6.9 Hess’s Law

Represent a chemical or physical process as a sequence of steps.

• Many processes can be broken down into a series of steps. Each step in the series has its own energy change.

Explain the relationship between the enthalpy of a chemical or physical process and the sum of the enthalpies of the individual steps.

• Because total energy is conserved (first law of thermodynamics), and each individual reaction in a sequence transfers thermal energy to or from the surroundings, the net thermal energy transferred in the sequence will be equal to the sum of the thermal energy transfers in each of the steps. These thermal energy transfers are the result of potential energy changes among the species in the reaction sequence; thus, at constant pressure, the enthalpy change of the overall process is equal to the sum of the enthalpy changes of the individual steps.
• The following are essential principles of Hess’s law:
  • i. When a reaction is reversed, the enthalpy change stays constant in magnitude but becomes reversed in mathematical sign.
  • ii. When a reaction is multiplied by a factor c, the enthalpy change is multiplied by the same factor c.
  • iii. When two (or more) reactions are added to obtain an overall reaction, the individual enthalpy changes of each reaction are added to obtain the net enthalpy change of the overall reaction.
  • Exclusion: The concept of state functions will not be assessed on the AP Exam.