Thermodynamics1
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Transcript of Thermodynamics1
THERMODYNAMICS.Elements of Physical Chemistry. By P. Atkins
Dr. H.A. Ellis18/1/05
Concerned with the study of transformation of energy:
Heat work
CONSERVATION OF ENERGY – states that:
Energy can neither be created nor destroyed in chemical reactions. It can only be converted from one form to the other.
UNIVERSE System – part of world have special interest in… Surroundings – where we make our observations
→ →
Open system Closed system Isolated system
Example:
↔ matter
↔ energy ↔ energy not matter matter × Energy ×
If system is themally isolated called Adiabatic system eg: water in vacuum flask.
WORK and HEAT
Work – transfer of energy to change height of the weight in surrounding eg: work to run a piston by a gas.
Heat – transfer of energy is a result of temperature difference between system and surrounding eg:
HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) - heat given off.
If heat released to surroundings – exothermic.If heat absorbed by surroundings – endothermic.
Example: Gasoline, 2, 2, 4 trimethylpentane
CH3C(CH3)2CH2CH(CH3)CH3 + 25/2 O2 → 8CO2(g) +H2O(l)
5401 kJ of heat is released (exothermic)
Where does heat come from?From internal energy, U of gasoline. Can represent chemical reaction:
Uinitial = Ufinal + energy that leaves system (exothermic)Or
Ui = Uf – energy that enters system (endothermic)
Hence, FIRST LAW of THERMODYNAMICS (applied to a closed system)
The change in internal energy of a closed system is the energy that enters or leaves the system through boundaries as heat or work. i.e.
∆U = q +w ∆U = Uf – Ui
q – heat applied to system W – work done on system When energy leaves the system, ∆U = -ve i.e. decrease internal
energy When energy enter the system, ∆U = +ve i.e. added to internal
energy
Different types of energies:
1. Kinetic energy = ½ mv2 (chemical reaction) kinetic energy
(KE) k T (thermal energy) where k = Boltzmann constant
2. Potential energy (PE) = mgh – energy stored in bonds
Now, U = KE + PE
3. Work (W)
w = force × distance moved in direction of forcei.e. w = mg × h = kg × m s-2 × m = kg m2 s-2
(m) (g) (h)
1 kg m2 s-2 = 1 Joule
- Consider work – work against an opposing force, eg: external pressure, pex. Consider a piston
Piston
Pex
pressure (P)
pex
A = area of piston
P
h
h is distance moved
w = distance × opposing force
w = h × (pex × A) = pex × hA
Work done on system = pex × ∆V
∆V – change in volume (Vf – Vi)
Work done by system = -pex × ∆V
Since U is decreased
Example:
C3H8(g) + 5 O2(g) → 3CO2(g) + 4H2O(l) at 298 K 1 atm
(1 atm = 101325 Pa), -2220 kJ = q
What is the work done by the system?For an ideal gas;
pV = nRT (p = pex)n – no. of molesR – gas constantT = temperatureV – volumep = pressure
V= nRT/p or Vi = niRT/pex
6 moles of gas:
Vi = (6 × 8.314 × 298)/ 101325 = 0.1467 m3
3 moles of gas:
Vf = (3 × 8.314 × 298)/ 101325 = 0.0734 m3
work done = -pex × (Vf – Vi)
= -101325 (0.0734 – 0.1467) = +7432 J
NB: work done = - pex (nfRT/pex – niRT/pex)
= (nf – ni) RT
Work done = -∆ngasRT
i.e. work done = - (3 – 6) × 8.314 × 298 = + 7432.7 J
Can also calculate ∆U∆U = q +w q = - 2220 kJ
w = 7432.7 J = 7.43 kJ
∆U = - 2220 + 7.43 = - 2212.6 kJ
NB:qp ∆U why?
Only equal if no work is done i.e. ∆V = 0
i.e. qv = ∆U
Example: energy diagram
C3H8 + 5 O2 (Ui)
3CO2 + 4H2O(l) (Uf)
U
U
progress of reaction
reaction path
Since work done by system = pex∆V
System at equilibrium when pex = pint (mechanical equilibrium)
Change either pressure to get reversible work i.e.
pex > pint or pint > pex at constant temperature by an infinitesimal change in either parameter
For an infinitesimal change in volume, dV Work done on system = pdV
For ideal gas, pV = nRT
p = nRT/ V work = p dV = nRT dV/ V
= nRT ln (Vf/Vi) because dx/x = ln x Work done by system= -nRT ln(Vf/Vi)
Vf
ViVf
Vi
Enthalpy, H
Most reactions take place in an open vessel at constant pressure, pex. Volume can change during the reaction
i.e. V 0 (expansion work).
Definition: H = qp i.e. heat supplied to the system at constant pressure.
Properties of enthalpy
Enthalpy is the sum of internal energy and the product of pV of that substance.
i.e H = U + pV (p = pex)
Some properties of H
Hi = Ui + pVi
Hf = Uf + pVf
Hf – Hi = Uf – Ui +p(Vf – Vi)
or
H = U + p V
Since work done = - pex V
H = (- pex V + q) +p V(pex= p)
H = ( -p V + q) + p V = q
H = qp
suppose p and V are not constant?
H = U + ( pV) expands to: H = U + pi V + Vi P + (P) (V)
• i.e. H under all conditions.
• When p = 0 get back
H = U + pi V U + p V• When V = 0:
H = U + Vi p
Enthalpy is a state function.
lattitude
A
Bpath 1
path 2
- does not depend on the path taken
NB: work and heat depend on the path taken and are written as lower case w and q. Hence, w and q are path functions. The state functions are written with upper case.
eg: U, H, T and p (IUPAC convention).
Standard States
By IUPAC conventions as the pure form of the substance at 1 bar pressure (1 bar = 100,000 Pa).
What about temperature? By convention define temperature as 298 K but could be at any
temperature.
Example:
C3H8(g) + 5 O2(g) → 3CO2(g) + 4H2O(l)
at 1 bar pressure, qp = - 2220 kJmol-1.
Since substances are in the pure form then can write
H = - 2220 kJ mol-1 at 298 K
represents the standard state.
H2(g) → H(g) + H(g), H diss = +436kJmol-1
H2O(l) → H2O(g), H vap = +44.0 kJmol-1
Calculate U for the following reaction:
CH4(l) + 2 O2(g) → CO2(g) + 2H2O(l), H = - 881.1kJmol-1
H = U + (pV)
= U + pi V + Vi p + p V
NB: p = 1 bar, i.e. p = 0
H = U + pi V
Since -pi V = - nRT,
U = H - nRT
calculation
U = - 881.1 – ((1 – 2)(8.314) 298)/ 1000
= - 881.1 – (-1)(8.314)(0.298) = - 2182.99 kJ mol-1
STANDARD ENTHALPY OF FORMATION, Hf
Defined as standard enthalpy of reaction when substance is formed from its elements in their reference state.
Reference state is the most stable form of element at 1 bar atmosphere at a given temperature eg.
At 298 K Carbon = Cgraphite
Hydrogen = H2(g)
Mercury = Hg(l)
Oxygen = O2(g)
Nitrogen = N2(g)
NB: Hf of element = 0 in reference state
Can apply these to thermochemical calculations
eg. Can compare thermodynamic stability of substances in
their standard state.
From tables of Hf can calculate H f rxn for any reaction.
Eg. C3H8 (g) + 5O2(g) → 3CO2(g) + 4H2O(l)
Calculate Hrxn given that:
Hf of C3H8(g) = - 103.9 kJ mol-1
Hf of O2(g) = 0 (reference state)
Hf of CO2(g) = - 393.5 kJ mol-1
Hf of H2O(l) = - 285.8 kJ mol-1
Hrxn = n H (products)- n H(reactants)
Hf(products) = 3 (- 393.5) + 4 (- 285.8)
= - 1180.5 -1143.2 = - 2323.7 kJ mol-1
Hf(reactants) = - 103.9 + 5 0 = - 103.9 kJ mol-1
Hrxn = - 2323.7 – (- 103.9) = - 2219.8 kJ mol-1
= - 2220 kJ mol-1
Answer same as before. Eq. is valid. Suppose: solid → gas (sublimation) Process is: solid → liquid → gas Hsub = Hmelt + Hvap
Ie. H ( indirect route) = . H ( direct route)
Hess’ Law
- the standard enthalpy of a reaction is the sum of the standard enthalpies of the reaction into which the overall reaction may be divided. Eg.
C (g) + ½ O2(g) → CO (g) , Hcomb =? at 298K
– From thermochemical data: C (g) +O2 (g) → CO 2(g) H
comb =-393.5 kJmol-1…………………………….(1)
CO (g) +1/2 O2 (g) →CO 2(g), Hcomb = -283.0 kJ mol-
1……………………. (2) Subtract 2 from 1 to give: C (g) + O2 (g) – CO (g) – 1/2 O2 (g) → CO2 (g) – CO2 (g)
C (g) + ½ O 2 (g) → CO (g) , Hcomb= -393.5 –
(-283.0) = - 110.5 kJ mol-1
Bond Energies
eg. C-H bond enthalpy in CH4
CH4 (g) → C (g) + 4 H (g) , at 298K.
Need: Hf of CH 4 (g) =- 75 kJ mol-1
Hf of H (g) = 218 kJ mol-1
Hf of C (g) = 713 kJ mol-1
Hdiss = nHf (products) - nHf
( reactants)
= 713 + ( 4x 218) – (- 75) = 1660 kJ mol-1 Since have 4 bonds : C-H = 1660/4 = 415 kJ mol-1
Variation of H with temperature
Suppose do reaction at 400 K, need to know
Hf at 298 K for comparison with literature value. How?
As temp.î HmÎ ie. H
m T
Hm = Cp,m T where Cp,m is the molar
heat capacity at constant pressure.
Cp,m = Hm/ T = J mol-1/ K
= J K-1 mol-1
HT2 = HT1 + Cp ( T2 - T1) Kirchoff’s equation. and Cp = n Cp(products)- nCp(reactants)
For a wide temperature range: Cp ∫ dT between T1 and T2. Hence : qp = Cp( T2- T1) or H = Cp T and.
qv = Cv ( T2 – T1) or Cv T = U
ie. Cp = H / T ; Cv =U /T For small changes: Cp = dH / dT ; Cv = du / dT
For an ideal gas: H = U + p V For I mol: dH/dT = dU/dT + R Cp = Cv + R
Cp / Cv = γ ( Greek gamma)
Work done along isothermal paths
Reversible and Irreversible paths ie T =0 ( isothermal)
pV = nRT= constant
Boyle’s Law : piVi =pf Vf
Can be shown on plot:
pV diagram
P
V
Piv
i
Pfv
f
pV= nRT = constant
Work done = -( nRT)∫ dV/V
= - nRT ln (Vf/Vi)
Equation is valid only if : piVi=pfVf and therefore: Vf/Vi = pi/pf and
Work done = -( nRT) ln (pi/pf) and follows the path shown.
pV diagram
An irreversible path can be followed: Look at pV diagram again.
V
P
Isothermal reversible process (ie. at equlb. at every stage of the process)
Irreversible reaction
PiVi
PfVf
An Ideal or Perfect Gas
NBFor an ideal gas, u = 0
Because: U KE + PE k T + PE (stored in bonds)
Ideal gas has no interaction between molecules (no bonds broken or formed)
Therefore u = 0 at T = 0
Also H = 0 since (pV) = 0 ie no work done
This applies only for an ideal gas and NOT a chemical reaction.
Calculation
eg. A system consisting of 1mole of perfect gas at 2 atm and 298 K is made to expand isothermally by suddenly reducing the pressure to 1 atm. Calculate the work done and the heat that flows in or out of the system.
w = -pex V = pex(Vf -Vi)
Vi = nRT/pi = 1 x 8.314 x (298)/202650= 1.223 x 10-2 m3
Vf = 1 x 8.314 x 298/101325 = 2.445 x 10-2 m3
therefore, w = -pex (Vf- Vi) = -101325(2.445-1.223) x 10-2 = -1239 J
U = q + w; for a perfect gas U = 0therefore q = -w and
q = -(-1239) = +1239 J
Work done along adiabatic path
ie q = 0 , no heat enters or leaves the system. Since U = q + w and q =0 U = w When a gas expands adiabatically, it cools. Can show that: pVγ = constant, where ( Cp/Cv =γ )
and: piViγ = pfVf
γ and since:
-p dV = Cv dT
Work done for adiabatic path = Cv (Tf- Ti)
For n mol of gas: w = n Cv (Tf –Ti)
Since piViγ = pfVf γ
piViγ/Ti
= pfVf γ/ Tf
Tf = Ti(Vi/Vf)γ-1
w =n Cv{ ( Ti( Vi/ Vf)γ-1 – Ti}
An adiabatic pathway is much steeper than pV = constant pathway.
Summary
piVi = pfVf for both reversible and irreversible Isothermal processes. For ideal gas: For T =0, U = 0, and H=0 For reversible adiabatic ideal gas processes: q=0 , pVγ = constant and Work done = n Cv{ ( Ti( Vi/ Vf)γ-1 – Ti}
piViγ = pfVf
γ for both reversible and irreversible adiabatic ideal gas.
2nd Law of Thermodynamics
Introduce entropy, S (state function) to explain spontaneous
change ie have a natural tendency to occur- the apparent driving force of spontaneous change is the tendency of energy and matter to become disordered. That is, S increases on
disordering.
2nd law – the entropy of the universe tends to increase.
Entropy
S = qrev /T ( J K-1) at equilibrium
Sisolated system > 0 spontaneous change
Sisolated system < 0 non-spontaneous change
Sisolated system = 0 equilibrium
Properties of S
If a perfect gas expands isothermally from Vi to Vf then since U = q + w = 0 q = -w ie qrev = -wrev and
wrev = - nRT ln ( Vf/Vi)
At eqlb., S =qrev/T = - qrev/T = nRln (Vf/Vi)
ie S = n R ln (Vf/Vi)
Implies that S ≠ 0 ( strange!) Must consider the surroundings.
Surroundings
Stotal = Ssystem + Ssurroundings
At constant temperature surroundings give heat to the system to maintain temperature.
surroundings is equal in magnitude to heat gained or loss but of opposite sign to make
S = 0 as required at eqlb.
Rem: dq = Cv dT and
dS = dqrev / T
dS = Cv dT/ T and
S = Cv ∫ dT /T between Ti and Tf
S = Cv ln ( Tf/ Ti )
When Tf/ Ti > 1 , S is +ve eg. L → G , S is +ve S → L , S is +ve and since qp = H
Smelt = Hmelt / Tmelt and
Svap = Hvap / Tvap
Third Law of Thermodynamics
eg. Standard molar entropy, SmThe entropy of a perfectly
crystalline substance is zero at T = 0
S
m/ J K-1 at 298 K ice 45 water 70 NB. Increasing disorder water vapour 189 For Chemical Reactions: S
rxn = n S (products) - n S
( reactants)
eg. 2H2 (g) + O2( g) → 2H2O( l ), H = - 572 kJ mol-1
Calculation
Ie surroundings take up + 572kJ mol-1 of heat
Srxn = 2S(H2Ol) - (2 S
(H2g ) + S (O2g) )
= - 327 JK-1 mol-1 ( strange!! for a spontaneous reaction; for this S is + ve. ).
Why? Must consider S of the surroundings also. S total = S system + S surroundings
S surroundings = + 572kJ mol-1/ 298K = + 1.92 x 103JK-1 mol-1
S total =( - 327 JK-1mol-1) + 1.92 x 103 = 1.59 x 103 JK-1 mol-1 Hence for a spontaneous change, S > 0
Free Energy, G
Is a state function. Energy to do useful work. Properties Since Stotal = Ssystem + Ssurroundings
Stotal = S - H/T at const. T&p
Multiply by -T and rearrange to give: -TStotal = - T S + H and since G = - T Stotal
ie. G = H - T S
Hence for a spontaneous change: since S is + ve, G = -ve.
Free energy
ie. S > 0, G < 0 for spontaneous change ;
at equilibrium, G = 0.
Can show that : (dG)T,p = dwrev ( maximum work)
G = w (maximum)
Properties of G
G = H - T S dG = dH – TdS – SdT H = U + pV dH = dU + pdV + Vdp Hence: dG = dU + pdV + Vdp – TdS – SdT dG = - dw + dq + pdV + Vdp – TdS – SdT dG = Vdp - SdT
For chemical Reactions:
For chemical reactions
G = n G (products) - n G (reactants) and
Grxn = H
rxn - T Srxn
Relation between Grxn and position of
equilibrium
Consider the reaction: A = B
Grxn = G
B - GA
If GA> G
B , Grxn is – ve ( spontaneous rxn)
At equilibrium, Grxn = 0.
ie. Not all A is converted into B; stops at equilibrium point.
Equilibrium diagram
For non-spontaneous rxn. GB > GA, G is + ve
Gas phase reactions
Consider the reaction in the gas phase: N2(g) + 3H2(g)→ 2NH3(g)
Q =( pNH3 / p)2 /( ( pN2/ p) (pH2/ p)3 ) where :
Q = rxn quotient ; p = partial pressure and p = standard pressure = 1 bar
Q is dimensionless because units of partial pressure cancelled by p .
At equilibrium: Qeqlb = K = (( pNH3 / p)2 / ( pN2/ p) (pH2/ p)3
)eqlb
Activity ( effective concentration)
Define: aJ = pJ / p where a = activity or effective concentration.
For a perfect gas: aJ = pJ / p
For pure liquids and solids , aJ = 1
For solutions at low concentration: aJ = J mol dm-3
K = a2NH3 / aN2 a3
H2
Generally for a reaction: aA + bB → cC + dD
K = Qeqlb = ( acC ad
D / aaA ab
B ) eqlb = Equilibrium constant
Relation of G with K
Can show that:
Grxn = Grxn + RT ln K
At eqlb., Grxn = 0
Grxn = - RT ln K
Hence can find K for any reaction from thermodynamic data.
Can also show that:
ln K = - G / RT
K = e - G / RT
eg H2 (g) + I2 (s) = 2HI (g) , H
f HI = + 1.7 kJ mol-1 at 298K; H
f H2 =0 ; Hf I 29(s)= 0
calculation
Grxn = 2 x 1.7 = + 3.40 kJ mol-1
ln K = - 3.4 x 103 J mol-1 / 8.314J K-1 mol-1 x 298K = - 1.37 ie. K = e – 1.37 = 0.25 ie. p2 HI / pH2 p =0.25 ( rem. p = 1 bar; p 2 / p = p )
p2 HI = pH2 x 0.25 bar
Example: relation between Kp and K
Consider the reaction:
N2 (g) + 3H2 (g) = 2NH3 (g)
Kp =( pNH3 / p)2 / ( pN2/ p )( pH2 / p)3
and
K = [( pNH3 / p)2 / ( pN2/ p )( pH2 / p)3] eqlb
Kp = K (p)2 in this case. ( Rem: (p)2 / (p)4 = p -2
For K >> 1 ie products predominate at eqlb. ~ 103
K<< 1 ie reactants predominate at eqlb. ~ 10-3
K ~ 1 ie products and reactants in similar amounts.
Effect of temperature on K
Since Grxn = - RT ln K = H
rxn - TSrxn
ln K = - Grxn / RT = - H
rxn/RT + Srxn/R
ln K1 = - Grxn / RT1 = - H
rxn/RT1 + Srxn/R
ln K2 = - Grxn / RT2 = - H
rxn/ RT2 + Srxn/ R
ln K1 – ln K2 = - H
rxn / R ( 1/ T1 - 1/ T2 ) 0r
ln ( K1/ K2) = - Hrxn / R ( 1/ T1 - 1/ T2 ) van’t Hoff equation