calculating enthalpy changes - nc state: www4...
TRANSCRIPT
![Page 1: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/1.jpg)
Temperature and pressure
dependence of K
Chemistry 201
NC State University
Lecture 10
![Page 2: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/2.jpg)
Factors that affect K
• How does the free energy relate to the
standard state?
• How does temperature change affect
energetics and equilibrium?
• What is the pressure dependence of K?
Text : Sections 4.7, 5.1
![Page 3: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/3.jpg)
Relationship of free energy
to standard state The free energy change for a reaction, DG,
is composed of individual free energies Gn
that correspond to each reactant. We can
write the free energy of each component as
where an is the activity of component n and
DGno is the standard free energy. Activity
means concentration, but it is used for “real”
solutions. See the next slide for comparison.
![Page 4: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/4.jpg)
Definition of activity
Activity is a concentration and has units of
molarity. We use activity to account for the
fact that there may an effective or non-ideal
concentration due to solvent effects. You
May also think of the free energy as follows:
where [n] is the effective concentration (activity).
![Page 5: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/5.jpg)
Combining free energies to make a
free energy change
The molar free energy change of a reaction
is given by the difference between the free
energy of the products minus the reactants.
For the hypothetical reaction
we have
![Page 6: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/6.jpg)
Combining free energies to make a
free energy change (cont’d)
Each free energy is related to the standard
free energy for that species so we have
We can rearrange this to
and finally
![Page 7: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/7.jpg)
The standard state
Thus, the standard free energy change is
the difference in free energy of each species
at its standard state,
And the reaction quotient is the ratio of each
concentration to the standard concentration,
which is 1 molar (so it does not appear).
![Page 8: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/8.jpg)
Defining the reaction quotient
The reaction quotient is unitless. The units of the
individual activities (concentrations) are cancelled
by the units of the standard state.
![Page 9: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/9.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
At what temp. does DGo = 0?
(K = 1)
Temperature dependence of K
![Page 10: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/10.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
At what temp. does DGo = 0?
(K = 1)
Temperature dependence of K
![Page 11: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/11.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
At what temp. does DGo = 0?
(K = 1)
Temperature dependence of K
![Page 12: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/12.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
At what temp. does DGo = 0?
(K = 1)
Temperature dependence of K
![Page 13: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/13.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
At what temp. does DGo = 0?
(K = 1)
Temperature dependence of K
![Page 14: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/14.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
What is K at 425 K?
Temperature dependence of K
![Page 15: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/15.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
What is K at 425 K?
Temperature dependence of K
![Page 16: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/16.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
What is K at 425 K?
Temperature dependence of K
![Page 17: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/17.jpg)
N2O4(g) 2 NO2(g)
DHo = +58.20 kJ DSo = +176.6 J/K
What is K at 425 K?
Temperature dependence of K
Remember to convert DHo to units of J (multiply by 1000)
![Page 18: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/18.jpg)
H2(g) + 1/2 O2 (g) H2O (g)
DHo = -241.82 kJ DSo = -44.38 J/K
At what T, if any, does K = 1?
Temperature dependence of K
![Page 19: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/19.jpg)
H2(g) + 1/2 O2 (g) H2O (g)
DHo = -241.82 kJ DSo = -44.38 J/K
At what T, if any, does K = 1?
Temperature dependence of K
The temperature corresponds to DGo = 0
![Page 20: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/20.jpg)
H2(g) + 1/2 O2 (g) H2O (g)
DHo = -241.82 kJ DSo = -44.38 J/K
Evaluate K at 1500 K.
Temperature dependence of K
![Page 21: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/21.jpg)
H2(g) + 1/2 O2 (g) H2O (g)
DHo = -241.82 kJ DSo = -44.38 J/K
Evaluate K at 1500 K.
Temperature dependence of K
![Page 22: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/22.jpg)
The temperature dependence of DGo
As we have shown previously, DG, will decrease until it
reaches 0. Then we have reached equilibrium. The
equilibrium condition is
DGo = -RT ln K
Next we consider the fact that we can use the temperature
dependence of the free energy to obtain information about
the enthalpy.
DHo - TDSo = -RT ln K
If we assume that DHo and DSo are independent of
temperature, then we can obtain the values of K at two
temperatures as follows,
DHo – T1DSo = -RT1 ln K1
DHo – T2DSo = -RT2 ln K2
![Page 23: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/23.jpg)
The temperature dependence of DGo
Then we can divide each equation by its respective
temperature to obtain,
DHo /T1 – DSo = - R ln K1
DHo /T2 – DSo = - R ln K2
We subtract temperature T2 from T1.
DHo (1/T1 – 1/T2) = - R ln(K1/K2)
ln(K2/K1) = -DHo /R(1/T2 – 1/T1)
This equation says that if we plot ln(K) vs 1/T, we obtain a
line, and the slope of that line is -DHo /R.
![Page 24: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/24.jpg)
2 NO2 (g) 2 NO (g) + O2 (g)
T1 at 190 K K1 = 18.4
T2 at 200 K K2 = 681
find DHo and DSo
Using equilibrium data to obtain DHo and DSo
![Page 25: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/25.jpg)
Solution: starting with the equation
Solve for DHo
Substitute in the given values
Using equilibrium data to obtain DHo and DSo
![Page 26: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/26.jpg)
Van’t Hoff plots
The standard method for obtaining the
reaction enthalpy is a plot of ln K vs. 1/T
Slope = -DHo/R
Note: DHo > 0
![Page 27: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/27.jpg)
Van’t Hoff plot for drug binding
A practical example of the application of the
van’t Hoff equation can be found in drug binding.
The equilibrium constant for drug binding to an
active site can be measured by fluorescence,
NMR, etc. at various temperatures. Then one
may plot ln K vs. 1/T and fit the result to a line.
In most cases the binding will be exothermic to
that DHo < 0 and then slope of the line will be
positive rather than negative as shown in the
previous slide.
![Page 28: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/28.jpg)
Van’t Hoff plot for drug binding
In this example, the slope is positive because the enthalpy
of binding is negative (i.e. binding is exothermic).
Slope = -DHo/R
Note: DHo < 0
![Page 29: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/29.jpg)
Example: preventing inflammation by binding to prostaglandin synthase 2
COX-2 crystal structure
DHo and DSo can be measured
using ln(K) as a function 1/T.
![Page 30: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/30.jpg)
Kinetic and Thermodynamic Assessment of Binding of Serotonin Transporter
Inhibitors. J Pharm Exp Tech (2008) vol. 327, pp. 991-1000
Example: preventing depression serotonin transport inhibitors
DHo and DSo can be measured using ln(K) as a function 1/T.
This article shows specific differences in the enthalpy of
binding of drugs based on analysis of so-called van’t Hoff
plots (i.e. plots of ln(K) vs. 1/T).
![Page 31: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/31.jpg)
Kinetic and Thermodynamic Assessment of Binding of Serotonin Transporter
Inhibitors. J Pharm Exp Tech (2008) vol. 327, pp. 991-1000
Basic conclusion: fluvoxamine binds exclusively based on entropic driving force
Entropically driven binding is relatively rare. Usually the
entropy of binding is unfavorable since a flexible drug
molecule (large W) will be forced to adopt a fixed
conformation (small W or W = 1) upon binding to a protein.
![Page 32: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/32.jpg)
Ligand binding in myoglobin Ligand binding to myoglobin is described as a chemical equilibrium
between a bound state MbCO, a dissociated state Mb:CO and a
solvent state Mb + CO.
Mb + CO Mb:CO MbCO
If we ignore the intermediate state the binding can be described by
An overall equilibrium process.
Mb + CO MbCO
with equilibrium constant K. We have:
The fraction bound is:
K =[MbCO]
[Mb][CO]
f =[MbCO]
[Mb] + [MbCO]
We use CO in
Many studies.
The same equations
Hold for O2.
![Page 33: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/33.jpg)
Ligand binding curve The fraction bound f can be related to the binding constant K:
This type of binding curve is plotted below.
K =[MbCO]
[Mb][CO]
f =[MbCO]
[Mb]+[MbCO]=
K[Mb][CO]
[Mb] + K[Mb][CO]=
K[CO]
1 + K[CO]
![Page 34: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/34.jpg)
Pressure dependence of species
We can see from the gas phase form of the equilibrium
constant that pressure of species depend on pressure.
For the general gas phase reaction,
we can write the equilibrium constant as
And the free energy is
From Dalton’s law
![Page 35: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/35.jpg)
Pressure dependence of species
If we substitute these mole fractions and total pressure into
the equilibrium constant we have
Which depends on the total pressure unless z – c – d = 0.
This expression shows that, in general, the free energy
depends on the total pressure. This means that for the
fixed pressure may affect the proportion of products
to reactants.
![Page 36: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/36.jpg)
Equilibrium of smog formation
![Page 37: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/37.jpg)
Why is the dissociation greater at low pressure?
Can the trend be explained in simple terms?
N2O4(g) 2 NO2(g)
![Page 38: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/38.jpg)
Why is the dissociation greater at low pressure?
Can the trend be explained in simple terms?
Yes, this is an example of Le Chatelier’s principle.
N2O4(g) 2 NO2(g)
![Page 39: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/39.jpg)
Why is the dissociation greater at low pressure?
Can the trend be explained in simple terms?
Yes, this is an example of Le Chatelier’s principle.
Can the trend be explained quantitatively?
N2O4(g) 2 NO2(g)
![Page 40: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/40.jpg)
Why is the dissociation greater at low pressure?
Can the trend be explained in simple terms?
Yes, this is an example of Le Chatelier’s principle.
Can the trend be explained quantitatively?
N2O4(g) 2 NO2(g)
![Page 41: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/41.jpg)
The Haber-Bosch Process
![Page 42: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/42.jpg)
The Haber-Bosch process
Involves both high temperature and pressure.
N2 (g) + 3 H2 (g) 2 NH3(g)
![Page 43: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/43.jpg)
The Haber-Bosch process
Involves both high temperature and pressure.
Why high pressure?
N2 (g) + 3 H2 (g) 2 NH3(g)
![Page 44: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/44.jpg)
The Haber-Bosch process
Involves both high temperature and pressure.
Why high pressure?
Forces the equilibrium to the right. This is an
example of Le Chatelier’s principle.
N2 (g) + 3 H2 (g) 2 NH3(g)
![Page 45: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/45.jpg)
The Haber-Bosch process
Involves both high temperature and pressure.
Why high pressure?
Forces the equilibrium to the right. This is an
example of Le Chatelier’s principle.
Why high temperature?
N2 (g) + 3 H2 (g) 2 NH3(g)
![Page 46: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/46.jpg)
The Haber-Bosch process
Involves both high temperature and pressure.
Why high pressure?
Forces the equilibrium to the right. This is an
example of Le Chatelier’s principle.
Why high temperature?
Although this is an exothermic reaction, it also has a
large barrier. The process uses a catalyst. But, what
does the temperature do to the equilibrium?
N2 (g) + 3 H2 (g) 2 NH3(g)
![Page 47: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/47.jpg)
The Haber-Bosch process
The enthalpy change of the reaction is equal to the
enthalpy of formation of NH3. DfHo = -45.9 kJ/mol
The entropy can be calculated from tabulated
absolute entropies.
1/2 N2 (g) + 3/2 H2 (g) NH3(g)
Mol So NH3 192.77 N2 153.3 H2 114.7
![Page 48: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/48.jpg)
The Haber-Bosch Process
Under standard conditions
![Page 49: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/49.jpg)
At high pressure there is a shift in the free energy for
the process is given by:
The entropy DrxnSo is obtained from the tables as well.
We note that the standard (tabulated) conditions
correspond to Q = 1 and therefore RTlnQ = 0.
Suppose we increase the pressure of N2 and H2
to 500 atm, while NH3 is maintained at 1 atm.
What happens to the free energy of reaction?
The Haber-Bosch Process
1/2 N2 (g) + 3/2 H2 (g) NH3(g)
![Page 50: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/50.jpg)
The Haber-Bosch Process
1/2 N2 (g) + 3/2 H2 (g) NH3(g)
The shift to high pressure will shift the reaction further
towards products. This is clear intuitively from
Le Chatelier’s principle, but it is quantified using the
equilibrium expressions we have learned.
Since the free energy change is negative, this might
seem unnecessary. However, the Haber-Bosch
Process is challenging since the reaction must occur
at high temperature as well. We will see how all of the
factors combine in future lectures.
![Page 51: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/51.jpg)
Hydrogenation: use of pressure
![Page 52: Calculating Enthalpy Changes - NC State: WWW4 Serverfranzen/public_html/CH201/lecture/Lecture_10.pdf · Definition of activity Activity is a concentration and has units of molarity](https://reader033.vdocuments.us/reader033/viewer/2022051720/5a7816197f8b9a4b538e72ea/html5/thumbnails/52.jpg)
Skills
• Calculate how DGo changes with
temperature and pressure
• Calculate how K changes with
temperature and pressure
• Calculate DHo from the temperature
dependence of K