Review of Enthalpy from CH 1010

Law of Conservation of Energy

energy cannot be created or destroyed; it can only be converted from one form to another. Also known as the First Law of Thermodynamics.

First Law of Thermodynamics

the principle that states that the energy of the universe is constant. (energy gained must equal the energy lost)

Thermal energy

the portion of total internal energy of a system that is proportional to its absolute temperature.

Heat

energy transferred from a higher-temperature object to a lower temperature object. (J or kJ)

Change in internal energy, ∆E

he loss and/or gain of energy by a system in the form of heat, q, and work, w.

System

the part of the universe that is the focus of a thermochemical study.

Surroundings

everything that isn’t part of the system

∆E

q + w

w

‒P∆V

1 L∙atm

101.3 J

Enthalpy change, ∆H

the quantity of heat transferred into or out of a system during a chemical
reaction or physical process such as a phase change at constant pressure (conditions under which most processes are run).

Exothermic change

one in which heat is given off as reactants form products (Energy flows out of the system)

Endothermic change

one in which heat is absorbed as reactants form products (Energy flows into the system)

Enthalpy of reaction, ∆Hrxn

the enthalpy change that accompanies chemical
reaction; also called “the heat of reaction.”

Enthalpy of vaporization, ∆Hvap

the energy required to convert 1.0 mol of a
liquid substance to its vapor state at some temperature.

Enthalpy of fusion, ∆Hfus

the energy required to convert 1.0 mol of a solid
substance to its liquid state.

Enthalpy of sublimation,
∆Hsub

the energy required to convert 1.0 mol of a solid
substance to its vapor state.

Enthalpy of solution, ∆Hsol

he enthalpy change that accompanies the
dissolution of a solute in a given solvent.

Enthalpy of combustion, ∆Hcombustion

the enthalpy change that accompanies the
reaction of a substance with oxygen, usually an exothermic process.

Changing the amount of reactants will change the amount of ____ absorbed or released.

heat

Thermodynamic Standard State

298 K; each gas has a partial pressure of 1.0 atm and each solute has a concentration of
1.0 M (∆H°rxn)

Breaking reactant bonds ____ energy

requires (endothermic and bond energies are positive)

Forming product bonds _____ energy

releases (exothermic reaction and bond energies are negative)

Bond dissociation energies

∆Hrxn ≈ D (product bonds) + D (reactant bonds)

Enthalpies of formation

∆H°rxn = Σnp ∆H°f (products) ‒ Σnr ∆H°f (reactants)

Calorimetry

For a reaction, the temperature change of the surroundings is determined and related to
the amount of heat that was gained or lost by the surroundings. The system’s heat change is the same magnitude but opposite sign.