Yes, generally speaking, an exothermic reaction has a negative enthalpy change (ΔH).
Understanding Exothermic Reactions and Enthalpy
An exothermic reaction is a chemical reaction that releases energy, usually in the form of heat. This means the energy of the products is lower than the energy of the reactants.
- Enthalpy (H): Represents the total heat content of a system at constant pressure.
- Enthalpy Change (ΔH): The difference in enthalpy between the products and reactants: ΔH = H(products) - H(reactants).
Why Exothermic Reactions Have Negative ΔH
Because exothermic reactions release energy, the products have less energy than the reactants. Therefore, when you subtract the higher enthalpy of the reactants from the lower enthalpy of the products, you get a negative value. This negative value indicates that energy has been released from the system into the surroundings.
Example
Consider the combustion of methane (natural gas):
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) + energy
In this reaction, heat is released, making it exothermic. The ΔH for this reaction is negative (approximately -890 kJ/mol), reflecting the release of energy.
Endothermic Reactions
For contrast, an endothermic reaction absorbs energy from the surroundings. In this case, the products have higher energy than the reactants, resulting in a positive ΔH.
Summary
| Reaction Type | Energy Change | Enthalpy Change (ΔH) |
|---|---|---|
| Exothermic | Releases Energy | Negative (-) |
| Endothermic | Absorbs Energy | Positive (+) |
