Hess's Law
Law of Constant Heat Summation | Thermodynamics
1. Statement & Principle
Hess's Law: The total enthalpy change for a chemical reaction is the same, regardless of whether the reaction takes place in one step or in several steps.
This law is a direct consequence of the First Law of Thermodynamics and the fact that Enthalpy ($H$) is a State Function (its value depends only on initial and final states, not the path taken).
$$ \Delta H_{total} = \Delta H_1 + \Delta H_2 + \Delta H_3 + \dots $$
2. Illustration
Consider the formation of $CO_2$ from Carbon and Oxygen. It can occur in two ways:
Path 1 (Direct):
$$ C(s) + O_2(g) \rightarrow CO_2(g) \quad (\Delta H = -393.5 \, kJ) $$Path 2 (Two Steps):
$$ C(s) + \frac{1}{2}O_2(g) \rightarrow CO(g) \quad (\Delta H_1 = -110.5 \, kJ) $$ $$ CO(g) + \frac{1}{2}O_2(g) \rightarrow CO_2(g) \quad (\Delta H_2 = -283.0 \, kJ) $$Sum of Path 2: $\Delta H_{total} = \Delta H_1 + \Delta H_2 = -110.5 + (-283.0) = -393.5 \, kJ$.
This matches Path 1 perfectly.
3. Rules for Thermochemical Calculations
Thermochemical equations can be treated like algebraic equations:
- Addition: If you add two equations, add their $\Delta H$ values.
- Reversal: If you reverse an equation, change the sign of $\Delta H$ (e.g., $-x$ becomes $+x$).
- Multiplication: If you multiply an equation by a coefficient $n$, multiply $\Delta H$ by the same factor $n$.
4. Applications
Hess's Law is extremely useful for calculating enthalpy changes that are difficult or impossible to measure directly in a calorimeter.
- Enthalpy of Formation: For compounds like $CO$, $CH_4$, or $C_6H_6$ which cannot be formed cleanly from elements in a single step.
- Enthalpy of Transition: E.g., $\Delta H$ for Graphite $\to$ Diamond (very slow process).
- Bond Energies: Calculating bond enthalpies from combustion data.
- Lattice Energy: Used in the Born-Haber Cycle to calculate lattice energy of ionic solids.
Practice Quiz
Test your logic on Hess's Law.
No comments:
Post a Comment