MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

Thermochemistry
Concept Summary

Systems and Processes

· Systems are classified based on what is or is not exchanged with the surroundings.

o Isolated systems exchange neither matter nor energy with the environment.

o Closed systems can exchange energy but not matter with the environment.

o Open systems can exchange both energy and matter with the environment.

· Processes can be characterized based on a single constant property.

o Isothermal processes occur at a constant temperature.

o Adiabatic processes exchange no heat with the environment.

o Isobaric processes occur at a constant pressure.

o Isovolumetric (isochoric) processes occur at a constant volume.

States and State Functions

· State functions describe the physical properties of an equilibrium state; they are pathway independent and include pressure, density, temperature, volume, enthalpy, internal energy, Gibbs free energy, and entropy.

· Standard conditions are defined as 298 K, 1 atm, and 1 M concentrations.

· The standard state of an element is its most prevalent form under standard conditions; standard enthalpy, standard entropy, and standard free energy are all calculated under standard conditions.

· Phase changes exist at characteristic temperatures and pressures.

o Fusion (melting) and freezing (crystallization or solidification) occur at the boundary between the solid and the liquid phases.

o Vaporization (evaporation or boiling) and condensation occur at the boundary between the liquid and the gas phases.

o Sublimation and deposition occur at the boundary between the solid and gas phases.

o At temperatures above the critical point, the liquid and gas phases are indistinguishable.

o At the triple point, all three phases of matter exist in equilibrium.

· The phase diagram for a system graphs the phases and phase equilibria as a function of temperature and pressure.

Heat

· Temperature and heat are not the same thing.

o Temperature is a scaled measure of the average kinetic energy of a substance.

o Heat is the transfer of energy that results from differences of temperature between two substances.

· The heat content of a system undergoing heating, cooling, or phase changes is the sum of all the respective energy changes.

Enthalpy

· Enthalpy is a measure of the potential energy of a system found in intermolecular attractions and chemical bonds.

· Hess’s law states that the total change in potential energy of a system is equal to the changes of potential energies of the individual steps of the process.

· Enthalpy can also be calculated using heats of formation, heats of combustion, or bond dissociation energies.

Entropy

· Entropy, while often thought of as disorder, is a measure of the degree to which energy has been spread throughout a system or between a system and its surroundings.

o Entropy is a ratio of heat transferred per mole per unit kelvin.

o Entropy is maximized at equilibrium.

Gibbs Free Energy

· Gibbs free energy is derived from both enthalpy and entropy values for a given system.

· The change in Gibbs free energy determines whether a process is spontaneous or nonspontaneous.

o ΔG < 0: reaction proceeds in forward direction (spontaneous)

o ΔG = 0: reaction is in dynamic equilibrium

o ΔG > 0: reaction proceeds in reverse direction (nonspontaneous)

· Gibbs free energy depends on temperature; temperature-dependent processes change between spontaneous and nonspontaneous, depending on the temperature.