Thermodynamics Flashcards
State function
Depend only on state of system and not how it reached
U changes when
Heat goes in or out
Work is done on or by it
Matter goes in or out
Why is U a state func?
Adiabatic work W(ad) required to bring change in state = diff btw value of U in 1 state & another state
Delta U = U2-U1=W(ad)
-ve sign for work done shows
Work is done on the system
U increases
Heat
Exchange of energy due to temp diff
Delta U =
q + w
1st law
Energy if an isolated system is constant
Pressure-volume work
W=P(ex) × (-dV)
= -p(ex)dV
= -p(ex)[Vf - Vi]
Work done on a gas =
W = -[integral p(ex) dV]
Reversible process
Ext pressure always < into pressure
P(ex) = [p(in) ± dp]
Work in reversible process
W(rev) = -2.303 nRT log Vf/Vi
Free expansion
Expansion of gas in vacuum
Hange in enthalpy =
dU + pdV
g in delta n(g)RT refers to
No of moles of (products-reactants)
Delta H =
dU + d n(g)RT
q =
Mc delta T
Relation btw Cv and Cp for an ideal gas
Qv = CvdT = dU
Qp = CpdT = dH
dH = dU + RdT
CpdT = CvdT + RdT
Cp - Cv = R
delta rH° =
Sum of enthalpies of products- sum of enthalpies of reactants
Standard enthaply of formation
Standard enthalpy change fir the formation of 1 mole of a compound form its elements on their most stable states of aggregation
Atomization enthalpy
Enthalpy change needed to break 1 mole of bonds to obtain atoms in gas phase
Lattice enthalpy
Enthalpy change which occurs when 1 mole of ionic compound dissociated into its ions in gas state
Born Haber cycle importance
Sum of enthalpy changes round a cycle is 0
Spontaneous process
Irreversible process & may only be reversed by some ext agency
Enthalpy-spontaneity relation
Enthalpy decrease, spontaneity increase
Entropy
Degree of randomness or disorderness in the system
Higher disorder higher entropy
What influences entropy?
Heat when added causes molecular motions increasing randomness
Heat added at lower temp increases entropy
Delta S =
Q(rev)/T
Delta S(total) =
Delta S(sys) + delta S(surr) > 0
Delta S(tot) for reversible or irreversible?
Only reversible
Gibb’s energy =
Delta H - T(delta S)
Gibb’s energy & spontaneity at const P and T:
Delta G < 0, spontaneous
Delta G > 0, Non spontaneous
3rd law
Entropy of any pur crystalline substance becomes 0 as temp becomes 0
Delta rG =
Delta rH° - T(Delta rS)° = -RT ln K
