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💏intro to chemistry review

Standard Enthalpy of Reaction

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

Definition

The standard enthalpy of reaction is the change in enthalpy that occurs during a chemical reaction under standard conditions, which are typically defined as a temperature of 25°C (298.15 K) and a pressure of 1 atm (101.325 kPa). It represents the energy released or absorbed when reactants are converted into products and is an important concept in understanding the thermodynamics of chemical processes.

5 Must Know Facts For Your Next Test

  1. The standard enthalpy of reaction is denoted by the symbol $\Delta H^\circ$, where the superscript '°' indicates the standard conditions.
  2. The standard enthalpy of reaction can be used to determine the energy changes associated with a wide range of chemical processes, including combustion, neutralization, and phase changes.
  3. The sign of the standard enthalpy of reaction indicates whether the reaction is exothermic ($\Delta H^\circ < 0$) or endothermic ($\Delta H^\circ > 0$).
  4. The standard enthalpy of reaction is a state function, meaning that it depends only on the initial and final states of the system and not on the pathway taken.
  5. The standard enthalpy of reaction can be calculated using Hess's law, which states that the overall enthalpy change for a reaction is independent of the intermediate steps, as long as the initial and final states are the same.

Review Questions

  • Explain the significance of the standard conditions in the context of the standard enthalpy of reaction.
    • The standard conditions of temperature (25°C or 298.15 K) and pressure (1 atm or 101.325 kPa) are important in the context of the standard enthalpy of reaction because they provide a consistent and well-defined reference point for comparing the energy changes associated with chemical reactions. By using these standard conditions, the standard enthalpy of reaction values can be directly compared and used in thermodynamic calculations, as the effects of temperature and pressure on the energy changes are minimized.
  • Describe how the sign of the standard enthalpy of reaction can be used to classify a chemical process as exothermic or endothermic.
    • The sign of the standard enthalpy of reaction, $\Delta H^\circ$, can be used to determine whether a chemical process is exothermic or endothermic. If $\Delta H^\circ$ is negative, the reaction is exothermic, meaning that it releases energy to the surroundings. Conversely, if $\Delta H^\circ$ is positive, the reaction is endothermic, meaning that it absorbs energy from the surroundings. This information is crucial for understanding the energy changes associated with chemical processes and their potential applications, such as in the design of energy-efficient chemical reactions or the prediction of the spontaneity of a reaction.
  • Explain how Hess's law can be used to determine the standard enthalpy of reaction for a multi-step process, and discuss the significance of this relationship.
    • Hess's law states that the overall enthalpy change for a reaction is independent of the intermediate steps, as long as the initial and final states are the same. This means that the standard enthalpy of reaction for a multi-step process can be calculated by summing the standard enthalpies of reaction for the individual steps, even if those steps occur through different pathways. The significance of this relationship is that it allows chemists to determine the standard enthalpy of reaction for complex processes without having to measure the enthalpy change directly. This is particularly useful when the direct measurement of the enthalpy change is difficult or impractical, as Hess's law provides a reliable method for calculating the overall energy change based on the known enthalpies of the individual steps.