hess's law

• What does Hess' law state about ΔH?

  The route does not affect ΔH
• How does Hess' law relate to the steps of a reaction?

  It allows breaking a reaction into steps
• How can you find the total ΔH⦵ for a reaction using Hess' law?

  Add up the enthalpy changes of steps
• What is the process to calculate ΔH⦵r using enthalpy of reaction data?

  1. Write the reaction equation.
  2. Draw an enthalpy cycle diagram.
  3. Apply Hess' law.
  4. Substitute and evaluate the enthalpy changes.
• What is the ΔH⦵ for the reaction of CH3COOH(l) with O2(g)?

  -875 kJ mol-1
• What is the ΔH⦵ for the reaction of C(s) with O2(g)?

  -394 kJ mol-1
• What is the ΔH⦵ for the reaction of H2(g) with ½O2(g)?

  -286 kJ mol-1
• How are standard enthalpies of formation (ΔH⦵f) used with Hess' law?

  • ΔH⦵f values help determine unknown ΔH⦵ values.
  • ΔH⦵f for elements in standard states is 0 kJ mol−1.
• What is the equation to calculate ΔH⦵r using standard enthalpies of formation?

  ΔH⦵r = ΣΔH⦵f(products) - ΣΔH⦵f(reactants)
• What is the ΔH⦵f for SO2(g)?

  297 kJ mol-1
• What is the ΔH⦵f for H2S(g)?

  20.2 kJ mol-1
• What is the ΔH⦵f for H2O(l)?

  286 kJ mol-1
• What are the steps to calculate ΔH⦵r using enthalpy of formation data?

  1. Write the reaction equation.
  2. Calculate ΣΔH⦵f(products).
  3. Calculate ΣΔH⦵f(reactants).
  4. Determine ΔH⦵ using the equation.
• What is the overall ΔH⦵ for the reaction SO2(g) + 2H2S(g) ➔ 3S(s) + 2H2O(l)?

  234.6 kJ mol-1
• How can standard enthalpies of combustion be used in Hess' law?

  • They help find unknown ΔH⦵r values.
  • Use the equation: ΔH⦵r = ΣΔH⦵c(reactants) - ΣΔH⦵c(products).
• What is the equation to calculate ΔH⦵r using standard enthalpies of combustion?

  ΔH⦵r = ΣΔH⦵c(reactants) - ΣΔH⦵c(products)