Chemistry – Chemical energetics - Exothermic and endothermic reactions | e-Consult

ΔH (Delta H) represents the heat absorbed or released during a chemical reaction at constant pressure. It's a measure of the energy change associated with the breaking and forming of chemical bonds.

• Exothermic reactions release thermal energy into the surroundings. This means the products have lower enthalpy than the reactants, resulting in a negative ΔH value. The heat released can be felt as an increase in temperature.
• Endothermic reactions absorb thermal energy from the surroundings. This means the products have higher enthalpy than the reactants, resulting in a positive ΔH value. The heat absorbed can be felt as a decrease in temperature.

In summary, a negative ΔH indicates an exothermic reaction (heat released), while a positive ΔH indicates an endothermic reaction (heat absorbed).