Hess's Law of Constant Heat Summation states that the total heat released or absorbed during a chemical reaction is independent of the path taken. This means th...
Hess's Law of Constant Heat Summation states that the total heat released or absorbed during a chemical reaction is independent of the path taken. This means that the overall heat change is equal to the heat change that would be observed if the reaction were performed under a constant pressure.
Formal Statement:
Hess's Law:
ΔH = ΔH°
where:
ΔH is the heat change under constant pressure
ΔH° is the heat change under a constant pressure
Examples:
When burning methane (CH4), the total heat released is equal to the heat change when burning methane in a constant pressure vessel.
When performing a combustion reaction between hydrogen (H2) and oxygen (O2), the heat released is the same as the heat change when performing the same reaction in a constant pressure vessel.
Significance of Hess's Law:
Hess's Law is a fundamental principle in chemical thermodynamics that helps predict the heat absorbed or released in a chemical reaction. By understanding this law, chemists can accurately calculate the total heat involved in a reaction, regardless of the path taken.
Limitations:
Hess's Law does not apply to all chemical reactions. Specifically, for exothermic reactions (where the heat is released), Hess's Law may not hold because the heat flow is not the same in both directions