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Understanding Hess's Law and Enthalpy Calculations

May 8, 2025

Lecture on Hess's Law and Enthalpy Calculations

Introduction to Hess's Law

  • Hess's Law allows the calculation of the enthalpy change of a reaction by using known enthalpy changes of other reactions.
  • The law states that the total enthalpy change for a reaction is the same, no matter how it is carried out, in multiple steps or one step.

Example 1: Calculating Enthalpy Change

  • Reaction 1: Nitrogen gas (N2) reacts with hydrogen gas (H2) to produce ammonia (NH3) with an enthalpy change of -92 kJ.
  • Reaction 2: Water decomposes to hydrogen gas and oxygen gas with an enthalpy change of +286 kJ.
  • Target Reaction: Ammonia reacting with oxygen gas to produce nitrogen gas and water.

Steps to Calculate:

  1. Reverse and adjust Reaction 1:
    • Reverse the reaction and multiply by 2 to focus on N2.
    • Adjust accordingly: 4 NH3 -> 2 N2 + 6 H2, Enthalpy: +184 kJ (92 x 2).
  2. Reverse and adjust Reaction 2:
    • Reverse and multiply by 6 to focus on H2O.
    • Adjust accordingly: 6 H2 + 3 O2 -> 6 H2O, Enthalpy: -1716 kJ (286 x 6).
  3. Add adjusted reactions to obtain the net reaction and enthalpy:
    • Net Reaction: 4 NH3 + 3 O2 -> 2 N2 + 6 H2O
    • Enthalpy Change: 184 + (-1716) = -1532 kJ.

Example 2: Water Decomposition and Atomic Oxygen

  • Given: Water decomposes to H2 and O2, requiring +576 kJ. Oxygen breaks into atomic oxygen with +495 kJ.
  • Target Reaction: H2 + O (atomic) -> Liquid water.

Steps to Calculate:

  1. Reverse and adjust water decomposition:
    • Reverse and divide by 2: H2 + 0.5 O2 -> H2O, Enthalpy: -286 kJ (576/2, reversed).
  2. Reverse and adjust oxygen breakdown:
    • Reverse and divide by 2: O -> 0.5 O2, Enthalpy: -247.5 kJ (495/2, reversed).
  3. Add reactions:
    • Net Reaction: H2 + O -> H2O
    • Enthalpy Change: -286 + (-247.5) = -533.5 kJ.

Example 3: Ozone Decomposition and Reaction with NO

  • Reactions Provided:
    • Ozone decomposing to O2: -427 kJ.
    • O2 decomposing to atomic O: +495 kJ.
    • NO reacting with ozone to produce NO2 and O2: -199 kJ.
  • Target Reaction: NO + O (atomic) -> NO2.

Steps to Calculate:

  1. Focus on desired substances (NO, O, NO2).
  2. Use Reaction 3 as is, as NO and NO2 are correctly positioned:
    • Enthalpy remains -199 kJ.
  3. Reverse and adjust Reaction 2:
    • O -> 0.5 O2, Enthalpy: -247.5 kJ (495/2, reversed).
  4. Reverse and adjust Reaction 1:
    • 1.5 O2 -> O3, Enthalpy: +213.5 kJ (427/2, reversed).
  5. Add reactions:
    • Net Reaction: NO + O -> NO2.
    • Enthalpy Change: -199 + (-247.5) + 213.5 = -233 kJ.

Conclusion

  • Hess's Law is a powerful tool for calculating the enthalpy changes of reactions using known data from other reactions.
  • Through the combination and manipulation of reactions, complex enthalpy calculations can be simplified.

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