Thermodynamics ❮ 137❯ Rapid Review
● Thermodynamics is the study of heat and its transformations.
● Kinetic energy is energy of motion, while potential energy is stored energy.
● The common units of energy are the joule, J, and the calorie, cal.
● A calorimeter is used to measure the heat released or absorbed during a chemical or
physical change. Know how a calorimeter works.
● The specific heat capacity is the amount of heat needed to change the temperature of
1 gram of a substance by 1 K, while the molar heat capacity is the heat capacity per mole.
● The heat lost by the system in calorimetry is equal to the heat gained by the surroundings.
● The specific heat (c) of a solid can be calculated by: -(csolid × msolid × DTsolid) = cwater ×
mwater × DTwater or by g = cmDT.
● The First Law of Thermodynamics states that the total energy of the universe is con-
stant. (Energy is neither created nor destroyed.)
● The Second Law of Thermodynamics states that all spontaneous processes move in a
way that increases the entropy (disorder) of the universe.
● The enthalpy change, DH, is equal to the heat lost or gained by the system under con-
stant pressure conditions.
● DH values are associated with a specific reaction. If that reaction is reversed, the sign of
DH changes. If one has to use a multiplier on the reaction, it must also be applied to
the DH value.
● The standard enthalpy of formation of a compound, DHf°, is the enthalpy change when
1 mol of the substance is formed from its elements and all substances are in their stan-
dard states.
● The standard enthalpy of formation of an element in its standard state is zero.
● DH°rxn = S DHf° products - S DHf° reactants. Know how to apply this equation.
● DH°rxn is usually negative for a spontaneous reaction.
● DS° = S S° products - S S° reactants. Know how to apply this equation.
● DS° is usually positive for a spontaneous reaction.
● The Gibbs free energy is a thermodynamic quantity that relates the enthalpy and
entropy, and is the best indicator for whether or not a reaction is spontaneous.
● If DG° > 0, the reaction is not spontaneous; if DG° < 0, the reaction is spontaneous; and
if DG° = 0, the reaction is at equilibrium.
● DG° = S DGf° products - S DGf° reactants. Know how to apply this equation.
● DG° = DH°rxn - T DS°rxn. Know how to apply this equation.
● For a system not at equilibrium: DG = DG° + RT ln Q = DG° + 2.303 RT log Q. Know
how to apply this equation.
● For a system at equilibrium: DG° = - RT ln K = - 2.303 RT log K. Know how to apply
this equation to calculate equilibrium constants.