Section 5.3 Enthalpy

Objectives Identify and apply the state function of enthalpy, H

Key Terms Work Pressure-volume (P-V) work Enthalpy Endothermicity Exothermicity

Work We can measure the work done by the gas if the reaction is done in a vessel that has been fitted with a piston. w = −P  V

Enthalpy If a process takes place at constant pressure and the only work done is this pressure-volume work, we can account for heat flow during the process by measuring the enthalpy of the system. Enthalpy is internal energy + product of pressure and volume: H = E + PV

Enthalpy When the system changes at constant pressure, the change in enthalpy,  H, is  H =  (E + PV) This can be written  H =  E + P  V

Enthalpy Since  E = q + w and w = −P  V, we can substitute these into the enthalpy expression:  H =  E + P  V  H = (q+w) − w  H = q So, at constant pressure the change in enthalpy is the heat gained or lost.

Endothermicity and Exothermicity A process is endothermic, then, when  H is positive.

Endothermicity and Exothermicity A process is endothermic when  H is positive. A process is exothermic when  H is negative.

The sign of enthalpy change,  H, associated with distillation of salt water is ________. 1.Positive 2.Negative 3.Zero

Correct Answer: As is evident in this figure of a distillation apparatus, a heat source is used; therefore the  H must be positive. 1.Positive 2.Negative 3.Zero

The sign of enthalpy change,  H, associated with a window washer dropping a squeegee from the top of a skyscraper is ________. 1.Positive 2.Negative 3.Zero

Correct Answer: The squeegee is falling, and potential energy is converted to kinetic energy. At the bottom, the squeegee will have lower potential energy, hence the enthalpy change is negative. 1.Positive 2.Negative 3.Zero

1. Internal energy changes are too small to measure reliably. 2. Enthalpy values typically yield the smallest numerical answers in our calculations. 3. Enthalpy is a state function and thus allows us to focus only on heat flow (not work) 4. Enthalpy content of reactants and products is an easily measured quantity.

1. Internal energy changes are too small to measure reliably. 2. Enthalpy values typically yield the smallest numerical answers in our calculations. 3. Enthalpy is a state function and thus allows us to focus only on heat flow (not work) 4. Enthalpy content of reactants and products is an easily measured quantity.