What is a state function? What are examples of a state function and when its not a state function?
1 Answer
Apr 9, 2018
A state function is simply one that depends only on the start and end point, and not the path. You know plenty that you may not have identified before...
- Pressure, e.g.
#P_1V_1 = P_2V_2# - Volume, e.g.
#V_1/T_1 = V_2/T_2# - Temperature, e.g.
#DeltaT = T_f - T_i# - Moles, e.g.
#K_P = K_c(RT)^(Deltan_"gas")# - Mass, e.g. conservation of mass!
You have used all of these in the ideal gas law, chemical reactions, etc. When have you ever cared how the volume changed? Have you ever needed to know how the mass changed, as long as it was conserved in a reaction?
And the more usual state function quantities are
- Enthalpy (
#H# ), e.g.#DeltaH_(rxn)^@# ,#DeltaH_"fus"# , etc. - Internal Energy (
#U# ), e.g.#DeltaU = q + w# (first law of thermodynamics) - Entropy (
#S# ), e.g.#DeltaS = q_(rev)/T# , Carnot engine - Gibbs' free energy (
#G# ), e.g.#DeltaG = DeltaH - TDeltaS#
Path functions are the flip side, and depend on the way the process is performed, even if the start and end points are the same. The most common ones are
- Heat flow (
#q# ), e.g.#q = mc_sDeltaT# - PV Work (
#w# ), e.g.#w = -int P dV#
