Enthalpy of combustion

Enthalpy of Combustion is the heat released when one mole of a substance completely burns or combusts in oxygen under standard conditions. It is a crucial concept in thermodynamics, physical chemistry, and energy engineering, providing insights into the energy content of various fuels and materials. The enthalpy of combustion is a specific type of enthalpy change, denoted as ΔH_c^o, where the subscript c represents combustion, and the superscript o indicates standard conditions (1 atm pressure and 298.15 K temperature).

Definition

The enthalpy of combustion is defined as the change in enthalpy of a system when one mole of a substance reacts completely with oxygen to form water, carbon dioxide, and other products, depending on the composition of the substance being combusted. It is measured under standard conditions of temperature and pressure (STP), which are 298.15 K (25°C) and 1 atmosphere, respectively. The process is exothermic, meaning it releases heat, resulting in a negative ΔH_c^o value.

Measurement

The measurement of the enthalpy of combustion is typically carried out using a calorimeter, a device that measures the amount of heat produced or consumed in a chemical reaction. The most common type used for this purpose is the bomb calorimeter, which allows the combustion reaction to occur in a sealed, constant-volume environment. The heat released by the combustion is absorbed by the calorimeter's surroundings, and the change in temperature is used to calculate the enthalpy of combustion using the heat capacity of the calorimeter.

Applications

The enthalpy of combustion is a fundamental parameter in various fields:

• In chemical engineering and fuel technology, it helps in evaluating the energy content of fuels, aiding in the selection of the most efficient fuel for a particular application.
• In environmental science, understanding the enthalpy of combustion of different substances can help in assessing their environmental impact, particularly in terms of carbon footprint and greenhouse gas emissions.
• In food science, the calorific value of food, which is closely related to its enthalpy of combustion, is used to determine its energy content.

Examples

Common examples of substances with their enthalpy of combustion values include:

• Glucose (C6H12O6): -2808 kJ/mol
• Methane (CH4): -890 kJ/mol
• Ethanol (C2H5OH): -1367 kJ/mol

These values indicate the amount of energy released when one mole of the substance is completely combusted in oxygen.

See Also

• Calorimetry
• Exothermic reaction
• Heat of reaction
• Standard enthalpy change