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AP Chem



Unit 6

6.2 Energy Diagrams of Reactions

3 min readaugust 22, 2020

Anika P


Dalia Savy


Dylan Black


Enthalpy, or heat energy, is represented by ΔH (Δ is the delta sign, which means change). If there is a negative change in energy, or -ΔH, an exothermic reaction is taking place and energy is released🔥 from the system to the surroundings. If there is a positive change in energy, or +ΔH, an endothermic reaction is taking place and energy is absorbed into the system from the surroundings.

Phase Changes

The basics of exothermic and endothermic reactions is understanding phase changes and which process is taking place.


For example, is H2O(s) →H2O(l) an endothermic process or an exothermic process?
Ice🧊 is melting into liquid water in this reaction but you should be thinking, "Does this involve absorbing energy or releasing energy? Is heat needed to melt the ice?"
Yes! In order to melt ice, heat is needed. Think about it, if you hold an ice cube in your hand, it'll melt because of your body warmth. Therefore, heat is going into the system, heat is being absorbed by the water, and melting is an endothermic process.


What about H2O(g) → H2O(l)?
This is how I would think about it: condensation is the opposite of boiling, right? Boiling requires energy/heat to happen, that's why it takes forever for water to boil🍜. Well, if boiling requires energy to happen and condensation is the opposite, condensation must be an exothermic process. Therefore, that reaction releases energy (from the system out to the surroundings).


Phase ChangeReactionProcess
MeltingSolid → Liquid Endothermic
Vaporization / BoilingLiquid → GasEndothermic
SublimationSolid → GasEndothermic
CondensationGas → LiquidExothermic
FreezingLiquid → SolidExothermic
DepositionGas → SolidExothermic

Energy Diagrams

Physical or chemical processes can be described through energy diagrams. As mentioned before, reactions can be categorized as endothermic or exothermic processes. The energy diagrams below show what should be known for the test.

Image Courtesy of Pinterest

Before looking at the specifics of each, you should be aware of a few terms:
  • PEreactants - The potential energy of the reactants
  • PEproducts - The potential energy of the products
  • Activation Energy - Energy necessary for the reaction to occur
  • Activated Complex - The maximum point of energy, where the reactants turn into the products and the reaction occurs.

Exothermic Reactions

In the graph for an exothermic reaction, you can see that the products have a lower potential energy, implying that energy has been released from the system. This further proves that ΔH is negative in an exothermic reaction.

Image Courtesy of SilaVula

Endothermic Reactions

In the graph for an endothermic reaction, you can see that the products have a higher potential energy, implying that energy has been put into the system. This further proves that ΔH is positive for an endothermic reaction.

Image Courtesy of SilaVula


Now that you've seen the graph and been introduced to a few terms, let's put the two together and do a practice problem.

Image Courtesy of Socratic

(a) What is the potential energy of the reactants?
Just look at the y-axis for the reactants: 40 kJ!
(b) What is the potential energy of the products?
Similar to part a, just look at the y-axis for the products: 20 kJ.
(c) What is the value of ΔH?
ΔH is the difference between the potential energy of the products and the potential energy of the reactants.
Simply do: (PEproducts) - (PEreactants)
(20 kJ) - (40 kJ) = -20 kJ
In other words, 20 kJ of energy is released during this reaction.
(d) What is the activation energy?
To find the activation energy, you should be looking for two numbers: the potential energy of the reactants and the energy of the activated complex (the maximum point).
(energy of activation complex) - (PEreactants)
(100 kJ) - (40 kJ) = 60 kJ
In other words, it takes 60 kJ of energy to complete the reaction.
(e) Is this an endothermic or exothermic reaction?
Since ΔH is negative and the potential energy of the products is lower than that of the reactants, this is an exothermic reaction.

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