AP Chem
Catalysts and Inhibitors
Catalyst have allowed us to build the massive chemical industry that supports modern life.
Along with their counterparts, the inhibitors, they play an important role in chemistry. The global
market in catalysts is around 30 billion dollars per year.
Catalysts work by lowering the activation energy of chemical reactions. As chemical species
bounce around at a given temperature they collide. Some of them collide with enough energy to react.
Catalysts lower the energy needed in order for a reaction to occur. A system where the process of
catalysis is key is the production of ammonia. Reacting hydrogen and nitrogen to produce ammonia is
exothermic. The entropy change for the reaction is negative. Because the entropy change for the
reaction is negative the reaction becomes less favorable at high temperature. At low temperature the
reaction is more favorable but too slow to be commercially viable. In order for the reaction to be fast
enough at lower temperatures where the reaction is exergonic a catalyst is needed. It has been found
that three elements are acceptable catalysts, uranium, ruthenium and iron. Not surprisingly iron is used
in industry. The process of producing ammonia from hydrogen and nitrogen has revolutionized human
life. Anywhere from 3-5 billion people on earth would starve if food supply had to rely upon natural
fertilizers. This process for producing ammonia is named the Haber process in honor of one of the
scientists who developed it. Sometimes it is called the Haber-Bosch process because a chemical
engineer, Carl Bosch, figured out how to scale the process to industrial levels. One of Bosch's major
innovations was to line the hydrogen pipes in the plant with cheap low quality iron or steel pipes. The
low quality metal absorbed hydrogen and wore out quickly due to embrittlement. These low quality
pipes lined the high quality tough steel pipes that contained the high pressure of the hydrogen. Had the
high quality high cost pipes not been lined with low quality metal the cost of the process would have
been higher as the high quality pipes would have needed replacement more often. Of historical interest
is the Haber process’ effect on World War One. A key part of the British strategy was to blockade
Germany from overseas sources of raw materials. One of these materials was nitrate deposits from
Chile, needed for explosives and fertilizers. Germany was able to build a large enough plant to supply
their need for explosives, but not fertilizer. Eventually Germany began to run out of food and hunger
was a factor in defeat. Today, the amount of nitrogen in the nitrogen cycle is doubled compared to preHaber as humans put as much nitrogen in as Nature does.
Fritz Haber
Carl Bosch
Chlorofluorocarbons (CFC's) were developed in order to provide a series of nontoxic relatively
inert gases that could be used as solvents and refrigerants. The CFC's were spectacularly successful in
fulfilling the hopes for them. Unfortunately they rise to the ozone layer. When the CFC's are hit by solar
photons they can break down and release chlorine atoms. These chlorine atoms are also called radicals
since they have an unpaired electron. The chlorine atoms catalyze the destruction of ozone. CFC's have
been banned in order to avoid this.
Why have levels of lead in children's blood dropped dramatically in the last thirty years? The
use of tetraethyl lead in gasoline has been phased out. Tetraethyl lead is an inhibitor sometimes added
to gasoline to improve engine performance. Spark ignition engines (often called gasoline engines) take
in a mixture of air and fuel. The fuel/air mixture is compressed by the piston inside the cylinder. Since
the piston does work on the fuel/air mixture its temperature increases. If the temperature gets too high
the fuel/air mixture can undergo combustion before the piston reaches the top of the cylinder. This
reduces the power of the engine and can even damage it. One solution to this is to use fuels that resist
predetonation better. This is expensive. It's much cheaper to add an inhibitor.
Tetraethyl lead inhibits the combustion process and keeps the fuel from burning until the spark plug
fires. Once the spark plug fires there is an overwhelming amount of activation energy added to the
cylinder and the inhibitor is overcome so that combustion occurs at the proper time. Unfortunately
tetraethyl lead is highly toxic. Modern gasoline no longer contains this dangerous additive. Another
issue with tetraethyl lead is deposits of lead metal and/or lead oxide in the engine. In order to prevent
these from building up 1,2-dibromoethane and/or 1,2-dichloroethane is added. These react with the
lead to form lead (II) chloride or lead (II) bromide which are volatile enough to leave with the other
exhaust gases.
Pop, we need to put more lead into the environment!
The removal of lead from gasoline is an absolute imperative. Some interesting stats from:
http://www.wired.com/thisdayintech/tag/tetraethyl-lead/
“After the efforts of concerned scientists such as Patterson and Phillip Landrigan, the newly
formed Environmental Protection Agency began to gradually phase out lead as a fuel additive
starting in 1976. Over the next 15 years, the concentration of lead in the bloodstream of
Americans dropped 78 percent.
So what’s wrong with a little lead between friends? Consider that researchers have tied even
small amounts of lead exposure to low IQ, aggression, attention disorders and delinquency. A
University of Pittsburgh study found that between 18 and 38 percent of crime could be tied to
lead exposure. Remember the New York crime wave of the 1970s and ’80s? Some researchers
say it was all due to lead poisoning.”
From: http://www.answers.com/topic/tetra-ethyl-lead#ixzz1cPZvS3GC
“Lead pollution from engine exhaust is dispersed into the air and into the vicinity of roads and
easily inhaled. Lead is a toxic metal that accumulates and has subtle and insidious neurotoxic
effects especially at low exposure levels, such as low IQ and antisocial behavior. It has
particularly harmful effects on children. These concerns eventually led to the ban on TEL in
automobile gasoline in many countries. Some neurologists have speculated that the lead phaseout
may have caused average IQ levels to rise by several points in the US (by reducing cumulative
brain damage throughout the population, especially in the young). For the entire US population,
during and after the TEL phaseout, the mean blood lead level dropped from 16 μg/dL in 1976 to
only 3 μg/dL in 1991.[17] The US Centers for Disease Control considered blood lead levels
"elevated" when they were above 10 μg/dL. Lead exposure affects the intelligence quotient (IQ)
such that a blood lead level of 30 μg/dL is associated with a 6.9-point reduction of IQ, with most
reduction (3.9 points) occurring below 10 μg/dL.[18]
A statistically-significant correlation has been found between the usage rate of leaded gasoline
and violent crime: taking into account a 22-year time lag, the violent crime curve virtually tracks
the lead exposure curve.[19][17] After the ban on TEL, blood lead levels in US children
dramatically decreased.[17]
Although leaded gasoline is largely gone in North America, it has left high concentrations of lead
in the soil adjacent to roads that were constructed prior to its phaseout. Children are particularly
at risk if they consume this.”
Note: The chemist Thomas Midgley developed both tetraethyl lead and CFC’s.
Questions:
1. Why are lead bromide and lead chloride more volatile than lead oxide and lead metal?
2. Use a table of thermodynamic values to calculate the enthalpy and entropy changes for the reaction
between nitrogen and hydrogen to produce ammonia.
3. Calculate the temperature at which the free energy change for the ammonia reaction is zero.
4. Why does the ammonia reaction have such a high activation energy?
5. Draw Lewis structures for the possible isomers of C2H2Cl2F2. Tell which are polar and which are
nonpolar.
6. Why does chlorine leave the CFC molecule more readily than fluorine when struck by a photon?
7. Is CF2Cl2 polar or nonpolar?
8. Which has a higher boiling point, CF2Cl2 or CFCl3?
9. The production of sulfuric acid requires that SO2 be reacted with O2 to produce SO3. Here are some
equations for this process:
2 SO2 + 4V5+ + 2 O2- → 2 SO3 + 4V4+
4 V4+ + O2 → 4 V5+ + 2 O2-
Add the two equations to get the overall process. Identify the catalyst(s) and the intermediate(s).
10. Look up the bond energy for a C-Cl bond. Calculate the minimum wavelength of the photon needed
to break the bond.
Answers:
1. Look up the melting and boiling points.
Lead metal: MP: 600K BP: 2022K
Lead (II) oxide: MP: 888K BP: 1477K
Lead (II) chloride: MP: 501C BP: 950C
Lead (II) bromide: MP: 373C BP: 916C
Note that it easier to boil the halides. The metallic bonding in lead is stronger than the ionic bonding in
the halides. The lattice energy of the halides is lower than the lattice energy of the oxide.
The most interesting thing we can pull out of the above data is the larger gap between the MP and BP of
lead vs its compounds. I would speculate that this is because when the lead melts there is still a
considerable degree of metallic bonding holding it together whereas in the compounds we disrupt the
ionic bonding a great deal when we melt them. Why the similarity in the bromide and chloride’s BP but
the difference in MP? My guess is they have different crystal structures due to the difference in relative
sizes of the anions vs the cations. Once they melt the difference in crystal structure has nothing to do
with the intermolecular forces. If the two halides have the same crystal structure then we need another
explanation. Light could be shed on this problem if we could find the lattice energy of the lead halides
or even better if some intrepid physical chemist out there would measure the affinity of one lead ion for
two bromide ions vs for two chloride ions.
2. For one mole of ammonia, Delta H = -46KJ/mol and Delta S = -99J/molK
3. 464K. Above this temperature the reaction is no longer spontaneous. We want the temperature we
run the reaction at to be lower than this. The reaction can only be profitably fast at lower temperatures
with a catalyst.
4. The triple bond in N2 has to be broken. This takes a large amount of energy and slows down the
reaction.
5.
It looks like all of them are at least a little bit polar except for the bottom left structure.
6. The carbon-chlorine bond is weaker than the carbon-fluorine bond.
7. It's slightly polar because the dipoles of the C-F and C-Cl bonds are weaker.
8. CFCl3 because it has higher molar mass and the strongest force in each is probably dispersion forces.
These are actually a lot harder than it seems. In some cases dispersion forces are stronger while in
others the dipole-dipole forces are stronger and there are cases where it's tough. Look at CCl2F2. Its
boiling point is −29.8C while CHCl2F has a boiling point of 8.9C. Here we can see that the
dipole-dipole forces of CHCl2F are stronger than the dipersion forces and very weak dipole
forces of CCl2F2.
9. V5+ is the catalyst, O2- is an intermediate. The overall equation is: 2SO2 + O2 = 2SO3
10. I found -327 KJ/mol for the bond. Convert to joules and divide by Avogadro’s number to get Joules
per bond. Plug this value in for energy into E = hc/λ. I got 366nm. 366nm light is common in the
atmosphere, especially the upper atmosphere where ozone damaging compounds can reach. Let’s also
look at a C-F bond. The C-F bond energy I found is 485KJ/mol, the wavelength here is 247nm which is
less common especially after the sunlight has been through any air at all. So, it seems more Cl will be
put into the atmosphere than F.