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