Hess's Law
Hess's Law states that the total enthalpy change of a chemical reaction is independent of the pathway taken, depending only on the initial and final states of the system. This principle allows chemists to calculate enthalpy changes for reactions that are difficult or impossible to measure directly by combining known thermochemical equations. It is a direct consequence of the First Law of Thermodynamics, making enthalpy a state function.
Key Formula
ΔH_rxn = Σ ΔH(products) - Σ ΔH(reactants)
LaTeX: \Delta H_{\text{rxn}} = \sum \Delta H_{\text{products}} - \sum \Delta H_{\text{reactants}}
| Symbol | Meaning | Unit |
|---|---|---|
| ΔH_rxn | Enthalpy change of the overall reaction | kJ/mol |
| ΔH(products) | Sum of enthalpies of products | kJ/mol |
| ΔH(reactants) | Sum of enthalpies of reactants | kJ/mol |
Worked Example
Problem
Calculate ΔH for the reaction C(s) + ½O₂(g) → CO(g), given: (1) C(s) + O₂(g) → CO₂(g), ΔH₁ = −393.5 kJ/mol; (2) CO(g) + ½O₂(g) → CO₂(g), ΔH₂ = −283.0 kJ/mol.
Solution
Step 1: Write the target reaction: C(s) + ½O₂(g) → CO(g). Step 2: Use reaction (1) as written: C(s) + O₂(g) → CO₂(g), ΔH = −393.5 kJ/mol. Step 3: Reverse reaction (2) so CO₂ cancels: CO₂(g) → CO(g) + ½O₂(g), ΔH = +283.0 kJ/mol. Step 4: Add the two adjusted equations: C(s) + O₂(g) + CO₂(g) → CO₂(g) + CO(g) + ½O₂(g). Step 5: Cancel CO₂ and ½O₂ from both sides to get C(s) + ½O₂(g) → CO(g). Step 6: Sum the enthalpies: ΔH = −393.5 + 283.0 = −110.5 kJ/mol.
Answer
ΔH = −110.5 kJ/mol
Key Properties Relevant to Hess's Law Calculations
| Reaction | ΔH (kJ/mol) | Direction Used | Sign Change? |
|---|---|---|---|
| C(s) + O₂(g) → CO₂(g) | −393.5 | Forward | No |
| CO(g) + ½O₂(g) → CO₂(g) | −283.0 | Reversed | Yes (+283.0) |
| H₂(g) + ½O₂(g) → H₂O(l) | −285.8 | Forward | No |
| N₂(g) + 3H₂(g) → 2NH₃(g) | −92.4 | Forward | No |
| 2SO₂(g) + O₂(g) → 2SO₃(g) | −198.4 | Forward | No |
Interactive Tools
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Related Terms
Enthalpy of Formation
The enthalpy of formation (ΔH°f) is the heat change when one mole of a compound is formed from its constituent elements in their standard states at 298 K and 1 atm pressure. Standard enthalpies of formation for elements in their most stable form are defined as zero. These values are tabulated and used extensively with Hess's Law to calculate enthalpy changes for any chemical reaction.
Standard Enthalpy
Standard enthalpy refers to the enthalpy change of a process measured under standard conditions: 298.15 K (25 °C) and 1 bar (approximately 1 atm) pressure, with all substances in their standard states. Standard enthalpy values provide a universal reference for comparing thermochemical data across different reactions and sources. The symbol ΔH° (read "delta H naught" or "delta H standard") denotes a standard enthalpy change.
Gibbs Free Energy
Gibbs free energy (G) is a thermodynamic potential that measures the maximum reversible work a system can perform at constant temperature and pressure, and determines the spontaneity of a process. A negative ΔG indicates a spontaneous reaction, a positive ΔG indicates a non-spontaneous reaction, and ΔG = 0 indicates a system at equilibrium. It combines enthalpy and entropy into a single criterion for spontaneity, making it one of the most powerful tools in chemical thermodynamics.
Named after Germain Henri Hess, a Swiss-Russian chemist who formulated the law in 1840. The term derives from his surname; the concept is also called the Law of Constant Heat Summation.