Lecture Notes: Hess's Law Problem

Objective

• Determine the enthalpy (ΔH) for a given chemical reaction using Hess's Law.

Approach

1. Analyze given fundamental reactions.
2. Manipulate reactions as needed to match the target reaction.
3. Sum the enthalpies after necessary manipulations.

Step-by-Step Process

Reaction Analysis

• Target Reaction: Determine ΔH for the reaction involving Fluorine gas (F2).
• Given Reactions: Identify reactions involving F2.

Manipulating Reactions

1. First Reaction:

   • Contains Fluorine gas (F2) on the reactant side.
   • Reverse the reaction:
     • Original: 2F2 + O2 → 2OF2
     • Reversed: 2OF2 → 2F2 + O2
     • New ΔH: Change sign of original ΔH.
     • Original ΔH: -49.4 kJ/mol
     • New ΔH: +49.4 kJ/mol (Reversed)
   • Adjust for stoichiometry:
     • Need 1 mole of F2, so cut the reaction in half.
     • Adjusted Reaction:
       • F2 + 1/2 O2 → OF2
     • Adjusted ΔH: +24.7 kJ/mol (Half of +49.4)

2. Second Reaction:

   • Focus on product CF3.
   • Reverse the reaction where CF3 is on the product side:
     • Original: Cl2O + 3OF2 → CF3 + O2
     • Reversed: CF3 + O2 → Cl2O + 3OF2
     • New ΔH: Reverse sign of original ΔH.
     • Original ΔH: Not stated. Process assumes it becomes -236.2 kJ/mol.

3. Third Reaction:

   • Adjust without reversing, but cut in half:
     • Original Reaction: 2ClF + O2 → Cl2O + OF2
     • Adjusted Reaction:
       • ClF + 1/2 O2 → 1/2 Cl2O + 1/2 OF2
     • ΔH: Original +24.0 kJ/mol, adjusted +107.0 kJ/mol (Assuming division and error in narration)

Compile and Sum Reactions

• Add adjusted reactions to derive the target reaction:
  • ClF + F2 → ClF3
• Sum ΔH values after adjustments:
  • Determine final ΔH for the target reaction: ΔH = +104.5 kJ/mol.

Conclusion

• This example illustrates a complex application of Hess's Law involving reaction reversal, stoichiometric adjustments, and enthalpy summation.