kJ/mol (kilojoules per mole) is a unit of energy used to quantify thermodynamic quantities, specifically expressing the energy involved in chemical reactions and physical processes on a molar basis.
Here's a breakdown:
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What it represents: kJ/mol represents the amount of energy (in kilojoules) absorbed or released when one mole of a substance undergoes a specific process. This process can be a chemical reaction, a phase change (like melting or boiling), or the formation of a solution.
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Thermodynamic Quantities: It is commonly used to express:
- Enthalpy Changes (ΔH): The heat absorbed or released during a chemical reaction at constant pressure. A negative ΔH indicates an exothermic reaction (releasing heat), while a positive ΔH indicates an endothermic reaction (absorbing heat).
- Gibbs Free Energy Changes (ΔG): Determines the spontaneity of a reaction. A negative ΔG indicates a spontaneous reaction, while a positive ΔG indicates a non-spontaneous reaction.
- Bond Energies: The energy required to break one mole of a specific bond in the gaseous phase.
- Lattice Energies: The energy required to separate one mole of an ionic compound into its gaseous ions.
- Heat of Solution: The enthalpy change when one mole of a substance is dissolved in a solvent.
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Why use "per mole"? Expressing energy in kJ/mol allows for easy comparison of energy changes across different reactions and processes, regardless of the mass or amount of the substances involved. It provides a standardized way to quantify energy changes on a molecular level.
Example:
The enthalpy change for the combustion of methane (CH4) is -890 kJ/mol. This means that when one mole of methane is burned completely, 890 kilojoules of heat are released.
In summary, kJ/mol is a crucial unit for quantifying energy changes in chemical reactions and physical processes, enabling scientists to understand and predict the behavior of matter.
