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Aula #06
Aula #06
Thermodynamics & Thermochemistry
SYSTEM……
Open
mass & energyExchange:
Closed
energy
Isolated
nothing
Thermodynamics & Thermochemistry
Exothermic process
is any process that gives off heat – transfers thermal energy from the system to the surroundings.
Endothermic process
is any process in which heat has to be supplied to the system from the surroundings.
2H2 (g) + O2 (g) 2H2O (l) + energy
H2O (g) H2O (l) + energy
energy + 2HgO (s) 2Hg (l) + O2 (g)
Thermodynamics & Thermochemistry
Thermochemistry
Thermodynamicsis the scientific study of the interconversion of heat and other kinds of energy.
State functions
are properties that are determined by the state of the system, regardless of how that condition was achieved.
Potential energy of hiker 1 and hiker 2 is the same even though they took different paths.
energy , pressure, volume, temperature
DE = Efinal - Einitial
DP = Pfinal - Pinitial
DV = Vfinal - Vinitial
DT = Tfinal - Tinitial
Thermodynamics & Thermochemistry
Thermodynamics & Thermochemistry
First Law of Thermodynamics
Thermodynamics & Thermochemistry
T1 T2 T3T0
Copper, 50 g
T= 50 0C Cp= 0.381 Jg-1K-1water, 200 g
T= 22 0C Cp= 4.814 Jg-1K-1 T= ??? 0C
Thermodynamics & Thermochemistry
First law of thermodynamicsenergy can be converted from one form to another, but cannot be created or destroyed.
DEsystem + DEsurroundings = 0
orDEsystem = -DEsurroundings
DE = q + w
DE is the change in internal energy of a system
q is the heat exchange between the system and the surroundings
w is the work done on (or by) the system… attention to the signal convention
Thermodynamics & Thermochemistry
Thermochemistry
DU = q + w
DE or DU…. is the change in internal energy of a system
q is the heat exchange between the system and the surroundings
w is the work done on (or by) the system
Is a state function…
w….Work and q …Heat are not a state functions!
Conversation between heat and work… !!!!!
Thermochemistry
DU = q + wDE or DU…. is the change in internal energy of a system
Thermochemistry
W = F x d
dW = - p.dV
DV > 0
-PDV < 0
wsys < 0
Work is not a state function!
DW = Wfinal - Winitial
initial final
Heat is not a state function! PV work
dW = - p.dV
W = - p.DV
Thermochemistry
Work is not a state function!
initial final
Heat is not a state function!
dW = - p.dV
Thermochemistry
A sample of nitrogen gas expands in volume from 1.6 L to 5.4 L at constant temperature. What is the work done in joules if the gas expands (a) against a vacuum and (b) against a constant pressure of 3.7 bar?
W = -pDV(a) DV = 5.4 L – 1.6 L = 3.8 L p = 0 atm
W = -0 bar x 3.8 L = 0 L•bar = 0 J
(b) DV = 5.4 L – 1.6 L = 3.8 Lp = 3.7 bar
W = -3.7 bar x 3.8 L = -14.1 L•bar
W = -14.1 kJ
dW = -pdV
Thermochemistry
Work ….p as function of V …
e.g. p=RT/Vp ..constant…isobaric expansion
W1…. W2…. - W2 > - W1dW = -pdV
Thermodynamics &Thermochemistry
H ≡ U + pV (Defined as…)DU = q + w If dV=0… constant volume
dU = dq + dwDU = q
dH ≡ dU + d(pV)
DH = q
If dp=0… constant pressure dH ≡ dU + d(pV)
Work …. dW = -pdV
If dV=0… constant volume
If dp=0… constant pressureDH = q
DU = qWork ….
dW = -pdV
Thermodynamics/Thermochemistry dU = dq + dw
W2….
A Comparison of DH and DU
2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g) DH = -367.5 kJ/molDH
DU
DH = DU + pDV
P = 1 bar, T = 25 ºC
Dn(g) = 1 mol
DH = q
Thermodynamics &Thermochemistry
A Comparison of DH and DU
2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g) DH = -367.5 kJ/molDH
DU DH = DU + pDV
DU = DH - pDV
DU ≡ DH - Dn(g) RT = -367.5 - 2.5 = -370.0 kJ/mol
(pDV = Dn(g)RT)
P = 1 bar, T = 25 ºC
Dn(g) = 1 mol
Thermodynamics &Thermochemistry
dW = -pdV
Thermochemistry
W = - pdVdW = -pdV W = - (area)
w….Work is not a state function!
Aula_#06. Stirling engine
A Stirling engine is a heat engine that operates by
cyclic compression and expansion of air or other gas (the working fluid) at different temperatures
e.g. Conversion of heat energy to mechanical work
Aula_#06. Energy tranformations !!! converting electric energy to light (radiative energy)
converting electric energy Work (mechanical energy)
Aula_#06. Energy tranformations !!!Heat pump !!
