General Chemistry/Thermodynamics/The First Law of Thermodynamics

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The First Law of Thermodynamics is simply a restatement of the Law of Conservation of Energy. There are a few ways of stating it, but they all mean the same thing:

  • "Energy is neither created nor destroyed. It can only change form."
  • "The change in the system's thermal energy is equal to the heat added to the system minus the work done by the system."

The first definition is self-explanatory. As an example, a propane grill does not "create" energy when it cooks food. The energy stored within propane molecules and oxygen atoms is released in the form of heat and light when the propane molecules reassemble themselves into carbon dioxide and water (the combustion reaction). Energy has not been created or destroyed. It has simply changed from chemical potential energy into light and thermal energy.

The second explanation comes from a physics point of view. It means that adding heat to a system increases its internal energy, and a system that does work (like breaking bonds or assembling large molecules from small ones) decreases its internal energy. Overall, the total amount of heat, work, and energy remains constant. This should make sense because heat is energy and work is energy being applied, so based on the first definition, all forms of energy must be accounted for.