Q.State first law of thermodynamics.

Internal energy $U$ is the total energy stored in a system's constituent atoms, ions and molecules -- the sum of translational, vibrational, rotational, bond, electronic and intermolecular-interaction energy. Only its CHANGE, $\Delta U$, is ever measured or used; $U$ is what physically distinguishes structurally different forms of the same substance (graphite vs diamond, both pure carbon). Characteristics: it is extensive (scales with amount); it is a state function (depends only on $T,P,V,n$, never on path); $\Delta U_{cyclic}=0$; and $\Delta U=U_f-U_i$ comes out negative if the final state has lower energy, positive if higher. …