Definition

Noun: - The enthalpy change: "heat of formation" refers to the amount of heat energy absorbed or released when one mole of a compound is formed from its constituent elements in their standard states under standard conditions (usually 298 K and 1 atm pressure). It is a specific type of standard enthalpy change.

Usage

• The term is used primarily in thermodynamics and chemistry to quantify the energy change associated with the formation of a compound from its elements.
• It is a key concept for understanding the stability of compounds and for performing thermochemical calculations, such as in Hess's Law.

Examples

• Noun:
  • The standard heat of formation for water is -285.8 kJ/mol, indicating the reaction is exothermic.
  • Scientists use the heat of formation to predict whether a chemical reaction will be spontaneous.
  • A negative heat of formation value means energy is released during the compound's formation.

Advanced Usage

• "Standard heat of formation (ΔH°f)": This specifies the enthalpy change when one mole of the compound is formed from its elements, with all substances in their standard states at 1 bar of pressure and a specified temperature (usually 25°C or 298.15 K).
  • The standard heat of formation of carbon dioxide is a well-known constant in thermochemical tables.
• "Enthalpy of formation": This is a synonymous term often used interchangeably with "heat of formation" in modern chemistry.

Variants and Related Words

• Formation enthalpy: Another term for heat of formation.
• Standard enthalpy of formation: The full, precise term.
• Heat of reaction (ΔHrxn): The broader category of enthalpy change for any chemical reaction, which can be calculated using heats of formation.

Synonyms

• Standard enthalpy of formation
• Formation enthalpy

Important Notes

• The heat of formation for a pure element in its standard state is defined as zero.
• This is a specific scientific term. In common language, the word "heat" alone does not convey this precise meaning.

Noun

1. the heat evolved or absorbed during the formation of one mole of a substance from its component elements