Enthalpy

Definition

Enthalpy is a measure of the total energy of a thermodynamic system, including both the internal energy of the system and the work done by or on the system due to changes in pressure and volume. It is a key concept in understanding the energy changes that occur during chemical reactions and phase changes.

5 Must Know Facts For Your Next Test

1. Enthalpy is represented by the symbol $H$ and is defined as the sum of a system's internal energy ($U$) and the work done on the system due to changes in pressure and volume ($pV$).
2. In a chemical reaction, the change in enthalpy ($\Delta H$) is the amount of heat released or absorbed at constant pressure, and it is a measure of the energy change of the reaction.
3. Exothermic reactions have a negative change in enthalpy ($\Delta H < 0$), meaning they release heat to the surroundings, while endothermic reactions have a positive change in enthalpy ($\Delta H > 0$), meaning they absorb heat from the surroundings.
4. Calorimetry is the experimental technique used to measure the change in enthalpy of a chemical reaction or a physical process, such as a phase change.
5. The concept of free energy, which combines enthalpy and entropy, is crucial in determining the spontaneity and feasibility of chemical reactions and physical processes.

Review Questions

• Explain how enthalpy is related to the concept of calorimetry.
  • Calorimetry is the experimental technique used to measure the change in enthalpy ($\Delta H$) of a chemical reaction or a physical process. By measuring the heat absorbed or released during a reaction or a phase change, and using the relationship $\Delta H = q_p$ (where $q_p$ is the heat absorbed or released at constant pressure), the enthalpy change can be determined. Calorimetry provides a direct way to quantify the energy changes associated with a system, which is essential for understanding the thermodynamics of chemical and physical processes.
• Describe the role of enthalpy in the concept of free energy and its importance in determining the spontaneity and feasibility of processes.
  • The concept of free energy, represented by the symbol $G$, combines the effects of enthalpy and entropy. Free energy is defined as $G = H - TS$, where $H$ is the enthalpy, $T$ is the absolute temperature, and $S$ is the entropy of the system. The change in free energy, $\Delta G$, determines the spontaneity and feasibility of a process: if $\Delta G < 0$, the process is spontaneous; if $\Delta G > 0$, the process is non-spontaneous. Enthalpy plays a crucial role in this relationship, as the change in enthalpy, $\Delta H$, reflects the energy released or absorbed during a process. The interplay between $\Delta H$ and $\Delta S$ (the change in entropy) determines the overall $\Delta G$ and, consequently, the spontaneity and feasibility of the process.
• Analyze how the concept of enthalpy is related to the idea of potential energy, free energy, and chemical equilibrium.
  • Enthalpy, which represents the total energy of a system, is closely linked to the concept of potential energy. In a chemical reaction, the change in enthalpy, $\Delta H$, reflects the difference in potential energy between the reactants and the products. This change in potential energy drives the reaction and determines whether it is exothermic ($\Delta H < 0$) or endothermic ($\Delta H > 0$). The concept of free energy, $G$, which combines enthalpy and entropy, is crucial in determining the spontaneity and feasibility of a chemical reaction. At equilibrium, the change in free energy, $\Delta G$, is zero, meaning that the system has reached a state where the forward and reverse reactions occur at the same rate. The relationship between enthalpy, free energy, and equilibrium is essential in understanding the thermodynamics of chemical processes and predicting their behavior.

Related terms

Internal Energy: The total energy contained within a system, including the kinetic energy of the particles and the potential energy of their interactions.

Heat:

The transfer of thermal energy from a hotter object to a cooler object, driven by a temperature difference.

Work:

The transfer of energy due to the movement of a force, such as the expansion or contraction of a gas.