Standard enthalpy of reaction

Standard Enthalpy of Reaction (ΔH°rxn) is a thermodynamic quantity that represents the heat absorbed or released during a chemical reaction under standard conditions. It is a specific case of enthalpy change in a system, which is a broader term encompassing various types of energy changes in a chemical process. The standard enthalpy of reaction is measured in joules per mole (J/mol) and is crucial for understanding the energetics of chemical reactions.

Definition

The standard enthalpy of reaction is defined as the change in enthalpy of a system when one mole of matter is transformed by a chemical reaction under standard conditions. Standard conditions typically mean a pressure of 1 atmosphere (atm) and a temperature of 298.15 K (25°C), although the temperature can vary in some definitions. The standard state of a substance is its pure form at 1 atm and the specified temperature.

Calculation

The standard enthalpy of reaction can be calculated using the equation: \[ \Delta H^\circ_{rxn} = \sum \Delta H^\circ_{f,products} - \sum \Delta H^\circ_{f,reactants} \] where:

• \( \Delta H^\circ_{rxn} \) is the standard enthalpy of reaction,
• \( \Delta H^\circ_{f,products} \) is the sum of the standard enthalpies of formation of the products,
• \( \Delta H^\circ_{f,reactants} \) is the sum of the standard enthalpies of formation of the reactants.

The standard enthalpy of formation (\( \Delta H^\circ_f \)) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states.

Significance

The standard enthalpy of reaction provides valuable information about the energetics of a chemical reaction. A negative value of \( \Delta H^\circ_{rxn} \) indicates an exothermic reaction, where heat is released to the surroundings. Conversely, a positive value indicates an endothermic reaction, where heat is absorbed from the surroundings. This information is crucial for the design and control of chemical processes, safety assessments, and thermodynamic studies.

Examples

1. Combustion reactions, where a substance combines with oxygen, are typically exothermic, releasing large amounts of heat. 2. Photosynthesis is an example of an endothermic process, where plants absorb sunlight to convert carbon dioxide and water into glucose and oxygen.

Applications

The standard enthalpy of reaction is used in various fields, including:

• Chemical engineering: for the design of reactors and the optimization of reaction conditions.
• Environmental science: to assess the energy impacts of chemical processes on the environment.
• Material science: to understand the formation and stability of materials.

See Also

• Thermodynamics
• Enthalpy
• Chemical reaction
• Standard state
• Standard enthalpy of formation

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