First Law of Thermodynamics

Statement

For a closed system,

$$dU=\delta Q-\delta W.$$

Here $U$ is internal energy, $\delta Q$ is heat added to the system, and $\delta W$ is work done by the system. For simple compressible matter,

$$\delta W=p\,dV,$$

so a reversible quasistatic process satisfies

$$dU=T\,dS-p\,dV.$$

Meaning

The first law does not say which processes occur spontaneously. It only conserves energy. Directionality enters through entropy and the second law.