Air is contained in a vertical piston-cylinder assembly by a piston with a mass of 15 kg and a face area of 0.01 m^2. The air has a mass of 5 g, and initially occupies a volume of 5 liters. The atmosphere exerts a pressure of 100 kPa on the top of the piston. Heat transfer of magnitude of 2.5 kJ occurs slowly from the air to the surroundings, and the volume of the air decreases to 2 liters. Neglecting friction between the piston and the cylinder wall, determine the change in specific internal energy of the air, in kJ/kg.