Exothermic reaction
In
Examples
Examples are numerous:
- 4Fe + 3O2 → 2Fe2O3 ΔH⚬ = - 1648 kJ/mol
A particularly important class of exothermic reactions is combustion of a hydrocarbon fuel, e.g. the burning of natural gas:
- CH4 + 2O2 → CO2 + 2H2O ΔH⚬ = - 890 kJ/mol
These sample reactions are strongly exothermic.
Uncontrolled exothermic reactions, those leading to
Measurement
The
The measured heat energy released in an exothermic reaction is converted to ΔH⚬ in Joule per mole (formerly cal/mol). The standard enthalpy change ΔH⚬ is essentially the enthalpy change when the stoichiometric coefficients in the reaction are considered as the amounts of reactants and products (in mole); usually, the initial and final temperature is assumed to be 25 °C. For gas-phase reactions, ΔH⚬ values are related to bond energies to a good approximation by:
- ΔH⚬ = total bond energy of reactants − total bond energy of products
In an exothermic reaction, by definition, the enthalpy change has a negative value:
- ΔH = Hproducts - Hreactants < 0
where a larger value (the higher energy of the reactants) is subtracted from a smaller value (the lower energy of the products). For example, when hydrogen burns:
- 2H2 (g) + O2 (g) → 2H2O (g)
- ΔH⚬ = −483.6 kJ/mol [3]
See also
- Chemical thermodynamics
- Differential scanning calorimetry
- Endergonic
- Exergonic
- Endergonic reaction
- Exergonic reaction
- Exothermic process
- Endothermic reaction
- Endotherm
References
- .
- ^ S2CID 98267946.
- ^ "Enthalpy (Chapter 5)". Archived from the original on 2013-07-08. Retrieved 2013-07-20.