Inorganic thermodynamics Flashcards
What is 2nd law thermo
In a spontaneous process the entropy of the universe increases
Entropy units
J per K (per mol if molar)
give equation for molecular basis of entropy
S=klnW, k=Boltzmann constant, W= possible arrangement of molecules among energy levels
Draw diagram of change of molecules arrangement in energy levels upon heating/expanding
see notes
Definition of entropy?
dS=δq(rev)/T
If system absorbs heat entropy increases
Entropy increase is greater the cooler the object to which heat is supplied
delta S universe =?
delta S surroundings+ delta S system
=delta Ssys - q(sys)/T(sys)
When delta S univ = 0 what happens?
If it is less than 0?
System is at equilibrium and there is no tendency for further change
Less than 0, not allowed by second law, process won’t take place
What is first law thermodynamics?
Energy cannot be created or destroyed just transformed from one form to another
delta U =?
q (heat absorbed by system) + w (work done ON system)
What is heat?
Means by which energy is transferred from a hotter body to a cooler one to equalise their temperatures.
What is work?
Done when moving against a force
What is internal energy
Possessed by an object, depends on temperature, pressure etc., it is ‘stored up’ energy, for an ideal gas all internal energy present as KE of particles (not energy of interaction between particles/chemical bond energy)
What is a state function?
Takes a value depending on the state of the substance under consideration and not how that state was arrived at
Internal energy what kind of function?
State
What is a path function?
depends on path A–>B
What are sign conventions for q and w?
q positive when heat absorbed by system, w positive when work done on system eg. when gas is compressed
Why can’t we use Δ for path functions?
We can’t talk about its absolute value at state A or B
In a gas expansion 𝛿w’ =?
P(ext)dV (see diagram in notes)
Against a constant external pressure w’=?
What does this expression assume?
P(ext) (Vf-Vi)
Assumes throughout expansion internal pressure always greater than external pressure, need to check to make sure ( as gas expands internal pressure will fall)
‘The work done in a reversible process…’
Is always at a maximum ie. internal and external pressures are only infinitesimally different and system is essentially at equilibrium
In a reversible isothermal expansion w’=?
Conditions?
nRTln(Vf/Vi)
Isothermal, reversible, ideal gas
heat involved in gas expansions, ΔU = ?
q-w’
If w’ is at a max what can you say about q?
What is the consequence of this?
It is also at a max, ie heat absorbed is at max
Heat must be supplied to maintain the temp of gas and keep process isothermal
Even if process is not reversible to work out dS…
Need to work out 𝛿q if process was reversible
In isothermal ideal gas expansion ΔS=?
nRln(Vf/Vi), valid for any isothermal expansion whether or not it is reversible
