Application of Hess's Law

Overview

• Hess's Law allows us to calculate the standard enthalpy change (ΔH°) of a reaction using the heats of formation of the reactants and products.
• Formula involves calculating the sum of the heat of formation of products minus the sum of the heat of formation of reactants.
• Key concept: Products minus Reactants.

Process

1. Balanced Chemical Reaction

   • Use stoichiometry to multiply the heat of formation values.

2. Heat of Formation (ΔHf)

   • Look up the ΔHf values in Appendix G or relevant resources.
   • Important to note the state of matter (e.g., aqueous, gaseous).

Example Calculation

• Example Reaction:
  • Components: NO, H₂O, HNO₃ (aq), and NO₂

Heat of Formation Values

• NO₂: 33.2 kJ/mol
• H₂O: -285.83 kJ/mol
• HNO₃ (aq): -207.49 kJ/mol
• NO: 90.2 kJ/mol

Calculation Steps

1. Sum of the Products:

   • NO: 1 mol × 90.2 kJ/mol = 90.2 kJ
   • HNO₃: 2 mols × (-207.49 kJ/mol) = -414.98 kJ
   • NO₂: 3 mols × 33.2 kJ/mol = 99.6 kJ
   • H₂O: 1 mol × -285.83 kJ/mol = -285.83 kJ

2. Overall Calculation

   • Ensure proper use of brackets and negative signs.
   • ΔH° = [Sum of products] - [Sum of reactants].
   • For this reaction, ΔH° = -138.4 kJ/mol.

Important Notes

• Proper use of brackets is crucial to ensure correct calculations.
• Cross-check each value with the appropriate resources for accuracy.