Phases and Drives Studysheet
► On a phase diagram:
Critical Point @ the critical temperature, critical pressure
Triple point; @ a temperature & pressure
Melting and/or Freezing Point; @ a temperature & pressure
Vapor Pressure: of solid, of liquid, vs. temp.
Boiling Point; @ a temp. & pressure. (“Normal B.P” @ 1 atm)
Sublimation Point; @ a temp. & pressure
►Energy factors in phase changes: Endothermic for s →liq, liq→g, s→g. Respectively, the Heat (enthalpy); of fusion, of
vaporization, of sublimation. ΔHfus, ΔHvap, ΔHsubl.
For a temperature change of ONE phaseL Q=mcΔt. For a phase change: J=g x ΔH or J=moles x ΔH
►Enthalpy, a specific kind of chemical potential energy, symbol=H. Change of enthalpy = ΔH.
ΔH positive = endothermic. ΔH negative = exothermic. Reactions more likely to go exothermic.
►Entropy, a measure of the molecular disorder of a system; the difficulty of an exact description of it; the probability of that
exact or equivalent system occurring. Symbol = S. Absolute entropy of a material of system, at standard conditions = Sº,
based upon the entropy of a perfect crystal at absolute zero = 0. Change of entropy = ΔS. When ΔS=positive, system is going
to a more orderly, less likely state – can occur only by the expenditure of energy.
Reactions more likely to go if ΔS is
positive. The entropy of the entire universe (system + surroundings) ALWAYS increases in EVERY reaction. Entropy
increases (ΔS+) if: solid → liquid, liquid → gas, gas → more gas (moles or volume), cold → hot, big chunks → little, pure
materials → mixture, simple → complicated, orderly → disorderly.
►Gibbs Free Energy, G, measures the NET driving force of a reaction, the useful work that it can do, the net balance
between the enthalpy and entropy drives. Change in Gibbs Free Energy, ΔG.
ΔG = ΔH –TΔS. T = absolute temperature.
Note: higher temperature increases the strength of the entropy drive. When ΔG = 0, no net driving force – the reaction is at
equilibrium, no net reaction in either direction. ΔG = negative; reaction will spontaneously react from left to right, →,
under the conditions for which ΔG was calculated. ΔG = positive; no reaction will try to run backwards, ←, under those
conditions, can’t react as written. Standard Conditions: 25ºC/ 298K, 1 atm (or, now, 100Kpa), elements in their standard
states. Values calculated as standard conditions indicated by “O”, as SO or ΔHO.
ΔHf, ΔGf, enthalpy and free energy of FORMATION: energy change when ONE MOLE of the substance is made
from its elements. Use in calculations by the rule “ +Right – Left.” To find the ΔH, ΔG, or ΔS of a reaction, add the values for
all products (things on right), subtract the values of all reactants (things on left). ΔH f, ΔGf, for elements are defined as zero.
Phases and Drives Studysheet
► On a phase diagram:
Critical Point @ the critical temperature, critical pressure
Triple point; @ a temperature & pressure
Melting and/or Freezing Point; @ a temperature & pressure
Vapor Pressure: of solid, of liquid, vs. temp.
Boiling Point; @ a temp. & pressure. (“Normal B.P” @ 1 atm)
Sublimation Point; @ a temp. & pressure
►Energy factors in phase changes: Endothermic for s →liq, liq→g, s→g. Respectively, the Heat (enthalpy); of fusion, of
vaporization, of sublimation. ΔHfus, ΔHvap, ΔHsubl.
For a temperature change of ONE phaseL Q=mcΔt. For a phase change: J=g x ΔH or J=moles x ΔH
►Enthalpy, a specific kind of chemical potential energy, symbol=H. Change of enthalpy = ΔH.
ΔH positive = endothermic. ΔH negative = exothermic. Reactions more likely to go exothermic.
►Entropy, a measure of the molecular disorder of a system; the difficulty of an exact description of it; the probability of that
exact or equivalent system occurring. Symbol = S. Absolute entropy of a material of system, at standard conditions = Sº,
based upon the entropy of a perfect crystal at absolute zero = 0. Change of entropy = ΔS. When ΔS=positive, system is going
to a more orderly, less likely state – can occur only by the expenditure of energy.
Reactions more likely to go if ΔS is
positive. The entropy of the entire universe (system + surroundings) ALWAYS increases in EVERY reaction. Entropy
increases (ΔS+) if: solid → liquid, liquid → gas, gas → more gas (moles or volume), cold → hot, big chunks → little, pure
materials → mixture, simple → complicated, orderly → disorderly.
►Gibbs Free Energy, G, measures the NET driving force of a reaction, the useful work that it can do, the net balance
between the enthalpy and entropy drives. Change in Gibbs Free Energy, ΔG.
ΔG = ΔH –TΔS. T = absolute temperature.
Note: higher temperature increases the strength of the entropy drive. When ΔG = 0, no net driving force – the reaction is at
equilibrium, no net reaction in either direction. ΔG = negative; reaction will spontaneously react from left to right, →,
under the conditions for which ΔG was calculated. ΔG = positive; no reaction will try to run backwards, ←, under those
conditions, can’t react as written. Standard Conditions: 25ºC/ 298K, 1 atm (or, now, 100Kpa), elements in their standard
states. Values calculated as standard conditions indicated by “O”, as SO or ΔHO.
ΔHf, ΔGf, enthalpy and free energy of FORMATION: energy change when ONE MOLE of the substance is made
from its elements. Use in calculations by the rule “ +Right – Left.” To find the ΔH, ΔG, or ΔS of a reaction, add the values for
all products (things on right), subtract the values of all reactants (things on left). ΔH f, ΔGf, for elements are defined as zero.