ChemBytes - Chapter 10

Thermochemistry and Thermodynamics

CHEMByte 20: A Scottish Chemist. Here is how James Black (1728-1799) described his results on mixing water and mercury:

" Here it is manifest that the quantity of heat which makes 2 volumes of water warmer by, say, 25 degrees is sufficient to make 3 volumes of quicksilver warmer by the same number of degrees. Quicksilver, therefore, has less capacity for heat (if I may be allowed to use this expression) than water."

CHEMByte 21: It All Began with a $500 Consulting Fee.
In the summer of 1854, a Dartmouth alumnus visiting his alma mater happened upon a bottle of oil from a spring in western Pennsylvania. Thinking there might be wealth in the spring that had yielded the bottle's contents, this young, entrepreneurial gentleman formed the Pennsylvania Rock Oil Company and bought the spring without quite knowing what to do with its product. The fledgling company sought out Benjamin Silliman, Jr., a Yale chemistry professor, to analyze a sample of their rock oil and see what it might be good for. Silliman was to be paid $500. After performing tests, he reported that the oil had wonderful lubricating properties and was "chemically identical with illuminating gas in liquid form... that the lamp burning this fluid gave as much light as any they had seen... the oil spent more economically... and the uniformity of the light was greater than in camphene, burning for twelve hours without a sensible diminution, and without smoke." Unfortunately, the Pennsylvania spring yielded very little oil. What there was had oozed up into the spring from below ground level. Was there more below? If so, why not drill for it? And drill they did, using technology then available for drilling into salt beds. In 1859, the first well was brought in by "Colonel" Edwin Drake. Wells followed in Ohio, Indiana and Illinois.

Neither Drake nor those who backed him got much out of their discovery. Leaving what quickly became the Pennsylvania Oil Fields, Edwin Drake went to Wall Street where he became a broker in oil stocks and lost everything he had in speculation. And although a grateful Pennsylvania legislature voted him a pension of $1500 a year, Drake died destitute and in obscurity (in 1880) in spite of the fact that the oil mania he started was second only to the California Gold Rush.
Oil was discovered in Texas in 1901 at just about the time the automobile was becoming more than just a toy gadget. The site was aside a pond on whose surface boys were fond of throwing matches "to see the lake catch fire." Flammable gas, it seemed, just bubbled up to the pond's surface from the salt dome whose limestone roof stood only 10-12 feet below the Earth's surface. Crowned by spindly pine trees -- hence the name "spindletop" -- drilling through the roof of the dome began late in 1900. By January 10, 1901, the drill was down 800 feet... and then things began to happen. As one of the drilling crew described that day.....
"We put the new bit on and had 700 feet of the drill pipe back in the hole when the rotary mud began flowing up through the rotary table. It came so fast and with such force that [the crew] had a hard time getting out of danger. [Then] the four inch pipe started up through the derrick, knocking off the crown block. It shot through the top of the derrick breaking off in lengths of several joints at a time as it shot skyward. It all happened in much less time than it can be told.
"After the water, mud and pipe were blown out, gas followed, but only for a short time. Then all became quiet... and... we boys ventured back, after the wild scramble for safety, to find things in a terrible mess -- at least six inches of mud on the derrick floor and damage to our equipment. We were disgusted. As we started shoveling the mud away.... without any warning a lot of heavy mud shot out of the well with the report of a cannon! Gas followed for a short time. Then oil showed up. In [seconds] oil was going up through the top of the derrick. Rocks shot hundreds of feet in the air. In a very few minutes the oil was shooting at a steady flow at more than twice the height of the derrick." For nine days the oil rained down before it could be capped, gushing 40,000 barrels a day. Spindletop proved to be the Klondike of oil.
Literally overnight the Texas oil rush had begun and with it the 20th century oil ride. It hasn't ended yet. It will, someday in the foreseeable future, closing out a couple of centuries of thermodynamic and economic dominance of the world's energy resources. As for Professor Silliman and his fee of $508.30, including expenses? There is no record of his ever being paid!

CHEMByte 22: Measuring Temperature
The actual value of the temperature of an object can be determined in a number of ways. For example, in our treatment of gases in Chapter 5 we discussed Charles's law and stated that the volume of an ideal gas held at constant pressure is directly proportional to the absolute (Kelvin) temperature. Therefore, measuring the volume of a gas is one way of determining the temperature and one such device for doing that is the gas thermometer. The functioning principle of ordinary household and laboratory thermometers is the property of thermal expansion possessed by most substances, expanding when heated and contracting when cooled. Mercury thermometers are the most common.

A typical mercury thermometer consists of a capillary tube -- a tube with a very narrow channel -- which is sealed at the top and has an enlarged cylindrical or spherical bulb at the bottom. The bulb is filled with mercury which, when heated, expands and rises up the channel. Because the channel is very narrow, even a small change in the volume of mercury causes it to rise significantly. The thermometer is calibrated between two fixed reference points, typically the freezing and boiling point of water at atmospheric pressure. The difference in level in the mercury column between these fixed points is divided into 100 equal parts for the Celsius scale or Kelvin scale, each division being 1°C. On the Fahrenheit scale, the difference between the two fixed points is divided into 180 parts, each division being 1°F. The boiling points and freezing points are the familiar values of 0°C and 100°C, and 32°F and 212°F.
A thermocouple can be made by soldering together the ends of two wires of dissimilar metals or metal alloys, for example copper and iron, or copper and constantan. If one soldered junction is kept at a constant temperature while the other is heated, an electric potential difference (voltage) develops between the two junctions. This potential difference is greater as the difference in temperature between the junctions is greater. The difference can be read on a voltmeter calibrated to give temperature readings.
Bimetallic thermometers are constructed of strips of dissimilar metals soldered together. Remember the bimetallic disks described in Chapter 1? The two metals have different coefficients of expansion, which means they undergo different increases in length on heating. To illustrate, consider the operating principle of the temperature-measuring device embodying a bimetallic spiral whose curvature varies with the temperature and causes a pointer to be deflected. The scale is calibrated by establishing the positions of the pointer at certain known temperatures and then marking the scale so that each division corresponds to a degree.
But keep in mind that for literally thousands of years before there was any kind of thermal transducer for converting the condition of temperature into some other physical effect, craftsmen needed to know the temperature. A Chinese porcelain kiln master had to judge 1300 ± 30 °C .... by eye... and commercial success depended on the ability to get it right or be left with ceramic pots reduced to stone masses. As a visit to the ceramics collections at the world's great museums reveals, some got it right. More recently, the town blacksmith and the furnace master in a steel mill could not be color-blind since both were taught to get the temperature right by the color of the object being tempered in the furnace. An experienced furnace master could distinguish 430°C (very pale yellow) from 460°C (straw yellow) .

CHEMByte 23: The Mechanical Theory of Heat
Count Rumford, while superintending the boring of cannon in the workshops of the military arsenal at Munich (1798), observed and documented the intense heat a brass gun acquired in a short time of being bored. Over 30 minutes of observation, he obtained a quantitative measure of the heating effect caused by rotating a hardened steel boring tool against a solid casting for "a brass six-pounder" from which he could only conclude that the heat produced had to do with the boring motion. The main action in cannon-boring was sliding friction, which was responsible for the heating and the temperature rise.

Others were stimulated by Rumford's conclusions, particularly two physicists. Julius Robert Mayer (German, 1842) noted that heat and motion (work) were quantitatively equivalent. At about the same time, James Prescott Joule (British, 1843), was busy studying the heating effect of an electrical current generated by a rotating coil in an electromagnetic field. By modifying his apparatus so that the coil was driven by falling weights, he was able to show that...
"the quantity of heat capable of increasing the temperature of a pound of water by one degree of Fahrenheit's scale is equal to, and may be converted into, a mechanical force capable of raising 830 lb to the perpendicular height of one foot."
Rumford, Mayer and Joule's experiments involved measurements of the heating effects of work -- the product of a force and the distance through which it moves. Joule succeeded in raising the temperature of a well-insulated quantity of water by the motion of a paddle wheel driven by a falling weight. The amount of work performed on the water could be calculated from w = Fd where d is the distance through which the weight fell and the force F is equal to the mass of the falling weight times the acceleration of gravity. Joule had found a proportional relationship between the amount of work done on the liquid by the paddle wheel and the increase in temperature of the water. Thus he was able to determine the molar heat capacity of the water in terms of the work performed on it.
Joule's experiments convincingly showed that heat and work are manifestations of the same thing, and that if work is done, and heat is not permitted to flow, the internal energy of the system must change. In fact, heat and work provide the only means for changing internal energy. Heat is motion or motion is a form of heat. Finally, since the elevation of a weight to some height and its acceleration to a steady velocity are mechanical effects, this concept has come to be known as the mechanical theory of heat. It is one of 19th century science's great conclusions. Though Mayer's work preceded Joule's, his published results aroused little interest and in the end it was Joule with his imposing experiments who received the lion's share of the credit for working out the mechanical equivalent of heat.

CHEMByte 24: The "Snow Tube" Carbon Dioxide Fire Extinguisher... and Making Artificial Snow as Illustrations of Thermodynamics in Action.
The BC-type carbon dioxide fire extinguishers commonly in use in chemical laboratories typically are charged with several liters of liquid carbon dioxide under very high pressure. If you shake an extinguisher that is fully charged, you can literally hear and feel the liquid sloshing around inside. When the safety pin is pulled and the triggering device is squeezed, the carbon dioxide is released and rapidly expanded to atmospheric pressure in the megaphone-shaped "snow tube" where the greater part rapidly vaporizes. Because of the huge difference in pressure between the system -- the contents of the extinguisher -- and the surroundings -- the atmosphere into which the carbon dioxide is ejected -- the response is instantaneous and we can reasonably assume that there is no heat exchanged between system and surroundings. There simply isn't time for heat to flow. Therefore q is zero, DE is equal to w, and the sign of w is negative because the system is doing work on the surroundings. The net result is therefore a decrease in the energy of the system. Now we also assume that for an ideal gas E is proportional to the absolute temperature T and so DE is proportional to DT. Because DE is negative, DT must also be negative. It is just this very cooling effect due to the negative DT that results in some of the carbon dioxide (about 30%) being cooled to -78°C, at which temperature it turns solid at atmospheric pressure. This "snow" of solid carbon dioxide is directed at the base of the fire where it serves to displace the oxygen needed to support combustion, suffocating the fire, and in many cases, lowers the temperature of the burning material to below its ignition temperature.

Interestingly, this same principle serves to explain how the snow-making equipment that has revolutionized the ski resort and winter vacation industry works, freeing the people who depend on snow-covered slopes for their livelihood from the vagaries and uncertainties of the weather. As long as the atmospheric temperature is around the freezing point of water, a mixture of compressed air and water sprayed into the atmosphere results in the production of something akin to snow.

CHEMByte 25: Geothermal Electric Power from Hot Dry Rock (HDR) and Hydrothermal Energy Technologies
Geothermal electric power is produced from the kinetic energy stored in the earth's crust where it is close enough to be extracted. Use of geothermal energy is thermodynamically based on the temperature difference between rock formations and water beneath the Earth's surface and water at the surface. Temperatures in the Earth increase to about 1000°C at the base of the crust and to perhaps 4000°C at the center of the Earth. The heat sources that produce these temperatures come from the flow of heat from the deep crust and mantle, and energy largely generated in the upper crust by radioactive decay of uranium, thorium and potassium isotopes.

Scientists at Los Alamos National Laboratory, have recently demonstrated that heat in sufficient quantity can be extracted from hot dry rocks under the surface of the earth to produce a limited but steady flow of electric power. By pumping water at rates up to 100 gallons per minute through cracks made in rocks at 235°C, 12,000 feet below the nearby Jemez Mountains, they were able to return water to the surface as steam at 175°C. That temperature differential in turn delivered a heat equivalent of 4 megawatts of power. While hot dry rock (HDR) technology offers the potential to vastly increase the supply of power derived directly from the earth's natural heat sink, hydrothermal energy is already being extracted from hot waters and steam circulating naturally through underground rock formations by power plants throughout California, Nevada, Utah and Hawaii -- some 70 plants in all, with a generating capacity of about 3000 megawatts or enough for about 1 million people. That is more commercial electricity than is presently being produced from the two more popular forms of renewable energy, solar and wind, combined.
The advantages geothermal -- HDR and hydrothermal -- technologies offer are impressive: no oil spills, no carbon dioxide added to the atmosphere or ash left over from burning coal and petroleum resources, no acid run-off from open pit coal mining, and no radioactive wastes to deal with. But, as always, there is no such thing as a free lunch. Geothermal may be less polluting, but it may not be cheaper, largely because commercial developers will have to bore holes a mile or perhaps two miles deep through bedrock. Current estimates are costs that are triple the current costs of gas turbine electricity from existing fossil fuel-based technologies. Nevertheless, the possibilities of geothermal energy cannot be denied and must be included in any mix of energy technologies that will see us through the 21st century and beyond.
CHEMBytes: Additional Problems on Thermochemistry / Thermodynamics
  1. Calculate the enthalpy of formation of Ca(OH)2(s) from the following data:
    H2(g) + 1/2 O2(g) H2O(liq) DH = -68.3 kcal
    CaO(s) + H2O(liq) Ca(OH)2(s) DH = -15.3 kcal
    Ca(s) + 1/2 O2(g) CaO(s) DH = -151.8 kcal
  2. Using available thermodynamic data from the chapter, calculate DH for each of the following reactions:
    Fe2O3(s) + 3 CO(g) 3 CO2(g) + 2 Fe(s)
    2 NO2(g) 2 NOg) + O2(g)
    N(g) + NO(g) N2(g) + O(g)
  3. A sample of solid naphthalene, C10H8, weighing 0.600 gram is burned to CO2(g) and H2O(liq) in a constant volume calorimeter. In this experiment, the observed temperature rise of the calorimeter and its contents is 2.255°C. In a separate experiment, the total heat capacity of the calorimeter was found to be 2550 cal/°C. What is DE for the combustion of one mole of naphthalene? What is the DH for this reaction? By using the tabulated values for DH°f(CO2,g) and DH°f(H2O,liq), calculate the enthalpy of formation of naphthalene.
  4. One mole of an ideal gas at 300K expands isothermally and reversibly from 5 to 20 liters. Calculate the work done and the heat absorbed by the gas. What is DH for the process?
  5. Ethylene (C2H4) and propylene (C3H6) can be hydrogenated according to the reactions
    C2H4(g) + H2(g) C2H6(g)
    C3H6(g) + H2(g) C2H6(g)
    to yield ethane (C2H6) and propane (C3H6), respectively. From data available in the chapter, calculate DH for these reactions. What do these answers suggest about DH for reactions that are generally of the type
    CnH2n(g) + H2(g) CnH2n+2(g)