Standard enthalpy of formation

Standard Enthalpy of Formation (ΔH_f^o) is a fundamental thermodynamic quantity that represents the enthalpy change during the formation of one mole of a substance from its constituent elements in their standard states. The standard states refer to the physical state of a substance under standard conditions of 298.15 K (25°C) and 1 atmosphere pressure. This concept is crucial in the fields of chemistry, chemical engineering, and thermodynamics for understanding and predicting the energy changes involved in chemical reactions.

Definition

The standard enthalpy of formation is defined for each substance and is measured in joules per mole (J/mol) or kilojoules per mole (kJ/mol). It is denoted as ΔH_f^o, where the subscript 'f' stands for formation, and the superscript 'o' indicates standard conditions (298.15 K and 1 atm pressure). For elements in their standard state, such as O_2(g), N_2(g), and S(s, rhombic), the standard enthalpy of formation is zero by definition.

Calculation

The standard enthalpy of formation can be calculated using Hess's Law, which states that the total enthalpy change for a chemical reaction is the same, regardless of the number of steps the reaction is carried out in. This allows for the calculation of ΔH_f^o for a compound by summing the enthalpies of formation of the products and subtracting the enthalpies of formation of the reactants, each multiplied by their respective stoichiometric coefficients.

Applications

The standard enthalpy of formation is used extensively in thermochemistry to calculate the enthalpy changes in chemical reactions using the equation: \[ \Delta H_{reaction}^o = \sum (\Delta H_{f,products}^o) - \sum (\Delta H_{f,reactants}^o) \] This equation is fundamental in predicting whether a reaction is exothermic or endothermic and in calculating the energy balance of chemical processes.

Examples

For example, the standard enthalpy of formation of water (H_2O(l)) is -285.8 kJ/mol, which means that 285.8 kJ of energy is released when one mole of water is formed from its elements, hydrogen and oxygen, under standard conditions.

See Also

• Hess's Law
• Thermochemistry
• Chemical reaction
• Enthalpy

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