$ ext{Δ H}_f^{ ext{°}}$

Definition

$ ext{Δ H}_f^{ ext{°}}$ is the standard enthalpy of formation, which represents the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states at a specified temperature, usually 25°C and 1 atm pressure.

5 Must Know Facts For Your Next Test

1. $ ext{Δ H}_f^{ ext{°}}$ is used to calculate the enthalpy change for any chemical reaction involving the formation of a compound from its constituent elements.
2. The standard enthalpy of formation for an element in its standard state is always zero, as there is no change in enthalpy when an element is formed from itself.
3. Negative values of $ ext{Δ H}_f^{ ext{°}}$ indicate an exothermic reaction, where energy is released, while positive values indicate an endothermic reaction, where energy is absorbed.
4. Tabulated values of $ ext{Δ H}_f^{ ext{°}}$ are commonly used in the application of Hess's law to determine the enthalpy change for a reaction.
5. The standard enthalpy of formation is a key parameter in the calculation of the standard enthalpy change for a reaction, which is essential for understanding the energetics of chemical processes.

Review Questions

• Explain the significance of $ ext{Δ H}_f^{ ext{°}}$ in the context of chemical reactions.
  • The standard enthalpy of formation, $ ext{Δ H}_f^{ ext{°}}$, is a crucial parameter in understanding the energetics of chemical reactions. It represents the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states. This value can be used to calculate the enthalpy change for any reaction involving the formation of the compound, which is essential for predicting the spontaneity and feasibility of chemical processes. $ ext{Δ H}_f^{ ext{°}}$ provides a standardized reference point for comparing the relative stabilities and reactivities of different compounds, making it a fundamental concept in the study of thermochemistry.
• Describe how the sign of $ ext{Δ H}_f^{ ext{°}}$ can be used to determine the nature of a chemical reaction.
  • The sign of the standard enthalpy of formation, $ ext{Δ H}_f^{ ext{°}}$, can be used to infer the nature of a chemical reaction. If $ ext{Δ H}_f^{ ext{°}}$ is negative, the reaction is exothermic, meaning that energy is released during the formation of the compound from its constituent elements. Conversely, a positive $ ext{Δ H}_f^{ ext{°}}$ value indicates an endothermic reaction, where energy is absorbed in the formation of the compound. This information is crucial for predicting the spontaneity of a reaction, as exothermic reactions are typically more favorable and occur naturally, while endothermic reactions require an input of energy to proceed. Understanding the sign of $ ext{Δ H}_f^{ ext{°}}$ allows chemists to anticipate the energy changes and thermodynamic behavior of chemical systems.
• Explain how tabulated values of $ ext{Δ H}_f^{ ext{°}}$ can be used in conjunction with Hess's law to determine the enthalpy change for a chemical reaction.
  • Tabulated values of the standard enthalpy of formation, $ ext{Δ H}_f^{ ext{°}}$, are commonly used in the application of Hess's law to calculate the enthalpy change for a chemical reaction. Hess's law states that the enthalpy change for a reaction is independent of the path taken, and is only dependent on the initial and final states of the system. By using the $ ext{Δ H}_f^{ ext{°}}$ values of the reactants and products, chemists can construct a thermochemical cycle that connects the initial and final states of the reaction. This allows for the calculation of the overall enthalpy change, even if the reaction cannot be carried out directly. The ability to determine the enthalpy change using tabulated $ ext{Δ H}_f^{ ext{°}}$ values and Hess's law is a powerful tool for understanding the energetics of chemical processes and predicting their feasibility.

Related terms

Enthalpy:

Enthalpy is a measure of the total energy of a system, including the energy required to establish the system's structure (a form of potential energy) and the energy required to make it flow or move (a form of kinetic energy).

Standard State: The standard state of an element or compound is the state in which it is most commonly found under standard temperature and pressure conditions.

Hess's Law:

Hess's law states that the change in enthalpy for a reaction is independent of the path taken, and is only dependent on the initial and final states of the system.