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 HEAT
q < 0 q > 0
Exothermic EndothermicNils Walter: Chem 260 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
•• TheThe changechange inin internalinternal energyenergy ((∆∆UU)) ofof aa closedclosed systemsystem isis equalequal to to the the sum sum of of the the heat heat ( (q)q) added added to to it it and and the the work work ( (w)w) done done upon upon it it •• The The internal internal energy energy of of an an isolatedisolated system system isis constantconstant
∆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 of state functionsNils Walter: Chem 260 Internal Energy can be exchanged with the surroundings as heat or work Closed system, ∆U = q + w expansion against external pressure Closed system, constant volume
Heat is stored as internal energy and released as q↑ heat introduced volume-pressure work [J] T↑ ∆U = q + w w = -Fdx = -p∆V ∆ = q - pex V w = 0; no work done Þ ∆ U = qv 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: V ∆ ò 2 U = q + w = qp - p dV V1 V ò 2 ∆ = qp - p dV = qp - p(V2-V1) = qp - p V V p independent of V 1 Enthalpy is the heat transferred ∆Η = ∆U + p∆V = q in a process at constant pressure p (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 HEAT
q < 0 q > 0
∆ Nils ∆Walter: Chem 260 Exothermic H < 0 Endothermic H > 0 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
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
s
e Standard Formation Reaction: Formation of
i
p l one mole of a substance from the elements in
a
h
t their standard states.
n
e
CS +87.86 kJ/mol →
n 2 (l) C + 2 S CS
r (graphite) (s, rhombic) 2(l)
o
a
i
l
t
o a C C
m (graphite) (graphite)
m 0
r
d
o O , S r 2 (g) (s, rhombic)
f
a
f
d
o n CO -110.52 kJ/mol → a (g) C(graphite) + 1/2 O2 (g) CO(g)
t
s
= ∅
H
f CO -393.51 kJ/mol → 2 (g) NilsC(graphite) Walter: + O Chem2 (g) 260 CO2(g)
∆ ∆
∆ ∆