## 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.

## Physical Chemistry Lesson of the Day – Basic Terminology in Thermodynamics

A system is the part of the universe of interest, and the surroundings is everything else in the universe.

The internal energy of a system is the sum of the kinetic and potential energies of all of the particles (atoms and molecules) in the system.  This cannot be measured, but changes in internal energy can be measured.

There are 2 ways in which the internal energy of a system can change: heat and work.

• Heat is the transfer of energy between 2 objects due to a temperature difference.  In chemistry, heat is commonly observed when a chemical reaction absorbs or releases energy.
• Work is force acting over a distance.  In chemistry, a common type of work is the expansion or compression of a gas.

In chemistry, it is conventional to take the system’s point of view in deciding the sign of heat and work.  Thus, if heat is entering the system or if work is done on the system, then the sign is positive.  If heat is exiting the system of if work is done by the system, then the sign is negative.