Lesson 5: Hess's Law

• Hess's Law

  If two or more thermochemical equations are added to give a final equation, then the enthalpies for each reaction can be added to give the enthalpy for the final equation
• State Function

  • A property of a system that is determined only by the current conditions of the system and is independent of the path taken by the system to reach these conditions
  • Internal energy and enthalpy of a system are state functions
  • Changes in either are independent of the reaction path chosen
• State Function

  • The gain in elevation in climbing from the base to the summit of a mountain is independent of the path chosen
• Hess's Law

  1. Reactions may proceed by a single step, or a series of steps, called the reaction pathway

  2. Each step is associated with a specific energy change

  3. No matter what the reaction pathway, the overall enthalpy change will be the same
• Standard Enthalpy of Formation, ΔHf:

  The enthalpy change that results from the formation of one mole of a compound from its elements at SATP (25C and 100 kPa)
• ΔHf for all free elements at SATP = 0 kJ, i.e., in their standard states including:

  the diatomic elements (HOFBrINCl), P4 (s), S8 (s), C(s) (as graphite or diamond), metals
• Steps to solving:

  1. Write the overall equation for the reaction, if it is not provided.
  2. If necessary, manipulate given thermochemical equations. (e.g. Reverse so a term is on the same side in the overall equation and change the sign of the enthalpy, or divide/multiply the equation by a factor that gives the same coefficients as the overall equation and divide/multiply the enthalpy by the same factor)
  3. Cancel terms that appear on both sides of the equation. Coefficients count!
  4. Add enthalpy values of the newly arranged equations to get the enthalpy change overall.
• molar enthalpy of reaction = the sum of molar enthalpy of the products - the sum of molar enthalpy of the reactants
• steps to solving the molar enthalpy of the reaction without thermochemical equations:

  1. write a balanced equation of the reaction if not provided
  2. find the molar enthalpy of formation for all reactants and products
  3. multiply each molar enthalpy value by the coefficient of that substance from the equation
  4. apply Hess's law