Hess' Law Revision Notes

Hess' law is a principle in chemistry that allows the calculation of the overall enthalpy change (ΔH) for a chemical reaction, regardless of the pathway taken.

Key Concepts

• Hess' Law Statement: The total enthalpy change for a reaction is the same, no matter how it occurs, whether in one step or multiple steps.
• Enthalpy (ΔH): Measure of heat energy in a system at constant pressure.
• Standard Enthalpies of Formation (ΔHf): Used to calculate ΔH for reactions using known values for compounds.
• Standard Enthalpies of Combustion (ΔHc): Similar to ΔHf, used to find unknown ΔH for reactions.

Calculating ΔH Using Hess' Law

Example 1: Reaction of Acetic Acid

• Reaction: [2C(s) + 2H_2(g) + O_2(g) \rightarrow CH_3COOH(l)]
• Given Equations:
  • CH₃COOH(l) + 2O₂(g) → 2CO₂(g) + 2H₂O(l), ΔH = -875 kJ/mol
  • C(s) + O₂(g) → CO₂(g), ΔH = -394 kJ/mol
  • H₂(g) + 1/2O₂(g) → H₂O(l), ΔH = -286 kJ/mol

Steps:

1. Draw Enthalpy Cycle Diagram: Visual tool to represent component steps.
2. Apply Hess' Law: ΔH(route 1) = ΔH(route 2).
3. Evaluate:
   • ΔH(route 1) = (394×2) + (286×2) - 875 = 485 kJ/mol.

Example 2: Reaction Involving Sulfur Dioxide and Hydrogen Sulfide

• Reaction: [SO_2(g) + 2H_2S(g) \rightarrow 3S(s) + 2H_2O(l)]
• Given ΔHf Values:
  • SO₂(g) = -297 kJ/mol
  • H₂S(g) = 20.2 kJ/mol
  • H₂O(l) = -286 kJ/mol

Steps:

1. Equation: ΔH = ΔHf(products) - ΔHf(reactants)
2. Substitution:
   • ΔHf(products) = 3(0) + 2(-286) = -572 kJ/mol
   • ΔHf(reactants) = -297 + 2(20.2) = -337.4 kJ/mol
3. Overall ΔH: -572 + 337.4 = -234.6 kJ/mol (rounded to 235 kJ/mol)

Example 3: Combustion of Ethanol

• Reaction: [C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)]
• Given ΔHc Values:
  • C₂H₅OH = -1367 kJ/mol
  • C = -394 kJ/mol
  • H = -286 kJ/mol

Steps:

1. Equation: ΔH = ΔHc(reactants) - ΔHc(products)
2. Substitution:
   • ΔHc(reactants): 2(-394) + 3(-286) = -1646 kJ/mol
   • ΔHc(products): -1367 kJ/mol
3. Overall ΔH: -1646 - (-1367) = -279 kJ/mol

These examples demonstrate how Hess’s Law is applied to determine the enthalpy changes of reactions using enthalpy cycles and known standard enthalpies of formation or combustion.