Chapter 2 presented an overview of energy transfer mechanisms, and Chapter 3 showed how to find numerical values for properties. In this chapter we expand on energy transfer by discussing relationships between heat, work, and thermodynamic energy properties. Conservation of energy, a statement of the relationship between energy transfer and energy as a property, is generally called the First Law of Thermodynamics. Very closely related to energy conservation is the concept of mass conservation. We will explore the implications of these relationships first for closed systems and then for open systems. Application of the First Law of Thermodynamics allows an engineer to analyze a wide range of practical devices.