Enthalpy, Standard Enthalpy Data, and Hess’s Law

All flashcards

First law of thermodynamics

For a system, the change in internal energy is $\Delta U = q + w$, where heat and work are counted as energy entering or leaving the system.

Enthalpy

Enthalpy is the state function defined by $H = U + PV$, where $U$ is internal energy.

Constant-pressure heat

When pressure is constant and only pressure-volume work occurs, the heat transferred is $q_p = \Delta H$.

Thermochemical equation

A balanced chemical equation paired with its enthalpy change; the enthalpy value applies to the amounts and physical states shown.

Hess’s law

The enthalpy change for an overall reaction equals the sum of the enthalpy changes for steps that add to that reaction.

Quiz questions

1. Which equation defines enthalpy?Answer: $H = U + PV$
2. When does the heat transferred equal the enthalpy change, $q_p = \Delta H$?Answer: At constant pressure when only pressure-volume work occurs.
3. A reaction has a negative enthalpy change. How is it classified?Answer: Exothermic; it releases heat.
4. What does the standard enthalpy of formation, $\Delta H_f^\circ$, describe?Answer: Formation of one mole of a substance from elements in their most stable forms.
5. Why can Hess’s law be used to add enthalpy changes for reaction steps?Answer: Enthalpy is a state function, so its change depends on the initial and final states.