Thermochemistry
The study of energy changes that occur during chemical
________ :
¾ at constant volume
ΔU = qV
no ________
¾ at constant pressure
ΔH = qP
only ______ work
For practical reasons most measurements are made at
constant _____, so thermochemistry mostly deals with ΔH.
ΔH reaction =
∑
H−
products
∑
H
reactants
If ΔH > 0 the reaction is ________ .
If ΔH < 0 the reaction is ________ .
For comparison purposes we need to refer ΔH 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
9/20/2006
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.
F
ΔH1
ΔH 2
F → B + G
A + D → E + G
ΔH 3
ΔH
A + B → C
C + D → E +
ΔH = ΔH1 + ΔH 2 + ΔH 3
Standard Reaction Enthalpy
ΔH o
reaction enthalpy at ________
o
ΔH 298
… and at ________ T
o
ΔH 500
or some other T
© Paul Percival
Modified by Jed Macosko
9/20/2006
Reaction Enthalpy 2
Standard (molar) enthalpy of ________
ΔH fo ≡ Δ f H o
Heat of formation of a substance from its elements, all
substances being in their standard state.
By definition, for all ________ ΔH fo = 0
Enthalpy of ________
ΔH° for total oxidation of a substance
e.g. C6H12O6 + 6O2 → 6CO2 + 6 H2O
ΔH co ≡ Δ c H o
ΔcH° = -2808 kJ mol-1
Enthalpy of ________
ΔH° when an unsaturated organic compound becomes fully
saturated
e.g. C6H6 + 3H2 → C6H12
ΔH° = -246 kJ mol-1
Enthalpy of ___________ ≡ Bond dissociation enthalpy
ΔH° for the dissociation of a molecule into its constituent
gaseous atoms
e.g. C2H6 (g) → 2C(g) + 6H(g)
ΔH° = 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. ΔH°(C2H6) = ΔH°(C-C) + 6 ΔH°(C-H)
© Paul Percival
Modified by Jed Macosko
9/20/2006
Temperature Dependence of ΔH°
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
∴ ΔH (T2 ) = ΔH (T1 ) + ∫ ___ C p dT
T1
where ΔC p =
∑
Cp −
products
∑
Cp
reactants
This general phenomenon is known as Kirchoff’s Law.
e.g.
A + B → C + D
↑
↑
↓
↓
A + B → C + D
ΔH ° (T1 )
ΔH ° (T2 )
ΔH °(T2 ) = ( C p (A) + C p (B) ) (T1 − T2 )
+ΔH °(___)
+ ( C p (C) + C p (D) ) ( ___ − ___ )
= ΔH °(T1 ) +
(∑C−
p
products
C
∑
reactants
p
) ___
assuming that the Cp values are ______ independent.
© Paul Percival
Modified by Jed Macosko
9/20/2006
Reactions at Constant Volume
ΔH r = ΔU r + ( PV )products − ( PV )reactants
For ________ and
liquids
For ideal gases
Δ ( PV ) ≈ 0, so ΔH ≈ ΔU
Δ ( PV ) = Δngas RT ,
so ΔH ≈ ΔU + Δngas RT
e.g. C3 H 6 (g) + 92 O 2 (g) → 3CO 2 (g) + 3H 2 O(l)
ΔH r = ΔU r + ( -___ ) RT
The relationship between ______ and ΔU is particularly
important when relating thermochemical enthalpies (ΔH)
to molecular properties (Umolecular),
e.g. for a single bond energy ΔU = ΔH – RT
as seen in the case of O2(g) → 2O(g).
In practice, ________ is usually so much smaller than ΔH
that it is often ignored.
© Paul Percival
Modified by Jed Macosko
9/20/2006
Enthalpies of Ions in Solution
Enthalpy of ________ ΔH° for solution of a substance in a
stated amount of solvent
Enthalpy of ________ ΔH° for dilution of a solution to a
lower concentration
o
Enthalpy of solution to ______ dilution ΔH soln
for an
infinite amount of solvent
The enthalpy of formation for a species in ________ can be
o
found by combining ΔH soln
with the ΔH fo of the ________
species:
1
1
ΔH fo = −92.31 kJ mol-1
2 H 2 (g) + 2 Cl 2 (g) → HCl(g)
HCl(g) → HCl(aq)
1
2
o
ΔH soln
= −75.14 kJ mol-1
o
H 2 (g) + 12 Cl2 (g) → HCl(aq) ΔH fo (ion) = ΔH fo + ΔH soln
= −167.45 kJ mol-1
ΔH 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 –
ΔH fo ( H a+q ) = 0
–
ΔH fo ( Claq
) = ΔH fo ( HClaq ) − ΔH fo ( Haq+ ) = ΔH 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
9/20/2006
Enthalpy of Formation of an Ionic Solid
Consider individual steps in the formation of NaCl.
1.
o
ΔH subl
( Na )
2.
ΔH o = __ ( Na ) + ________
1
2
3.
ΔH o ( Cl-Cl )
ΔH o = − __ ( Cl ) − RT
4.
5.
≡ NaCl(aq)
o
ΔH sol
( Na + ) + ΔH solo ( Cl− )
ΔH fo ( NaClaq )
Na(s) + ½Cl2(g) → NaCl(aq)
o
ΔH fo ( NaClaq ) = ΔH subl
( Na ) + I (Na) + 12 ΔH o ( Cl-Cl )
o
− EA (Cl) + ΔH sol
( Na + ) + ΔH solo ( Cl− )
Step 5 could be creation of solid NaCl instead of solution
o
ΔH lattice
( NaCl )
5'.
leading us to the enthalpy of formation of solid NaCl:
Na(s) + ½Cl2(g) → NaCl(s)
o
ΔH fo ( NaCls ) = ΔH subl
( Na ) + I (Na) + 12 ΔH o ( Cl-Cl )
o
− EA (Cl) + ΔH lattice
( NaCl )
© Paul Percival
Modified by Jed Macosko
9/20/2006
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
ΔH subl
( Na )
Na+(g) + Cl–(g)
Na(s) + Cl(g)
1
2
ΔH o ( Cl-Cl )
Na(s) + ½Cl2(g)
o
ΔH sol
o
ΔH lattice
ΔH fo ( NaClaq )
NaCl(aq)
ΔH 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
9/20/2006