thermodynamics Flashcards
what is an isolated system
no exchange of matter or energy with surroundings e.g. a vacuum
what is a closed system
exchange of heat but not matter with surroundings e.g. the earth
what is an em system
exchange of matter and energy with the surroundings
difference between intensive/extensive properties
intensive - independent on amount id material in the system e.g. T and p
extensive- dependent on the amount of material in the system e.g. V, U, H, S and G
what is the first law of thermodynamics
energy is neither created or destroyed but just transformed from one kind to another
change in U= q+w
(energy heat and work)
w is negative when work is done by system and positive when work is done by the system
what does the sign q tell us
heat (q) tells the direction of heat transfer
if change in temp in -ve, heat is transferred out and q is -ve (exothermic)
if change in temp is +ve, heat is transferred into substance and q is +ve (endothermic)
what is enthalpy
the change in enthalpy (H) of a system is the heat transferred from the surroundings to the system in a constant pressure
change in H=qp
qp is heat at constant pressure
H=U+pv
what is heat capacity
Cp- heat required to raise T by one degree
ised to relate change in enthalpy to a chnage in temp at constant pressure.
for infinitesimal changes in temp; dH=CpdT
for larger variations of temp (Cp remains constant); change in H = Cp change in T
relationship between enthalpy and heat
if change in enthalpy is positive the hear is absorbed by the system and process is endothermic
if change in enthalpy is negative then heat is released by the system and process is exothermic
standard state
the standard state of a substance at a specified temp is its pure form at a pressure of 1 bar
what can enthalpy changes also be associated with
enthalpy is a state function and can be measured by heat changes at a constant pressure, enthalpy changes are also associated with a change of state
types of standard states
adiabatic- no hear enters or leaves the system therefore q=0
isothermal- no temperature change to change in V=O
differences between reversible and irreversible reactions
reversible- small ch ages reverse process, max amount of work obtained, occurs slowly
irreversible- small changes don’t reverse reaction, less than max quantity of work obtained, process occurs rapidly
work equation
w=-p2 x change in V
what is entropy
entropy (S) is the tendency for energy to spread out in a system of atoms and molecules. amount of disorder is called entropy. (increased disorder increased entropy)
disordered states are more probable than ordered states
the change in entropy of a system is given by
change in entropy= SFinal-Sinitial
how is entropy a state function
positive values of delta S indicate an increase in disorder
negative values of delta S indicate a decrease in disorder
what is the second law of thermodynamics
within the context of entropy, the second law of thermodynamics states that in any spontaneous process, there is always an increase in entropy somewhere in the universe
for an isothermal process
change in S = qrev/T
qrev = heat transferred reversibly
for a non isothermal process
S=k ln W
k= boltzmanns constant and W = no of says of arranging energy
entropy of phase transitions
when a substance melts or boils>increase in disorder> increase in S
able to substitute heat of fusion or heat of vaporisations for qrev
for all phase transition
deltaSTrans= deltaHtrans/Ttrans
when processes tend to occurs (system and surroundings)
> 0 tends to occur
<0 tends to occur in reverse direction
0 reversible
in order for a reaction to occur spontaneously in a forward direction:
delta Ssys+ delta Ssurr ( entropy of universe) must increase
what is the third law of thermodynamics
the enthalpy of all pure crystalline solids at the absolute temperature zero is zero
>i.e. S=0 when T=0K
what is Gibbs free energy
Gibbs free energy (G)
energy which is, or can be, available to do work
G=H-TS
shows balance between enthalpy and entropy in determining the direction of a process
delta G= delta H - T delta S
thermodynamic criteria for a reaction to occur
delta G is negative (spontaneous)
preferably a large negative value
this will increase heat to the surroundings (entropy eill increase)
when process will occur in regards to Gibbs free energy
value of delta G <0 will occur >0 will occur in reverse 0 reversible when reaction at equilibrium delta G is 0
what is chemical potential
the contribution the component makes to the overall G of the system
chemical potential = G/n
so chemical potential is the molar free energy or free energy per mole
activities (a)
a enables us to wuantify hoe thr chemical potential of a pure substance chnages when it becomes a component in a mixture
activities are related to concentrations by the activity coefficient (lambda)
ai= lambda i x mi
mi = conc of i
chemical equilibrium delta G
delta G = sum of products-sum of reactants
k in chemical equilibrium
k less than 1, equilibrium lies to LHS
K greater than 1, equilibrium lies to RHS
the van’t hoff reaction isochore
equilibrium constants are temp dependant
what can van’t hoff reaction isochore be used for
determine variation of equlibrium constant with T
