Thermodynamics: the second law Flashcards
Define spontaneous change
Spontaneous change is change that requires no work to bring about. The mixing of gases in an isolated system
Define entropy, S
Entropy, S, is a measure of molecular disorder
Give the second law of thermodynamics
The total entropy (ie the entropy of the system, ΔS, + the entropy of the surroundings, ΔSSur) increases in the course of a spontaneous change.
Note: ΔS can be negative as long as ΔSSur is positive enough so that overall Stotal > 0
Give the equation for dStotal, taking temperatures of the system and the surroundings into account
Define Th, the higher temperature, as the temperature of the system and Tc, the lower temperature, as the temperature of the surroundings. dq is the heat transfer.
Give the equation linking dS and dqreversible
Give the equation of ΔS as a state function
Derive the equation of ΔS for the isothermal expansion of an ideal gas
Derive the equation for ΔTrsS at constant temperature and pressure
(Trs = phase transition)
Remember that reversible phase transitions of pure substances take place at constant temperatures, and dq = dH at constant pressure
Describe Trouton’s rule
Trouton observed that the standard molar entropy of vaporisation. ΔvapS0m ~ 85JK-1mol-1 for a wide range of liquids.
This is due to similar changes in disorder when going from a liquid to a gas. Water is an exception due to the structural order (ie lower entropy) of hydrogen bonds.
Derive the equation of ΔS when a material is heated at constant pressure
Note that Cp is assumed to be independent of temperature
State the third law of thermodynamics
The entropy of all perfect crystalline substances is zero at T = 0
Derive an equation to work out the entropy for any given temperature
Combining entropy changed upon heating and during phase transitions, we can calculate the entropy for any given temperature
Give the equation for standard reaction entropy
Give the equation for the standard reaction enthalpy at T2 if we know the quantity at T1
Describe the total entropy change for any irreversible reaction
Describe the total entropy change for any reversible reaction
Give and describe the Clausius inequality
This is a criterion for irreversible/ spontaneous change. It is very useful in judging the spontaneity of chemical reactions
Define G, the Gibbs free energy
G = H - TS
dG = dH - TdS
Temperature and pressure are constant
Give dG for spontaneous processes
dG < 0
Derive dG = Vdp - SdT by combining the first and second laws of thermodynamics
Note: use the product rule
Derive the equation of the Gibb’s free energy at pressure p, where T is held constant.
Denote G0 as the standard Gibbs free energy
Modify the equation for G(p) for a mixture of gasses, denoting pA and pB as the partial pressures of reactants
