Notes #2 Chem 341
Isothermal
Isothermal Reversible
Isochoric
U = Q + W (if just PV work then)
Isobaric
U = Q + W
(if just PV work then)
U = Qp + PV





and Qp = 
Adiabatic
U= Q + W but Q =
Specific Processes
Phase Changes:
Defined changes in terms of changes in thermodynamic state, but is what happens at the phase
boundary?
......................................................................................Think about the Phase Diagram
Phase Change
Name
Gas -> liquid
Condensation
Solid -> liquid
Melting, Fusion
Liquid -> Solid
Freezing
Liquid -> Gas
Vaporization
Solid -> Gas
Sublimation
Solid 1 -> Solid 2
Liquid 1 -> Liquid 2
Overheads:
Phase Change
Phase Change
Changes occurring are reversible. If heating or cooling to initiate a phase change, the temp.
remains constant while all of the material is being converted.
For example, at sea level if one takes water at 25oC and heats it at constant pressure, it will heat
up changing temperature to 100oC, its temperature will remain the same until all liquid is
converted to gas, and then the gas is superheated. Extra energy is required to convert the liquid
to gas, before the temp can change.
The extra energy required to convert the liquid water to water vapor is called the _______
_______ ____________________.
Furthermore, since Qp = ........................., then the heat needed for vaporization is just equal to
the
The Work during the phase change will be Wp = - PV
where V is the volume difference between the two phases.
and the internal energy difference for the phase change is just:
Often U or H for the phase change needs to be known at another temperature:
One can apply a HESS’ LAW to determine these values:
HESS’ LAW says: the enthalpy change for an overall process is the sum of the enthalpy
changes of the individual processes into which a reaction may be divided.
Example:
Suppose the enthalpy of vaporization of water at 100oC and 1 atm is know and equal to 40.1
kJ/mol. What is the value of the heat of vaporization of water at 250C and 1atm.
Realize that you can create the process of interest from the follow three process steps:
H2O(l) 25C -> H2O(l)100C
get H from
H2O(l)100C -> H2O(g)100C
Hvap 100C
H2O(g)100C -> H2O(g)25C
get H from
________________________
is the sum
Chemical Reactions can be considered as Processes
A process where there is a chemical change in the system.
Generalize this as:
nA A + nB B  nC C + nD D
Must specify the thermodynamic states of the reactants and the products by specifying variables
like P,V, and T for each.
In general, for any of the state variables F, one can write
F = nC FC + nD FD - nA FA + nB FB
More specifically for the enthalpy:
H = nC HC + nD HD - nA HA - nB HB
THERMOCHEMISTRY
Measurement of the energy a chemical rxn produced as heat yields H or U. Conversely if we
know H or U, then the amount of heat that is required or generated by a process ( a physical
change or a chemical rxn. is known)
Bomb calorimetry
Diagram.
Put Reaction into a bomb calorimeter.
Heat effect is measured by measuring a temp. change in the liquid surrounding the reaction
vessel. The heat capacity of the calorimeter (surroundings) is determined in a separate
experiment by passing a given amount of electrical energy through a heating coil immersed in
the same reaction vessel.
Exothermic? .......................... Endothermic?
Biochemical reactions necessary to sustain life in an adult person produce about ___________
of heat, this must be replenished by the food we eat.
Even in bed, __________ to _____________ kJ/day is needed.
Each gram of food
Type
Energy Provided
Protein
Carbohydrate
Fat
Note energy unit used by nutritionists is Cal 1Cal = 1000 cal = 1kcal
Ideally one should have some tabulated values for the internal energy and enthalpy changes. Use
a Standard State enthalpy change as the change as the enthalpy when its initial and final
substances are in a standard state, its pure form at a specific temp.
Thermodynamicists have adopted the convention of assigning zero enthalpy to all of the
elements in their most stable states at 1 atm or 1 bar pressure. These standard states are
The Standard State of a substance at a specified temperature is its purest form at 1 bar pressure
and some temperature. (usually 298 K) 1 bar = 100,000 Pa = 0.986923 atm
Enthalpies of Transition and Bond Enthalpy Tables contain some of this information.
Enthalpy Changes of Physical Transformation
A(s) -> A(l)
A(l) -> A(g)
A(s) -> A(g)
The last one, osub , is the ___________ of the __________ two.
This is an example of Hess’ Law:
The overall process (reaction) enthalpy is the sum of the enthalpies of the individual ............
Processes Into which the Rxn Enthalpy or Process Enthalpy is Divided
Formation of Solutions:
.................................... A+(g) + B-(g) ->
Formation of Positive Gaseous Ions: (Ionization)
....................................C(g) -> C+(g) + e-(g)
Formation of Negative Ions
.................................. D(g) + e-(g) ->
Lattice Enthalpy: Std. Rxn Enthalpy for the formation of gaseous ions from the crystalline solid:
................................... MX(s) -> M+(g) + X-(g) .................... (all lattice enthalpies are pos.)
Non Ionic Processes (Bond Breaking and Making) Approximate methods
................. A-B (g)
The bond energies can be used to give approx. values for the reaction.
Hess’ Law .......... Hrxno = Ho(bonds broken) - Ho (bonds made) ..............................(why?)
Example: ................................. N2(g)..+ ....3H2(g) ....-> ....2NH3 (g)
See Table
in the book.
Standard Enthalpies of Combustion
Combustion:
................................ Combustible + O2(g) -> products
hydrocarbon + O2(g) ..... -> ...... CO2(g) + ..... H2O(l)
C6H12O6(s) ..+
2CH4 (g) + 3O2 -> 2CO + 4H2O want this one.
CH4 + 2O2 -> CO2 + 2H2O
2CO + O2 -> 2CO2
(all gases?)
Standard Enthalpies of Formation
H0f is the reaction enthalpy for formation of one mole of the product from its elements
in their most stable state at the specified temperature and 1 bar of pressure
........................ 6 Cgraph(s) + 3H2(g) -> C6H6(l) ............................................... TABLES
NOTE: By definition, the std. enthalpies of formation of the elements in their most
stable form at 1 bar are zero at all temps.
HESS’ LAW ALLOWS US TO COMBINE these enthalpies of formation for the
reactants and products in a reaction together to come up the Hrxno for a chemical
reaction of interest.
Example:
Calculate the approximate value of Hrxn for reacting liquid ammonia with gaseous
nitrogen monoxide to make hydrogen peroxide and nitrogen gas. ( Hfo (H2O2(l) = 896.3 kJ/mol.)
More generally:
HESS’ LAW ALLOWS US TO USE ALL OF THESE DIFFERENT TYPES OF
REACTION ENTHALPIES TOGETHER
That is, the enthalpies of formation or combustion, ionization, etc for the reactants and
products can be used to give the enthalpy change for a reaction. This means then that
there may be several ways to calculate the enthalpy change for the reaction.
Examples:
Remember that for a cyclical process, including a cyclical set of reactions, the value of
H and/or U is _________________________.
TEMP DEPENDENCE OF ENTHALPY REVISITED
In the absence of explicit measurements on the enthalpy changes of reaction at different
temps., the reaction enthalpies are determined from the heat capacities of the
_____________ _______
_______________ and from the enthalpy of reaction at some other temp.
For an individual reactant or product:
......................................................... H = ..... 
For the entire reaction then:
............................................ orxn (T2) = Horxn(T1) + ....  rxnCp
............................................... Kirchhoff’s Law
The quantity rxnCp is the difference of the molar heat capacities of .......
For example, for the reaction aA + bB -> cC + dD
rxnCpo = c * Cpm(C) + d * Cpm(D) - a * Cpm(A) - b * Cpm(B)
If Cp has no explicit temp dependence and we know information at 298K, then this
simplifies to:
o (T) = o(298K) + rxnCpo(T - 298)