Skip to main content
Chemistry LibreTexts

6: The First Law

  • Page ID
    • 6.1: Internal Energy
      In order to study the exchange of energy between a system and its surroundings, we need to define the boundaries of the system. We then need to determine how much energy the system possesses. This energy is known as the total energy of the system or the internal energy of the system. In this section, we will describe the basic parameters of the system, distinguish between heat and work, define internal energy, and determine changes in the internal energy.
    • 6.2: Enthalpy
      Enthalpy is defined as the sum of the internal energy and the product of the pressure times the volume: H = U + PV. Many experimental studies of enthalpy occur at constant pressure, under which conditions it can be shown that the change in enthalpy is the change in heat. In this section we will derive the relationship between enthalpy and heat, and then describe constant-pressure calorimetry experiments, introduce molar heat capacity at constant pressure, and then relate the constant-pressure an
    • 6.3: Thermochemistry
      Thermochemistry is an extremely useful application of thermodynamics because it allows us to use previously gathered data to estimate the enthalpy changes of processes that we have not yet done, or that are unknown to us. The two most common applications are the use of Hess's Law to combine the enthalpy changes of entire chemical equations, and the combination of the enthalpy changes of formation of the pure substances involved in a process.

    Thumbnail: A thermite reaction using iron(III) oxide. The sparks flying outwards are globules of molten iron trailing smoke in their wake. Image used with permission (CC SA-BY 3.0; Nikthestunned)