Transcript Thermochemistry The study of energy changes that occur during chemical ________ :  at constant volume DU = qV no ________  at constant pressure DH =

Thermochemistry
The study of energy changes that occur during chemical
________ :
 at constant volume
DU = qV
no ________
 at constant pressure
DH = qP
only ______ work
For practical reasons most measurements are made at
constant _____, so thermochemistry mostly deals with DH.
DH reaction 

H
products

H
reactants
If DH > 0 the reaction is ________ .
If DH < 0 the reaction is ________ .
For comparison purposes we need to refer DH to the
same ____ and ____ . To define a standard reaction
enthalpy each component of the reaction must be in its
________ ________ – the most stable form at 1 bar
pressure and (usually) 25°C.
1 bar = 105 Pa
© Paul Percival
1 atm = 1.01325 bar
Modified by Jed Macosko
4/26/2020
Reaction Enthalpy 1
Hess’s Law
The standard enthalpy change in any reaction can be
expressed as the ________ of the standard enthalpy changes,
at the same temperature, of a ________ of reactions into which
the overall reaction can be formally divided.
Combine chemical equations as if ___________ equations,
e.g.
DH1
A + B  C
F
DH 2
 B + G
DH 3
A + D  E + G
DH
C + D  E +
F
DH  DH1  DH 2  DH3
Standard Reaction Enthalpy
DH o
reaction enthalpy at ________
o
DH 298
… and at ________ T
o
DH 500
or some other T
© Paul Percival
Modified by Jed Macosko
4/26/2020
Reaction Enthalpy 2
Standard (molar) enthalpy of ________
DH fo  Df H o
Heat of formation of a substance from its elements, all
substances being in their standard state.
By definition, for all ________ DH fo  0
Enthalpy of ________
DH° for total oxidation of a substance
e.g. C6H12O6 + 6O2  6CO2 + 6 H2O
DH co  Dc H o
DcH° = -2808 kJ mol-1
Enthalpy of ________
DH° when an unsaturated organic compound becomes fully
saturated
e.g. C6H6 + 3H2  C6H12
DH° = -246 kJ mol-1
Enthalpy of ___________  Bond dissociation enthalpy
DH° for the dissociation of a molecule into its constituent
gaseous atoms
e.g. C2H6 (g)  2C(g) + 6H(g)
DH° = 2883 kJ mol-1
Bond ________  single bond enthalpy
An average value taken from a series of compounds and
often combined for a ________ estimate
e.g. DH°(C2H6) = DH°(C-C) + 6 DH°(C-H)
© Paul Percival
Modified by Jed Macosko
4/26/2020
Temperature Dependence of DH°
The temperature dependence of reaction enthalpies can be
expressed in terms of the T dependence of the enthalpies
of the reaction __________ :
T2
H (T2 )  H (T1 )   ___dT
T1
T2
 DH (T2 )  DH (T1 )   ___ C p dT
T1
where DC p 

Cp 
products

Cp
reactants
This general phenomenon is known as Kirchoff’s Law.
e.g.
A  B  C  D



DH  T1 

A  B  C  D
DH  T2 
DH (T2 )   C p (A)  C p (B)  T1  T2 
DH (___)
  C p (C)  C p (D)   ___  ___ 
 DH (T1 ) 
C
p
products

reactants

C p ___
assuming that the Cp values are ______ independent.
© Paul Percival
Modified by Jed Macosko
4/26/2020
Reactions at Constant Volume
DH r  DU r   PV products   PV reactants
For ________ and
liquids
For ideal gases
D  PV   0, so DH  DU
D  PV   Dngas RT ,
so DH  DU  Dngas RT
e.g. C3H6 (g)  92 O2 (g)  3CO2 (g)  3H2O(l)
DH r  DU r   -___  RT
The relationship between ______ and DU is particularly
important when relating thermochemical enthalpies (DH)
to molecular properties (Umolecular),
e.g. for a single bond energy DU = DH – RT
as seen in the case of O2(g)  2O(g).
In practice, ________ is usually so much smaller than DH
that it is often ignored.
© Paul Percival
Modified by Jed Macosko
4/26/2020
Enthalpies of Ions in Solution
Enthalpy of ________ DH° for solution of a substance in a
stated amount of solvent
Enthalpy of ________ DH° for dilution of a solution to a
lower concentration
o
Enthalpy of solution to ______ dilution DH soln
for an
infinite amount of solvent
The enthalpy of formation for a species in ________ can be
o
found by combining DH soln
with the DH fo of the ________
species:
1
1
DH fo  92.31 kJ mol-1
2 H 2 (g)  2 Cl 2 (g)  HCl(g)
HCl(g)  HCl(aq)
1
2
o
DHsoln
 75.14 kJ mol-1
o
H 2 (g)  12 Cl2 (g)  HCl(aq) DH fo (ion)  DH fo  DH soln
 167.45 kJ mol-1
DH fo for individual ions in solution can only be found if one
is arbitrarily fixed. By convention this is ________ .
1
2
H 2 (g)  H + (aq)  e –
DH fo  H aq   0
–
DH fo  Claq
  DH fo  HClaq   DH fo  Haq   DH fo  HClaq 
The ________ state for a substance in solution (not just ions)
is a concentration of 1 mole solute in 1 kg solution (1 molal).
© Paul Percival
Modified by Jed Macosko
4/26/2020
Enthalpy of Formation of an Ionic Solid
Consider individual steps in the formation of NaCl.
1.
o
DH subl
 Na 
2.
DH o  __  Na   ________
1
2
3.
DH o  Cl-Cl 
DH o   __  Cl   RT
4.
5.
 NaCl(aq)
o
DH sol
 Na    DH solo  Cl 
DH fo  NaClaq 
Na(s) + ½Cl2(g)  NaCl(aq)
o
DH fo  NaClaq   DH subl
 Na   I (Na)  12 DH o  Cl-Cl 
o
 EA (Cl)  DH sol
 Na    DH solo  Cl 
Step 5 could be creation of solid NaCl instead of solution
o
DH lattice
 NaCl 
5'.
leading us to the enthalpy of formation of solid NaCl:
Na(s) + ½Cl2(g)  NaCl(s)
o
DH fo  NaCls   DH subl
 Na   I (Na)  12 DH o  Cl-Cl 
o
 EA (Cl)  DH lattice
 NaCl 
© Paul Percival
Modified by Jed Macosko
4/26/2020
A ____________ Cycle for NaCl
Enthalpy changes can also be expressed in a diagram, e.g.
Na+(g) + e– + Cl(g)
I  Na   RT
EA  Cl   RT
Na(g) + Cl(g)
o
DH subl
 Na 
Na+(g) + Cl–(g)
Na(s) + Cl(g)
1
2
DH o  Cl-Cl 
Na(s) + ½Cl2(g)
o
DH sol
o
DH lattice
DH fo  NaClaq 
NaCl(aq)
DH fo  NaCls 
NaCl(s)
Since H is a state variable, the sum of enthalpy changes
around the cycle must be _______ . Consequently, if all but
one of the enthalpy changes is known, it can be readily
calculated.
This is equivalent to using _______ Law to sum reaction steps.
© Paul Percival
Modified by Jed Macosko
4/26/2020