Science For Everyone - Haber, Nitrogen, and Chlorine

Copyright © Karl Dahlke, 2019

Once you have a background in chemistry, physics, and biology, most of the elements seem like old friends. You know their properties, how they behave, the compounds they create, and their role in industry and in life on earth. You may even know how they look, or feel, or smell. Surely the smiths of a thousand years ago knew the smell of iron, copper, tin, nickel, gold, silver, and lead, in the heat of the forge, and as cold nuggets in the hand. Such familiarity was illustrated by an episode of The Big Bang Theory, when Sheldon asked Leonard, “Which element do you feel is too big for its britches?” as though elements had personality, and even an ego. He actually had an answer in mind, argon, though he never offered an explanation. It's a joke of course, but it underscores the way scientists anthropomorphize the world around them.

In the previous chapter we explored some of the properties of sulfur. There are many others of course, including an essential role in amino acids and life on earth, that we may get to later. When someone says sulfur, you might picture the thiols, and imagine the smell of a skunk, or a gas leak, or a rotten egg, or a lit match. When someone says hydrogen, you might think of a light invisible odorless gas that burns furiously with oxygen to make water, or the hydrocarbons of varying lengths that heat your home, cook your food, run your car, and comprise your candles. I'm not going to spend 90 chapters going over the properties of 90 different elements in detail, that would be rather tiresome, and we have other fish to fry. Still, various elements will be introduced from time to time, when they are central to the topic at hand. this chapter, for instance, would not be complete without a basic understanding of chlorine and nitrogen, 2 of the 11 elements that are gases at room temperature.

You already know the smell of chlorine from a swimming pool, or certain disinfectants. In pure form it is a green gas, very pretty, and very deadly. Fritz Haber used this chemical in world war I to murder thousands, but he also saved billions from starvation. He cared about neither, as best we can tell; he was merely interested in the science.

Oxygen

Our atmosphere is 78% nitrogen, 21% oxygen, and 1% trace gases such as CO2. In our atmosphere, two oxygen atoms join together to make one oxygen molecule, written O2. The two oxygen atoms connect via a double bond, sometimes denoted O=O. The resulting molecule is small, symmetric (like a dumbbell), and self-contained. such molecules are usually gases, because they don't interact with one another, i.e. they don't stick together. They are free to fly around the room. Indeed, oxygen will not liquify until it is chilled to -183 °C. That's pretty damn cold.

Given a little push, O2 will separate into its two constituent oxygen atoms, which can then burn (oxidize) fuel such as paper or wood or propane. The resulting combustion products, water, carbon dioxide, sulfur dioxide, etc, are in a lower energy state, thus heat is released. This is called a fire. It takes a little energy to break the O2 double bond and start the fire, energy from a match or a spark, but once oxygen is released and the chain reaction begins, the resulting fire produces plenty of heat and light. The match pushes the reaction up over an energy hump, whence it can roll down the energy mountain. Invest a little energy at the start, and you get a lot back.

Catalyst

A catalyst is a compound that flattens the energy hump, so you can just roll right down the mountain. You don't need the match or the spark any more. The picture on the left has no catalyst; you have to put some energy into the system before the reaction begins. The picture on the right assumes a catalyst.

Energy curves with and without a catalyst

Your body is full of catalysts. After all, there are no flames or sparks inside you, yet somehow you burn your food and derive energy at a slow and steady pace, all at body temperature. These catalysts are called enzymes, and we may get to those later. I will instead focus on a much simpler catalyst, driving a much simpler reaction.

Imagine a room full of hydrogen and oxygen. The molecules are H2 and O2, two atoms of hydrogen forming a hydrogen molecule, and two atoms of oxygen forming an oxygen molecule. If you want a chemical reaction, the H-H bond or the O=O bond must be broken by a flame or a spark. This little spark allows some hydrogen and oxygen to burn, producing water and heat. The heat, like a match, pushes nearby hydrogen and oxygen over its activation hump and allows it to burn. This in turn generates more heat, and soon the flame spreads throughout the entire room. In fact it is more like an explosion, so stand back!

If the room was 99% hydrogen and 1% oxygen, there would not be enough oxygen to sustain a chain reaction. The spark would create a little bit of water and heat, but the oxygen molecules aren't close enough together to spread the flame. In the same way, a room consisting of 99% oxygen and 1% hydrogen will not ignite. The optimal mix, the most explosive mix, is ⅔ hydrogen and ⅓ oxygen, as per the following chemical reaction.

2H2 + O2 → 2H2O

The mixture will combust with anywhere from 4 to 74 percent hydrogen. Of course some hydrogen, or oxygen, will be left over if the ratio is not precisely 66.6%. This holds for any two gases that combine: hydrogen and chlorine, ammonia and hydrogen sulfide, methane and oxygen, etc. There is a minimum, optimal, and maximum ratio of reactants.

The ignition temperature of hydrogen and oxygen is 500 °C. This is the temperature of an open flame. But what if we could lower that energy hump? What if these two gases could burn at room temperature?

Enter the magic of platinum. Platinum is right next to gold on the periodic table, and both metals are rare, and extremely valuable. Platinum can cost almost twice as much as gold per ounce, though both metals are commodities, and prices vary. These metals have many industrial applications, and they are prized for their beauty in jewelry and as ornamentation.

Catalysis is perhaps the most important application of platinum in engineering and technology. For example, platinum is part of your car's catalytic converter, to finish burning any unburnt or partially burned hydrocarbons in the exhaust, before they enter the atmosphere as smog. In addition, platinum catalyzes the aforementioned hydrogen oxygen reaction, so that it takes place at room temperature. This is used in fuel cells that burn hydrogen for power and/or electricity - the fuel cells on the Space Shuttle for instance, or the fuel cells in a hydrogen powered car, or the fuel cells that power a small African village. As a prank, your friend might give you a beautiful platinum bracelet and tell you to stroll casually into the hydrogen oxygen room. That's a bit more than a prank however; you would probably die in the resulting explosion.

Nitrogen

Change the experiment just a bit, so that the room contains ¾ hydrogen and ¼ nitrogen. The reaction you are hoping for is this, having an end product of ammonia.

N2 + 3H2 → 2NH3

The reaction produces heat and energy, but the activation hump is very high. The two nitrogen atoms in a nitrogen molecule are held together by a triple bond, denoted N≡N. This bond is not easy to break. You can walk in with a platinum bracelet, or even an open flame, and the room will not ignite. This was a serious problem at the beginning of the 20th century for two reasons.

  1. There are three types of macronutrients in your food: carbohydrates, fats, and proteins. Of these, only proteins contain nitrogen. You can't transmute one element into another, thus you cannot turn carbohydrates and fats (containing no nitrogen) into proteins (containing nitrogen). You have to eat protein, end of story. Since proteins break down in all animals, how does nitrogen reenter the food chain? The answer is legumes, a class of plants that pull nitrogen out of the air, break the triple bond, and build protein molecules. Legumes include alfalfa, carob, peas, beans, lentils, soybeans, peanuts, and tamarind. Vegetarians are encouraged to eat legumes, in concert with other vegetables, since they don't get any protein through meat. The nitrogen fixing capabilities of legumes were sufficient when the earth's population was small, but as humans spread across the globe, we needed more nitrogen than soybeans could provide. We needed a nitrogen rich fertilizer to nourish our crops, one that could be produced on a massive scale. Creating ammonia from nitrogen and hydrogen is the first step in this process.

  2. All military and industrial explosives depend on nitrogen. This began with gun powder in the 13th century, consisting of charcoal, sulfur, and potassium nitrate, and it continues today with nitroglycerin (also called nitro), and its more stable cousin TNT, trinitrotoluene. Once again, ammonia is the first step in creating these explosives.

    Unfortunately, fertilizer can substitute for an explosive, as illustrated by the improvised bomb in Oklahoma City in 1995.

In the 1800's, ammonia was synthesized using several different processes, all of them inefficient. It was cheaper to mine nitrates from the ground. Chile was rich in salt peter deposits, but these were largely controlled by Great Britain. If Germany wanted nitrogen for its munitions, it had to develop a better process. In 1909, Fritz Haber did just that. His process involved high temperatures (400 °C), high pressures (200 atmospheres), and of course a catalyst. Haber used Osmium, noting that uranium is superior, but prohibitively expensive. Even osmium is expensive when scaled up to industrial levels, so Haber, Bosch, and Mittasch developed an iron catalyst, which is still used today. This is not pure iron, but iron mixed with K2O, CaO, SiO2, and Al2O3. The catalyst lowers the activation energy needed to break the triple bond and bind nitrogen to hydrogen. Still, high pressures and temperatures are required, and the Haber process is not a trivial endeavor.

If you look at the reaction, you can see why pressure drives the equilibrium to the right. One liter of nitrogen and three liters of hydrogen gives two liters of ammonia, hence the product is half the volume of the reactants. High pressure will favor such a reaction. As for the catalyst, and the other details of the process, well, those are less obvious, and we must defer to Haber's genius in that regard.

It is difficult to overstate the importance of the Haber process. The resulting fertilizers raise crops that feed half the people on earth. As of the year 2000, 15% of all ice-free land was used to feed the world. This efficiency is made possible by the Haber process, along with pesticides and other agricultural advances. If crop yields were the same as they were in the year 1900, half of all land would be required to feed the world. This ratio is clearly impossible, since most land is not arable, deserts and mountains etc, and farms must necessarily compete with cities and factories for the remaining realestate. Without Haber, billions of people would starve. That's the bottom line. He is indeed the man who fed the world. This was recognized as early as 1918, when he was awarded the Nobel prize in chemistry for his accomplishments.

Today, the Haber process produces half a billion tons of nitrogen fertilizer annually. This consumes 3 to 5 percent of the world's natural gas output, representing 1 to 2 percent of the world's energy. These are resources well spent, since the alternative is mass starvation followed by societal collapse.

With a few more agricultural advances, we can probably eke out enough food to feed the next billion or two, but as I watch the world's population soar, I have this sinking feeling that Malthus was right. Population explosion is every bit as ominous as global warming - perhaps more so since the former drives the latter. The greenest thing you can do is not have more than 2 children. Haber bought us some time, a century or two - but we really need to get ourselves and our population under control.

Chlorine

You have probably experience chlorine in trace amounts in your municipal drinking water, and in larger concentrations in public swimming pools. This is typically hypochlorous acid, HClO, rather than pure chlorine. The weak chlorous acid is gentler on human and animal tissue, and is still an effective disinfectant, deriving its antibacterial properties from chlorine, which is gradually released into the water over time. It is a bit like a time-released drug. Some of this chlorine evaporates into the air, and the smell of a swimming pool, especially an indoor pool, is unmistakable.

The pure element chlorine, Cl2, or Cl-Cl, is never found in nature, because chlorine is highly unstable. We saw this with sodium earlier; sodium is too reactive to remain in pure form. It readily combines with chlorine to make common table salt, or with bromine, or sulfur, or carbonate, or any other nonmetal it comes in contact with. In the same way, chlorine gloms on to any metal it can find, and is thus never found alone in nature. However, being curious creatures, we have produced pure chlorine in the lab, and discovered that it is a yellow-green gas. Its gaseous state is not surprising, since the Cl2 molecule is small, self contained, and symmetric. Chlorine gets its name from the Greek chloris, for pale green. Other green words include chlorophyl (making plants green) and chloroplast. The chlorophyl molecule contains no chlorine; they just both happen to be green.

Chlorine liquifies at -34 °C, which is cold, but within the realm of a cold winter's day in northern latitudes. Alternatively, chlorine liquifies at room temperature under a pressure of 7.5 atmospheres. This is within the capabilities of a reenforced truck or train car for purposes of transport. The greatest risk here is an accident, leading to a spill. Remember that the volume of a liquid expands by a factor of 1,000 when it expands to a gas. Imagine 1,000 trucks of chlorine gas floating around a populated area. Chlorine is a bit heavier than air, so it hugs the ground. Furthermore, chlorine is highly toxic, so this is not a good situation. Fortunately such spills are rare.

Chlorine Acids

Chlorine combines with hydrogen and oxygen to make several acids. These don't really become acids until they are dissolved in water, whereupon the hydrogen ion is set free. For example, the first compound, hydrogen chloride, is actually a gas, but it becomes hydrochloric acid when dissolved in water. The names below assume the compounds are dissolved in water.

Hydrochloric Acid: HCl
Hypochlorous Acid: HClO
Chlorous Acid: HClO2
Chloric Acid: HClO3
Perchloric Acid: HClO4

The first is a strong acid, and in concentrated form it dissolves flesh. This is precisely why free chlorine is so toxic. It enters the lungs, combines with water to make hydrochloric acid, and eats away at the tissue, until the victim drowns in his own juices. In 1915, during World War I, Lance Sergeant Elmer Cotton described the effects of chlorine gas this way.

“It produces a flooding of the lungs - it is an equivalent death to drowning only on dry land. The effects are these - a splitting headache and terrific thirst (to drink water is instant death), a knife edge of pain in the lungs and the coughing up of a greenish froth off the stomach and the lungs, ending finally in insensibility and death. The colour of the skin from white turns a greenish black and yellow, the colour protrudes and the eyes assume a glassy stare.”

Chemical Warfare

At this point we are ready to circle back to Fritz Haber. In 1915 he orchestrated the first use of poison gas, chlorine, in the context of war. He did more than develop the chemistry, he coordinated its deployment on the battlefield during World War I. This gave him no moral qualms. He felt that if you were going to kill enemy soldiers, which is the whole point of war, then it hardly mattered how you did it. He explained, “During peace time a scientist belongs to the World, but during war time he belongs to his country.” Thus Haber developed methods to produce, store, transport, and deploy chlorine gas in large quantities. German soldiers would wait for the wind to blow towards the enemy, open the canisters, and run like hell. Chlorine hugged the ground, and quickly filled the trenches where the Allies were ensconced. What would that be like? As you watched the green cloud rolling towards you, like the last plague in The Ten Commandments, you could lay low in the trenches and die a gruesome death, or jump up and run for higher ground under heavy German fire. There is a third option that isn't perfect, but might work if the chlorine dissipates quickly. Take off your shirt, urinate on it, hold it up to your mouth, and breathe through it. The chlorine reacts with the urea, and does not enter your lungs. Once this trick was discovered it spread quickly. A few intrepid soldiers even tried to fight on, with rifles at the ready and urine soaked cloths tied to their faces. However, these improvised urine masks were imperfect at best. The new chemical weapon killed thousands, and injured or blinded most of the survivers. Some of the casualties were German, as the shifting winds blew some of the chlorine back across the battle lines. Over the next few months both sides developed gas masks with activated charcoal to absorb the chlorine, whereupon the Germans replaced chlorine with phosgene, COCl2, a deadlier gas that is invisible, and starts to damage the lungs even before you notice the distinctive odor of freshly cut hay or grass. Better gas masks, deadlier chemicals, and so it goes even into the modern era. Recall the use of sarin gas in a Tokyo subway on March 20, 1995, killing 12 and injuring 1050, and Saddam Hussein's attack on the Kurds at Halabja, killing 3 to 5 thousand and injuring 7 to 10 thousand. We have indeed perfected our techniques. Still, Fritz Haber got the ball rolling, and is considered the father of chemical warfare. He killed thousands, perhaps tens of thousands throughout World War I, but his nitrogen fixation process has fed, and continues to feed, billions.

Haber's wife Clara killed herself in their garden with Haber's service revolver on May 2, 1915. Many believe this was a direct result of her husband's role in the war. Her death took place shortly after an argument with Fritz, and just ten days after the infamous Battle of Ypres began, where chlorine was deployed with devastating effect.

Electrolysis

If you're determined to make chlorine gas at home, it might be possible. Recall the earlier experiment wherein we separated water into hydrogen and oxygen using an AA battery. An electrolyte is required, and I suggested baking soda, because table salt will not work. This time you want to use table salt, but there's another problem. First let's review the setup.

Cut 2 insulated wires about a foot in length and strip the ends. Attach the end of one wire to the positive node of an AA battery, and the end of the other wire to the negative node of the battery. Place the other ends of the wires in a bowl of water, a couple inches apart. If you use a metal pan, don't let the wires touch the pan, or you have a short circuit. Place a piece of plastic, or any insulator, between the wires and the pan. Add salt (was originally baking soda) to the water until it tastes salty. Since you're not going to drink it, you can use less expensive rock salt, e.g. the kind you might put down on your sidewalk in the winter. Place inverted glasses over the ends of the wires to catch the bubbling hydrogen and chlorine. Glasses should be clear so you can see what is going on and measure the gas volumes. At the outset, the jars must be full of water, with no air.

Sounds easy, but there's a problem. One glass accumulates hydrogen as expected, but at the anode, the liberated chlorine immediately attacks the metal wire and creates copper chloride. Once the wire is coated in a layer of copper chloride it doesn't conduct any more. You get little to no chlorine gas in the jar. You must use a different conductor. Platinum will work, but nobody can afford that. Titanium will work, but that's hard to come by. Graphite will work, and that's the only thing you might have around the house. Cut the eraser end off of a pencil and tape the end of the wire to the exposed graphite. Insert the other end of the pencil into the water, with the point under the glass. If all goes well, transparent hydrogen bubbles up into the wire glass, and light green chlorine bubbles up into the pencil glass. Volumes are exactly equal, confirming the atomic theory of matter. Although, if you can measure volumes to the milliliter, volumes may not be equal, because chlorine, as a gas, is more soluble in water than hydrogen. Some of the chlorine gas is stuck in the water. There are always caveats in real world science.

Warning: chlorine gas is dangerous, as described above. Don't inhale it or try to sniff it directly. Release it outdoors or in a well ventilated room.

Warning: if considerable chlorine is produced, like a couple of liters, the remaining solution in the bowl contains sodium hydroxide, the active ingredient in Draino, and is caustic. Keep away from mouth and eyes, and wash hands after contact. Throw the solution down the toilet and rins the bowl.