THE UNIVERSITY
OF QUEENSLAND
Foundation Year
THERMOCHEMISTRY II
Lesson Overview
Thermochemistry
Heat
Capacity
Specific heat
capacity
Endothermic and
Exothermic Equations
Calorimetry
Thermochemical
Equations
Heats of Changes
of State
Hess’s Law
Standard Heats of Formation
Thermochemical Equations 1
• A thermochemical equation is one that
includes energy changes.
• In exothermic reactions, heat is a product (it's
being formed), so a reaction of this kind might
look like this:
A + B ---> C + D + heat
• And similarly, if a reaction is endo, then it acts
like a reactant (goes on the left side):
A + B + heat ---> C + D
Thermochemical Equations 2
• There are two ways to write a
thermochemical equation:
• 2 C2H6(g) + 7 O2(g) 4 CO2(g) + 6 H2O(g)
H = -2855 kJ
or
• 2 C2H6(g) + 7 O2(g)  4 CO2(g) + 6 H2O(g)
+ 2855 kJ
• Are these reactions exothermic or
endothermic?
Thermochemical Equations 3
• For endothermic reactions, energy must
be added to the reactants to make it
happen.
• Heat may be considered as a reactant.
• 2 NH3(g) + 92 kJ  N2(g) + 3 H2(g)
or
• 2 NH3(g)  N2(g) + 3 H2(g) H = +92 kJ
• When writing thermochemical equations,
the state symbols must be included. This
is because changing the state of a
Heat and Changes of State (1)
• All solids absorb heat in melting to liquids. The
heat absorbed by one mole of a substance in
melting from a solid to a liquid at a constant
temperature is called the molar heat of fusion
(Hfus.).
• The heat lost when one mole of a liquid changes
to a solid at a constant temperature is the molar
heat of solidification (Hsolid. )
Heat and Changes of State (2)
• The amount of heat absorbed by one mole
of a liquid that is undergoing evaporation is
called the molar heat of vaporisation.
( Hvap)
• The condensation of 1 mole of vapour
releases heat as the molar heat of
condensation (Hcond).
Vapour
 Hcond = -ve
 Hvap = +ve
Enthalpy
Liquid
 Hsolid = -ve
 Hfus = +ve
Solid
Heat of Solution
• Heat changes can occur when a substance is
dissolved in a solvent. The heat change
caused by dissolution of one mole of
substance is the molar heat of solution,
Hsoln.
Exothermic Solvation
http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnProc/Hsolv1gif.gif
Endothermic Solvation
http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnProc/SolnProc.htm
http://www.wou.edu/las/physci/ch412/heatsoln.gif
Thermochemical Equation
Calculations
• Thermochemical equations obey several simple
rules that make computation of the enthalpy
change in a reaction easy.
1. The magnitude of H is directly proportional
to the amount of reactants used in the reaction
2. H for a reaction is equal in magnitude but
opposite in sign to the reverse reaction.
3. Hess' Law: The value of H for a reaction is
the same no matter what path is used to get
from reactants to products.
Hess’s Law of Summation
•
Hess's Law of Heat Summation states:
If you add two or more thermochemical
equations to give a final equation, then you
can also add the heat changes to give the
final heat changes.
Hess’s Law of Summation(2)
To find the enthalpy for:
• C(s, diamond)  C(s, graphite)
• C(s, graphite)+ O2(g)  CO2(g)  H = -393.5kJ
• C(s, diamond)+ O2(g)  CO2(g)  H =-395.4kJ
Write the reverse of equation (a) to give
• CO2(g)  C(s, graphite)+O2(g)
H
= +393.5kJ
Hess’s Law of Summation(3)
• Adding the equations should give us our
original equation.
• Now do the same thing with the enthalpy
values :i.e.: H = -395.4kJ
+ H = +393.5kJ
H = -1.9kJ
Heats of Formation
• The standard heat of formation (Hf) of a
compound is the change in enthalpy that
accompanies the formation of one mole of the
compound from its elements with all substances in
their standard states at 25oC.
• The H for a reaction is the difference between the
standard heats of formation of all the reactants and
products.
• ie: H
=  Hf(products) -  Hf(reactants)
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References
1.
2.
3.
4.
5.
6.
http://www.chem.vt.edu/RVGS/ACT/notes/Chap_8_Trip
tik.html
http://apchem.virtualave.net/concepts/thermochem.html
http://chemed.chem.purdue.edu/demos/movies/small_mo
vies/5.2small.mov
http://www.wou.edu/las/physci/ch412/heatsoln.gif
http://wine1.sb.fsu.edu/chm1046/notes/SolnProp/SolnPr
oc/Hsolv1gif.gif
http://cwx.prenhall.com/petrucci/medialib/media_portfol
io/text_images/
End of Lecture