Thermodynamics: The First Law
Atkins, Chapter 2
• Open System: Mass, heat, energy flow freely
• Closed System: Heat, energy flow freely
• Isolated System: No mass, heat, or energy flow
System
System
System
HEAT
q<0
Exothermic
HEAT
q>0
Nils Walter: Chem 260
Endothermic
Internal Energy U
Internal Energy U:
The sum of all of the kinetic and potential energy contributions to the energy
of all the atoms, ions, molecules, etc. in the system
He gas
Methanol Gas
Translational Energy
Rotational Energy
Electronic Energy
Vibrational Energy
Nuclear Energy
Bond Energy
Nils Walter: Chem 260
The First Law of Thermodynamics:
Internal Energy is Conserved
••The
∆∆U)
Thechange
changein
ininternal
internalenergy
energy(∆
(∆
U)of
ofaaclosed
closedsystem
systemisis
equal
equalto
tothe
thesum
sumof
ofthe
theheat
heat(q)
(q)added
addedto
toititand
andthe
thework
work(w)
(w)
done
doneupon
uponitit
••The
Theinternal
internalenergy
energyof
ofan
anisolated
isolatedsystem
systemisisconstant
constant
∆U = q + w
For a Closed System
∆U = 0
For an Isolated System
Internal energy U is a state function
Þ Quantity is independent of path
Volume, Temperature, Pressure,
and Quantity are other examples
Nils Walter: Chem 260
of state functions
Internal Energy can be exchanged with
the surroundings as heat or work
∆U = q + w
Closed system,
constant volume
q↑ heat introduced
Closed system,
expansion against
external pressure
Heat is stored as internal
energy and released as
volume-pressure work [J]
T↑
↑
w = -Fdx = -p∆
∆V
w = 0; no work done
Þ ∆U = q v
∆U = q + w
= q - pex∆V
Nils Walter: Chem 260
Internal Energy and Enthalpy
Enthalpy definition:
Η = U + pV
Most convenient for processes at constant pressure:
•Cooking dinner
• Digesting dinner
•Drying the laundry
•Synthesizing a compound in lab
At constant pressure, if only pV work is done:
V2
∆U = q + w = qp - ò p dV
V1
V2
p independent of V
= qp - pòò dV = qp - p(V2-V1) = qp - p∆
∆V
V1
∆Η = ∆U
∆V = q p
∆ + p∆
Enthalpy is the heat transferred
in a process at constant pressure
(assuming only pV work)
Nils Walter: Chem 260
Enthalpy
• Open System: Mass, heat, energy flow freely
• Closed System: Heat, energy flow freely
• Isolated System: No mass, heat, or energy flow
System
System
System
HEAT
q<0
Exothermic ∆H < 0
HEAT
q>0
Nils ∆
Walter:
H > 0Chem 260
Endothermic
Enthalpy and Internal Energy are
State Functions
We only need be concerned with the change in enthalpy (∆
∆H) or
change in internal energy (∆
∆U), not the path of how we got there
∆fH = standard molar enthalpies
of formation
Þ We can arbitrarily assign H = 0 for each element in its
standard state = state of aggregation at p = 1 bar, T = 298.15 K
∅
CS2 (l)
Standard Formation Reaction: Formation of
one mole of a substance from the elements in
their standard states.
+87.86 kJ/mol
C(graphite) + 2 S(s, rhombic) → CS2(l)
C(graphite) C(graphite)
O2 (g),
S(s, rhombic)
CO (g) -110.52 kJ/mol
CO2 (g) -393.51 kJ/mol
0
C(graphite) + 1/2 O2 (g) → CO(g)
C(graphite)
+O
CO2(g)
Nils
Walter:
Chem
260
2 (g) →