Energy Diagrams (College Board AP® Chemistry): Study Guide
Energy Diagrams
The energy changes that accompany physical and chemical processes can be described with an energy diagram
The overall energy change of the reaction, ΔH, is the energy of the products minus the energy of the reactants
For an exothermic process, the reactants are at a higher energy than the products, so ΔH is negative (ΔH < 0)
For an endothermic process, the reactants are at a lower energy than the products, so ΔH is positive (ΔH > 0)
Exothermic reactions
In an exothermic reaction, heat is released from the system to the surroundings
A good example is the combustion of hydrogen gas to produce water
2H2 (g) + O2 (g) → 2H2O (l) + energy
In representing exothermic reactions, energy is included as a part of the products
Energy diagram for an exothermic reaction
Endothermic reactions
For endothermic reactions, heat is absorbed by the system from the surroundings
An example of this is the thermal decomposition of mercury(II) oxide, HgO:
2HgO (s) + energy → 2Hg (l) + O2 (g)
In the reaction above, the difference between the energy of the products and the energy of the reactants is equal to the heat supplied to the system by the surroundings
Observe that energy is a part of the reactant in the equation
Energy diagram for an endothermic reaction
Examiner Tips and Tricks
On an energy diagram, check which level is higher before you name the process:
Products below reactants means exothermic and ΔH < 0
Products above reactants means endothermic and ΔH > 0
Draw the ΔH arrow from the reactants to the products and label it with its sign
Worked Example
A 2.00 g sample of liquid water at T = 287.5 K absorbs 92.0 J of heat and warms to T = 298.5 K. Is the process endothermic or exothermic?
Answer:
The process is endothermic because heat is added to the system to raise the temperature of water
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