Physical Chemistry Lesson of the Day – The First Law of Thermodynamics

The change in internal energy of a system is defined to be the internal energy of a system in its final state subtracted by the internal energy of the system in its initial state.

\Delta U = U_{final} - U_{initial}.

However, since we cannot measure the internal energy of a system directly at any point in time, how can we calculate the change in internal energy?

The First Law of Thermodynamics states that any change in the internal energy of a system is equal to the heat absorbed the system plus any work done on the system.  Mathematically,

\Delta U = q + w.

Recall that I am using the sign convention in chemistry.

The value of q and w can be positive or negative.

  • A negative q denotes heat released by the system.
  • A negative w denotes work done by the system.

Your thoughtful comments are much appreciated!

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