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💏intro to chemistry review

$ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$

Written by the Fiveable Content Team • Last updated September 2025
Written by the Fiveable Content Team • Last updated September 2025

Definition

$ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ represents the standard enthalpy change, or the amount of heat energy released or absorbed during a chemical reaction under standard conditions of temperature and pressure. It is a fundamental concept in thermochemistry that quantifies the energetic changes associated with chemical transformations.

5 Must Know Facts For Your Next Test

  1. $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ is a state function, meaning its value depends only on the initial and final states of the system and not on the pathway taken.
  2. The sign of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ indicates whether the reaction is exothermic (negative) or endothermic (positive).
  3. $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ can be used to predict the spontaneity of a reaction, as reactions with negative $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ values are typically spontaneous.
  4. The magnitude of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ reflects the energy released or absorbed per mole of reactants, which is important for understanding the energetics of chemical processes.
  5. $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ values can be determined experimentally or calculated using standard enthalpies of formation and Hess's law.

Review Questions

  • Explain the significance of the standard conditions in the context of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$.
    • The standard conditions of temperature (25°C) and pressure (1 atm) are important in the context of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ because they provide a consistent reference point for measuring and comparing the enthalpy changes of chemical reactions. By using these standard conditions, the values of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ become independent of the specific experimental setup and can be used to make reliable predictions and comparisons about the energetics of different reactions.
  • Describe how the sign of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ can be used to determine the spontaneity of a reaction.
    • The sign of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ provides information about the spontaneity of a chemical reaction. Reactions with a negative $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ value are exothermic, meaning they release energy to the surroundings, and are typically spontaneous. Conversely, reactions with a positive $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ value are endothermic, meaning they absorb energy from the surroundings, and are typically non-spontaneous. This relationship between $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ and spontaneity is a crucial concept in understanding the thermodynamics of chemical processes.
  • Analyze how the magnitude of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ can be used to compare the energetics of different chemical reactions.
    • The magnitude of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ reflects the amount of energy released or absorbed per mole of reactants during a chemical reaction. By comparing the $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ values of different reactions, you can determine which reactions are more or less energetically favorable. Reactions with larger magnitudes of $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ (either positive or negative) release or absorb more energy per mole of reactants, making them more or less energetically significant compared to reactions with smaller $ ext{Δ H}_{ ext{rxn}}^{ ext{°}}$ values. This information is crucial for understanding the energy requirements and potential applications of various chemical processes.