Energy Diagrams
- The energy changes that accompany physical and chemical processes can be described with an energy diagram
- The difference between the energy of the products and those of the reactants gives the overall energy change of the reaction
- For an exothermic process, the reactants are at a higher energy than the products
- For an endothermic process, the reactants are at a higher energy than the products
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
Endothermic Reactions
Energy profile for the thermal decomposition of mercury (II) oxide
Exothermic reactions
- For exothermic reactions, a considerable amount of 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
Exothermic Reactions
Energy profile for the combustion of hydrogen gas to give water
Worked example
Consider the process illustrated by the figure below. Is the process endothermic or exothermic?
Answer:
The process is endothermic because heat is added to the system to raise the temperature of water
