The internal energy of a substance equals the energy of all the atoms and molecules that it contains. Kinetic energy is due to the actual movement of atoms or molecules.

Molecules may be stationary but they can have Rotational and Vibrational energy.

Temperature is a measure of the mean Kinetic Energy of Molecules

Internal energy increases as the temperature increases.

Enthalpy is Energy Transferred by Heat

If Heat is produced the reaction is called Exothermic.

If Heat is absorbed by the reaction it is called Endothermic.