because it separates two ideas that are easy to mix up: how hard a reaction is to start and whether it releases or absorbs energy overall. A reaction with a high peak may be slow to begin, but once it gets over that peak, the energy drop to products can still be large.
In Gen Chem II, you often use enthalpy diagrams with thermochemistry problems, lab data, and reaction comparisons. You may be asked to label ΔH, identify reactants and products, compare two reactions, or explain why a catalyst changes the path but not the overall enthalpy difference. The diagram is a visual shortcut for the energy story of the reaction.
Enthalpy diagrams show up anywhere General Chemistry II asks you to connect energy, reaction direction, and reaction behavior. They make thermochemistry less abstract because you can see the sign of ΔH instead of only writing it as a number.
They also help you separate the overall energy change from the activation energy. That matters when a problem asks why a reaction needs heat or a spark to get started even though it is exothermic overall. The diagram makes it clear that the barrier to begin a reaction and the energy released after the reaction are not the same thing.
You will also use enthalpy diagrams to compare reactions qualitatively. For example, if two processes both release heat, the one with the bigger drop has a larger magnitude of negative ΔH. If one diagram shows a taller peak, that reaction has a larger activation energy, which often hints at a slower start under the same conditions.
This concept connects directly to later thermodynamics work in the course, especially when you relate enthalpy to spontaneity, heat flow, and reaction pathways. It gives you a visual framework you can reuse instead of memorizing isolated numbers.
Keep studying General Chemistry II Unit 6
Visual cheatsheet
view galleryExothermic Reaction
An exothermic reaction is the situation an enthalpy diagram shows when products sit lower than reactants. The downward difference corresponds to a negative ΔH, meaning heat leaves the system. When you read a diagram, this is the pattern that tells you the reaction releases energy overall.
Endothermic Reaction
An endothermic reaction appears on an enthalpy diagram when the products are higher than the reactants. That upward change means the system absorbs heat from the surroundings, so ΔH is positive. This is the visual pattern to look for when a problem asks whether energy is taken in or given off.
Activation Energy
Activation energy is the height of the barrier the reactants must climb before forming products. On an enthalpy diagram, it is the peak between the reactant side and the product side. A reaction can have a small or large activation energy without changing the overall ΔH.
Potential Energy Diagram
A potential energy diagram looks very similar to an enthalpy diagram, which is why the two get mixed up. In General Chemistry II, both diagrams are used to show energy changes along a reaction coordinate. The wording matters, but the reading skills are nearly the same when you compare peaks and energy differences.
A quiz question may give you an enthalpy diagram and ask you to label the reactants, products, activation energy, and ΔH. Your job is to read the vertical differences, not the shape alone. If the products are lower than the reactants, you identify an exothermic reaction and a negative ΔH. If the products are higher, you identify an endothermic reaction and a positive ΔH.
You may also be asked to compare two reaction profiles. In that case, look for which one has the larger energy barrier and which one has the larger heat change. If a catalyst is mentioned, the diagram usually changes in the size of the peak but not in the starting or ending energy levels. That is the move instructors want you to make: use the diagram to explain heat flow, reaction initiation, and the effect of a catalyst.
These terms are often used almost interchangeably because both show a reaction path with energy on the y-axis. In General Chemistry II, an enthalpy diagram focuses on enthalpy change, while a potential energy diagram is a broader energy-profile label. On homework and tests, the reading strategy is the same, but the vocabulary may differ depending on the chapter or instructor.
An enthalpy diagram shows how the energy of a reaction changes from reactants to products.
The vertical gap between reactants and products is the enthalpy change, ΔH.
Products below reactants mean exothermic reaction, while products above reactants mean endothermic reaction.
The peak on the diagram represents activation energy, the energy barrier the reaction must overcome.
A catalyst can lower the peak on the diagram, but it does not change the overall ΔH.
It is a reaction-energy graph that shows the enthalpy of reactants, the activation energy peak, and the enthalpy of products. You use it to tell whether the reaction is exothermic or endothermic and to read the size of ΔH.
Compare the height of the products to the reactants. If products are lower, the reaction is exothermic and ΔH is negative. If products are higher, the reaction is endothermic and ΔH is positive.
The peak is the activation energy, which is the energy barrier the reactants must get over before the reaction can proceed. A higher peak means more energy is needed to start the reaction, even if the overall reaction still releases heat.
They are very similar in how you read them, since both show an energy profile for a reaction. In class, the exact label may vary, but you still look for reactants, products, activation energy, and the overall energy difference.