Transcript chapter2-4.ppt

Chapter 2
The First Law
Unit 4 thermochemistry
Spring 2009
Thermochemistry
• The study of the energy transferred as heat
during the course of chemical reactions is
called thermochemistry.
Thermochemistry
• we can use calorimetry to measure the energy
supplied or discarded as heat by a reaction.
• We can identify q with a change in internal
energy DU (if the reaction occurs at constant
volume) or a change in enthalpy DH (if the
reaction occurs at constant pressure).
Thermochemistry
• an exothermic process at constant pressure
∆H < 0.
• an endothermic process at constant pressure
∆H > 0.
Standard state
• The standard state of a substance at a specified
temperature is its pure form at 1 bar.
• standard state of liquid ethanol at 298 K is pure liquid
ethanol at 298 K and 1 bar;
• the standard state of solid iron at 500 K is pure iron at
500 K and 1 bar.
• The standard enthalpy change for a reaction or a
physical process is the difference between the products
in their standard states and the reactants in their
standard states, all at the same specified temperature.
Enthalpies of physical change
• The standard enthalpy change that accompanies a
change of physical state is called the standard
enthalpy of transition and is denoted ∆trsH .
The standard enthalpy of vaporization,∆vapH .
The standard enthalpy of fusion, ∆fusH .
Exercise 2.16b
• A certain liquid has ∆vapH = 32.0 kJ mol−1.
Calculate q, w, ∆H, and ∆U when 0.75 mol is
vaporized at 260 K and 765 Torr.
Enthalpies of physical change
• Because enthalpy is a state
function, a change in enthalpy
is independent of the path
between the two states.
• the conversion of a solid to a
vapour either as occurring by
sublimation or as occurring in
two steps, first fusion (melting)
and then vaporization of the
resulting liquid.
Enthalpies of physical change
The standard enthalpy changes of a
forward process and its reverse
differ in sign.
Enthalpies of chemical change
Standard enthalpy of combustion,
o
∆ cH
• standard enthalpy of combustion, ∆cHo, is the
standard reaction enthalpy for the complete
oxidation of an organic compound to CO2 gas
and liquid H2O.
Standard enthalpy of combustion,
o
∆ cH
Hess’s law
• The standard enthalpy of an overall reaction is
the sum of the standard enthalpies of the
individual reactions into which a reaction may
be divided.
Example 2.5 Using Hess’s law
The standard reaction enthalpy for the hydrogenation of
propene is −124 kJ mol−1.
The standard reaction enthalpy for the combustion of
propane is −2220 kJ mol−1.
Calculate the standard enthalpy of combustion of propene.
Self Test 2.6
Calculate the enthalpy of hydrogenation of
benzene from its enthalpy of combustion and
the enthalpy of combustion of cyclohexane.
C6 H 6 (l ) 
15
O 2 (g)  6 CO 2 (g)  3 H 2 O (l )
2
C6 H12 (l )  9O 2 (g)  6 CO 2 (g)  6 H 2 O (l )
C6 H6 (l )  3H 2 (g)  C6 H12 (l )
Standard enthalpy of formation
∆fHo
Standard enthalpy of formation, ∆fHo, of a
substance is the standard reaction enthalpy for
the formation of the compound from its
elements in their reference states.
The standard enthalpies of formation of elements in their reference states are zero at
all temperatures
the hydrogen ion in solution has zero standard enthalpy of formation at all
temperatures
Standard enthalpy of formation
o
∆fH
Illustration 2.7
Using standard enthalpies of formation
Calculate The standard reaction enthalpy of
2 HN3(l) + 2 NO(g) → H2O2(l) + 4 N2(g)
Exercise 2.17b
The standard enthalpy of formation of phenol
(C6H5OH) is −165.0 kJ mol−1. Calculate its
standard enthalpy of combustion.
Exercise 2.18a
The standard enthalpy of combustion of
cyclopropane is −2091 kJ mol−1 at 25°C. From
this information and enthalpy of formation data
for CO2(g) and H2O(g),
(a) calculate the enthalpy of formation of
cyclopropane.
(b) The enthalpy of formation of propene is
+20.42 kJ mol−1. Calculate the enthalpy of
isomerization of cyclopropane to propene.
Exercise 2.18a
• From the following data, determine ∆fH for
diborane,B2H6(g), at 298 K:
The temperature-dependence of
reaction enthalpies
Kirchhoff’s law
Example 2.6 Using Kirchhoff’s law
The standard enthalpy of formation of gaseous H2O
at 298 K is −241.82 kJ mol−1.
Estimate its value at 100°C given the following
values of the molar heat capacities at constant
pressure:
H2O(g): 33.58 J K−1 mol−1;
H2(g): 28.84 J K−1 mol−1;
O2(g): 29.37 J K−1 mol−1.
Assume that the heat capacities are independent of
temperature.
• Estimate the standard enthalpy of formation
of cyclohexene at 400 K.