Hess's Law Hess's law states that the enthalpy of a reaction is independent of the path taken between the initial and final states as long as the initial an...
Hess's Law
Hess's law states that the enthalpy of a reaction is independent of the path taken between the initial and final states as long as the initial and final states are at the same temperature. This means that the total amount of heat absorbed or released in a reaction is the same regardless of how it is achieved.
Enthalpy of Processes
The enthalpy of a process is the total amount of heat energy absorbed or released during a reaction at constant pressure. It is a state function, meaning that its value depends only on the initial and final states of the system, not on the path taken to get there.
Hess's law applies to both exothermic and endothermic reactions. Exothermic reactions release heat, while endothermic reactions absorb heat. The enthalpy of an exothermic reaction is negative, while the enthalpy of an endothermic reaction is positive.
Examples
When burning methane (CH4(g)) at 25 °C, the enthalpy of combustion is -880 kJ/mol. This means that 880 kJ of heat is released when 1 mole of methane burns completely.
When freezing water (H2O(l)) at 0 °C, the enthalpy of fusion is 336 kJ/mol. This means that 336 kJ of heat is absorbed when 1 mole of water freezes.
When burning hydrogen (H2(g)) and oxygen (O2(g)) to form water vapor (H2O(g)), the enthalpy of combustion is 484 kJ/mol. This means that 484 kJ of heat is released when 1 mole of hydrogen burns completely with oxygen