Enthalpy change

Definition

Enthalpy change refers to the heat content difference of a system during a chemical reaction at constant pressure. It indicates whether a reaction is exothermic, releasing heat, or endothermic, absorbing heat, and provides insight into the energy landscape of reactions, especially when visualized through potential energy surfaces and reaction coordinates.

5 Must Know Facts For Your Next Test

1. Enthalpy change is typically represented by the symbol ΔH, where a negative value indicates an exothermic process and a positive value indicates an endothermic process.
2. The relationship between enthalpy change and potential energy surfaces can be visualized by analyzing the energy difference between reactants and products along the reaction coordinate.
3. During a reaction, if the products have lower potential energy than the reactants, the enthalpy change is negative, indicating that energy has been released.
4. Conversely, if the products have higher potential energy than the reactants, the enthalpy change is positive, suggesting that energy has been absorbed from the surroundings.
5. Understanding enthalpy change is essential for predicting reaction spontaneity and stability, as it helps determine the feasibility of a reaction based on energy considerations.

Review Questions

• How does enthalpy change affect the shape of potential energy surfaces for different reactions?
  • Enthalpy change directly influences the shape of potential energy surfaces by determining the relative energy levels of reactants and products. In exothermic reactions, the surface typically slopes downward from reactants to products, reflecting a decrease in enthalpy. For endothermic reactions, the surface slopes upward, indicating an increase in enthalpy. By analyzing these slopes, one can visualize how energy is exchanged during chemical transformations.
• Compare and contrast exothermic and endothermic reactions in terms of their enthalpy changes and implications on potential energy surfaces.
  • Exothermic reactions involve a negative enthalpy change (ΔH < 0), meaning they release heat and result in products with lower potential energy than reactants. This is depicted on potential energy surfaces as a downward slope. In contrast, endothermic reactions have a positive enthalpy change (ΔH > 0) since they absorb heat and lead to products with higher potential energy. This is represented as an upward slope on potential energy surfaces. Understanding these differences helps in predicting reaction behavior.
• Evaluate the role of enthalpy change in determining reaction spontaneity using Gibbs free energy concepts.
  • Enthalpy change plays a crucial role in determining reaction spontaneity when combined with entropy considerations through Gibbs free energy (ΔG = ΔH - TΔS). A negative ΔG indicates a spontaneous process. If enthalpy change is negative (exothermic), it often favors spontaneity, while a positive ΔH could be offset by an increase in entropy (positive ΔS) at high temperatures. Thus, assessing both enthalpy and entropy allows for a comprehensive understanding of whether a reaction will proceed naturally under given conditions.

Related terms

exothermic reaction: A reaction that releases heat to its surroundings, resulting in a decrease in enthalpy.

endothermic reaction: A reaction that absorbs heat from its surroundings, leading to an increase in enthalpy.

activation energy: The minimum energy required for a chemical reaction to occur, which affects the shape of potential energy surfaces.