Hess’ Law

Limitations of Calorimetry Calorimetry is an accurate technique for determining enthalpy changes of a system. But there are limitations… How do chemists deal with chemical systems that cannot be analysed using calorimetry?

Hess’ Law of Summation “For any reaction that can be written in a series of steps, the enthalpy change of the reaction will be the same as the sum of the enthalpy changes for the steps” The net changes of a system are independent of the way in which the system changes from the initial state to the final state If a set of reactions occur in different steps but the initial reactants and final products are the same, the overall enthalpy change will be the same as well

Hess’ Law of Summation

Hess’ Law of Summation ∆H1 = ∆H2 + ∆H3

Manipulating Thermochemical Equations Ensure all equations are balanced Write the equations so that the arrows line up. Compare the given equations to the target equation All equations need to add together to give the target equation “Flip” equations to place reactants and products on the correct sides Remember to also “flip” the sign for the enthalpy value Multiply molar coefficients by an integer or fraction as required Remember to also multiply the enthalpy value Add up reactants and products canceling out substances that appear on both sides (watch your states of matter!) Add the enthalpy values

Examples

Practice Hess’ Law Worksheet P. 326-329 #1-5 P. 330 #1-4