Chem 162 - Ch 9 Thermochemisty (Hess’s Law of Summation)

Chapter 1: Introduction to Hess's Law of Summation

  • Overview of methods to calculate heats of a reaction

    • Specific heat/heat capacity: Involves mass, temperature change, and specific heat to calculate heat (Q or enthalpy change).

    • Hess's Law: The main focus of this chapter.

    • Enthalpy of formation: Covered later as another method to calculate enthalpy changes.

Chapter 2: Hess's Law of Summation

  • Definition of Hess's Law: Allows for the summation of known reactions to deduce the heat of an unknown reaction.

    • Useful for reactions that are impractical to measure directly in the lab.

  • Example of Application: Calculating enthalpy changes for the reaction of carbon graphite with oxygen to form carbon monoxide.

    • Direct measurement of this reaction's enthalpy is difficult.

    • Use known reactions for manipulation:

      • Reaction 1: C (graphite) + O2 -> CO2; ΔH = -390.3 kJ

      • Reaction 2: 2 CO2 -> 2 CO + O2; ΔH = -566.0 kJ

Manipulating Reactions

  • Aim: Combine known reactions to derive the unknown reaction.

    • Process: Adjust coefficients and reactions while ensuring reactants and products are aligned with the target reaction:

      • If coefficients are changed (e.g., multiplying or reversing reactions), the enthalpy changes must also be adjusted accordingly.

    • Different methods may lead to the same solution.

Chapter 3: Worked Example

  • Initial Setup: Verify that reactants and products are correctly aligned.

    • Focus on canceling out by aligning known reactions with the desired target reaction:

      • Reaction involved: 2 C (graphite) + O2 -> 2 CO

  • Steps to Solve:

    • Adjust Reaction 1 by multiplying by 2:

      • New reaction: 2 C + 2 O2 -> 2 CO2; new ΔH = -780.6 kJ

    • Cancel out elements between the adjusted reactions:

      • Combine resulting reactions carefully while adding enthalpy changes.

    • Final calculation of enthalpy change: Combine enthalpy changes of known reactions, yielding: -1350.3 kJ for the target reaction.

Summary of Hess's Law of Summation

  • Hess's Law provides a systematic method to deduce unknown enthalpy changes by using known reactions.

  • Focus on manipulating reactions to achieve the desired chemical equation.

  • Several practice worksheets and examples are available to aid understanding and mastery of this concept.