Chemistry 1020Lecture 9Notes
This chapter opens with a discussion of the energy "crisis" of 1974. How many of you were alive in 1974? That was the year I switched from a medium sized American car getting 15 mpg on the highway to a smaller German car getting 25 mpg. It was a year that saw shrinking gasoline availability, and increasing prices. How many of you have ever traveled in Europe? Do you wonder why the cars there are much smaller than cars in the US? Why public transportation is so much more common?
We have grown accustomed to dependence upon petroleum products in the form of gasoline, heating oil, and natural gas in order to run not only our transportation system but our industrial economy. That is not to speak of the developments in the chemical industry which learned how to convert crude petroleum into artificial materials such as nylon,--miracle fabrics and plastics we have come to depend on.
Figure 4.2 compares the average energy consumption per capita of 18 major countries. Notice where the US is. Figure 4.4 shows a relatively brief historical perspective of the sources of this consumption in the US. In the last century, following the civil war, wood was a major source of energy. Note as we come into the 20th century that coal begins to replace woodand coal mining became a major industry. With the development of the steam engine, use of coal began to be the means by which our rail transportation grew, the development of steel mills for the production of steel was possible, and the wheels of our industrial revolution began to turn.
As electrical power became a reality for most of the country, some of it was powered by hydroelectric energy sourcesdamming up of streams and rivers to harness the potential energy available from water as it flows downhill. Note that was and is a small part of our consumption. Finally oil and natural gas began to overtake coal as the major energy source, with a small contribution from nuclear energy over the past 30 years. We will come back to nuclear energy later in the term.
Coal, oil and natural gas all have in common that they derive from living organisms created by photosynthesis in the history of the planet. The term "liquid sunshine" for oil is a reflection of this history. We cannot forget, though, that these substances have built up over millions of years, and our consumption of them is a relatively recent blip in the overall history of the planet. These are "non-renewable" energy sources. When we use them up, they are gone. We would have to wait another million years to regenerate them.
There have been various predictions of the amount of oil reserves. There is probably enough to last your lifetime at the current rate of consumption. There is probably 40-50 times as much coal reserves, but we shall see that coal use generates more problems that petroleum use. Even so, are we justified in consuming most of these reserves in the relatively short span of the 20th and 21st century?
Before we discuss these issues further, it is probably necessary to have a better understanding of the concept of energy, and the rules regarding its use.
We all have some concept of the thing we call energy. "Gee, Im not feeling very energetic today". "Maybe you should take your energy pills?" "You can feel the energy flow through his body". We use the term in many different ways, most of them without a clear definition.
The development of the concept of energy was one of the great intellectual breakthroughs that undergird modern science. In some ways, we might view energy as a construct of our imagination. We never measure it directly. It is always calculated as a product of two measurementsand the unifying principle behind the calculation is that there exists this property called energy which is conserved in any process. In other words, the First Law of Thermodynamics is that energy is neither created nor destroyed. It only changes form. (We will discuss a more encompassing law in Chapter 8, a combination of this Law of conservation of energy and the Law of conservation of mass which allows for an interconversion of mass and energy).
We can define energy as the capacity to do work. But what is work? That is defined as a force acting on an object causing movement over a distance. Note that work (and hence the energy associated with work) is here defined as the product of two terms:
Force x Distance = work
Force is called an "intensive" variable, and distance and "extensive" variable. When a force acts on an object it changes its condition such that the object then possesses energy equivalent to the amount of work done. There are two forms of such energy, referred to as mechanical energy.
According to Newtons laws of motion, a force acting on an object will put it in motion, causing it to accelerate. After the force has acted, the object in motion possesses a form of energy called kinetic energy. It can be shown from Newtons laws and a bit of calculus that the kinetic energy depends on the mass of the object and its velocity according to the relationship:
kinetic energy = mv2/2
Note here we are also "measuring" energy as the product of two variables, mass and velocity (or perhaps more correctly, "momentum", which is defined as mv, and velocity).
Alternatively, our force can simply change the relative position of objects that have an attraction or repulsion of each other. Your book is attracted to the center of the earth by gravity. It takes a force to lift it. Therefore its energy is increased. This is also mechanical energy and we refer to it as potential energy.
Consider a ball tossed in the air. Our work first produces kinetic energy of motion. As the ball rises, the kinetic energy is converted to potential energy. At the top of the arc, all the energy is potential. As it falls back, the potential energy is converted back to kinetic energyuntil it hits the floor. Then what happens to the energy?
That is where we have to get inventive if we are going to preserve the law of conservation of energy. There must be other forms of energy which the kinetic energy was converted to.
This brings us to a remarkable breakthrough in the history of science, the recognition that something we call heat is just such a form of energy.
We can define heat as that which flows from a hot body to a cold body, until the temperature of the two bodies becomes the same. Heat was first conceived of as some kind of fluid, referred to as a caloric, i.e. that it was some form of matter involved in the transfer. We owe our idea of the equivalence of work and heat to experiments done by a British scientist named James Prescott Joule. Joule noted that you could put a paddle in water, and turn a crank causing the paddle to spin, and the result of this "work" was an increase in the temperature of the water. He could then using an electric heater measure the amount of electrical energy necessary to cause the same increase in temperature, and therefore define the heat as an equivlent form of energy. Our current unit of energy, the Joule, is named after him, and is defined in terms of electrical energy as the product of voltage (in volts, an intensive variable) and current (in coulombs, an extensive variable).
Even heat is not measured directly, but must be measured in terms of its effect on the temperature of an object. We use a device called a calorimeter to measure heat. (See figure 4.6). In fact, a commonly used energy term is defined just on this basisthe calorie was originally defined as the amount of heat energy needed to raise one gram of water by one degree Celsius (specifically from 14.5o to 15.5o). The calorie is now defined as 4.184 Joules exactly. The food Calorie is actually a kilocalorie (i.e. 103 calories).
The property of a substance that describes its ability to absorb heat is called its heat capacity. Water has one of the highest heat capacities of substances, which makes it such a useful fluid in many heat exchange devices, such as your car radiator. It also helps your body to maintain a fairly constant temperature.
What is heat, precisely? Where has that "energy" gone to? The study of such energy changes comprises the field of thermodynamics. All that is necessary is that we define for any substance that it contain a certain amount of energy we call internal energyessentially a ploy to preserve the law of conservation of energy. When you add energy to an object and its energy increases, then we say its internal energy has increased. When it cools, it loses some of its internal energy. We do not require a molecular explanation for this formal treatment to be valid. Nevertheless, we have a molecular interpretation. We relate temperature to the motion of molecules, and hence the heat energy an object contains is related to the kinetic energy of the molecules. We also consider the electrons in motion about a nucleus, indicating that it possesses both kinetic energy (from motion) and potential energy (dependent on the distance from the nucleus), and that these "mechanical" energies represent the "internal energy" of a substance.
So now, what about energy changes associated with a chemical reaction? Remember the balloon explosion, which was simply the combustion of hydrogen, represented in the following reaction:
2 H2 + O2 -----> 2 H2O
You observed energy in the form of heat, light, and sound being produced. In this case q (and w) were done on the surroundings, so carried a negative value, which means that the internal energy of the system (i.e. the hydrogen and oxygen atoms) must have decreased. We can describe this process by writing energy as a product of the reaction, and we say that the reaction is exothermic. (The actual energy produced depends upon whether the water ends up as a liquid or a gas, since gaseous water gives up some additional heat as it condenses.) We can write a properly balanced equation, including energy, as follows:
2 H2(g) + O2(g) -----> 2 H2O(g) + 483.6 kJ
This means that 483.6 kJ of heat energy are produced when two moles of hydrogen and one mole of oxygen react to form two moles of water. We might find the quantity expressed more often as 241.8 kJ of energy per mole of water produced.
It is possible to decompose water into hydrogen and oxygen, but it takes energy to do so. One way of accomplishing this reaction is to pass an electric current through water to carry out an electrolysis, and one must put in a quantity of electric energy equivalent to that produced in the above reaction, in other words:
So where is this energy coming from, and where is it going? Our molecular interpretation lies in the energy change electrons undergo when they take part in the formation of chemical bonds. When two electrons pair up to form a chemical bond, their overall energy (a combination of kinetic and potential) actually decreases, meaning that the chemically bound state is a state of lower energy than that of the individual atoms. Therefore we could conceptually conceive of the above reaction as the sum of three steps:
Even though the actual reaction doesnt proceed that way, the internal energy change can be calculated as if it did. Just like if you climb to the summit of a mountain, your potential energy increases by a specific value, no matter what the path you take to get to the summit.
A lot of thermochemical data can be summarized in the form of bond energies as shown in table 4.1, and the process can be described as follows:
2 H2 -----> 4 H + 2 ( 432 kJ)
O2 ------> 2 O + ( 494 kJ)
4 H + 2 O -----> 2 H2O - 4 ( 459 kJ)
Sum - 478 kJ
Note this is close to the measured value given above of 483.6 kJ.