thermodynamics (chem) Flashcards
State functions/state variables
Any measurable property which can be defined for a system present in a state is called state function. doesnt depend upon path followed
e.g. moles, temperature, pressure, density, mass
Path functions
Quantities defined at path as well as values depending on path
Heat (Q), Work (W)
Isothermal process
Temperature remains constant throughout the process
dT = 0
ΔT = if∫dT = 0
Isobaric process
Pressure remains constant throughout the process
dP = infinitesimally small change in P = 0
Isochoric process
Volume remains constant throughout the process
dV = 0
Adiabatic process
No heat exchange throughout the process
dq = 0
Cyclic process
System undergoes a series of processes and finally comes back to its original state, then the process is collectively known as cyclic process
Change in state function is zero in cyclic processes
Heat capacity of substance (c)
The amount of heat required/released by a substance to increase or decrease its temperature by 1 Celsius or 1K for a substance
C= dq/dT (heat req or released/ Change in temp)
unit: J/K or cal/K
Specific heat capacity (Cs)
The amount of heat required/released by 1g of a substance to change its temperature by 1oC
Heat capacity per unit mass of a substance
Cs = C/m
= dq/m dT
Molar heat capacity (Cm)
intensive property (mass independent/non-additive)
Heat capacity per mole of a substance
Cm = C/n
= dq/n dT
molar heat capacity at constant pressure, volume
heat capacity at
constant pressure: Cp
constant volume: Cv
molar heat capacity at
constant pressure: Cpm
constant volume: Cvm
Relation between Cpm and Cvm for ideal gas
Cpm - Cvm = R (gas constant)
Cp - Cv = nR (gas constant)
Cpm/Cvm = γ (specific heat ratio)
Cvm = R/(γ -1)
only valid for molar heat capacities and not for specific heat capacity
Internal energy (U or E)
- It is a state function
- It is an extensive property (mass dependent)
- for a system internal energy is the sum of all types of molecular energies as follows= KE of particles + PE of particles
First law of thermodynamics
- Related to conservation of energy
- Can neither be created nor destroyed, only transformed from one form to another
- the total energy of the universe remains conserved
dU = dq + dw
ΔU = q + w
IUPAC sign convention
1. heat given to the system
2. heat absorbed by the system
3. heat released by the surroundings
all +ve
IUPAC sign convention
1. Heat given by the system
2. Heat released by the system
3. Heat absorbed by the surroundings
All -ve
IUPAC sign convention
1. work done on the system
2. work done by the surroundings
3. work done by the system
4. work done on the surroundings
1 and 2 are +ve
3 and 4 are -ve
Calculation of work donr
dW = -pextdV
compression: dV —> -ve
expansion dV —> +ve
irreversible work done
done against constant external pressure
W = -pext(V2 - V1)
reversible work done
dW = PintdV
for ideal gas
dW = -PgdV
dW = -nRT/V dV
integrating both sides from V1 to V2
W = -2.303nRT log(V2/V1)
IDEAL GAS, REVERSIBLE, ISOTHERMAL WORK
work done from graph
area under P-V curve
for an ideal gas del U is a function of ___?
Temperature
pressure in vacuum
zero
change in internal energy for an ideal gas
ΔU = CvΔT
ΔU = nCvmΔT
ΔU = mCvsΔT
Enthalpy (H)
- form of energy
- state function
- extensive property
- for a system it is defined as the total heat (energy) content of the system and it is given as
H = U + PV
enthalpy for ideal gas
H = U + PV
H = U + nRT
both U and nRT are f(t)
therefore, for ideal gas H is a function of temperature
relation between ΔH and ΔU for gaseous species
ΔH = ΔU + RTΔng
Δng = ng,p - ng,r
Entropy
- state function
- extensive property
- measure of randomness/disorderness/disturbance
- third law
entropy of pure substance vs mixture
S of pure < S of mixture
In solids, what has more entropy crystalline or amorphous
amorphous because crystalline substances have regular arrangement of particles
Second law of thermodynamics
- Complete conversion of heat into work is not possible
- Heat cant flow from a low temp body to a high temp body without application of external help
- Spontaneous process cannot proceed in reverse direction in similar conditions
- During a sponteneous process entropy of the universe increases and at equilibrium it remains constant
ΔS (univ) >= 0
If ΔS
1. > 0
2. = 0
3. < 0
- spontaneous ΔS > 0
- equilibrium ΔS = 0
- non spontaneous ΔS < 0
Third law of thermodynamics
At absolute zero temperature (0K) the entropy of a crystalline substance is zero
Gibbs free energy (G)
- state function
- extensive property
ΔG = ΔH - TΔS
