18 minutes reading time (3674 words)

    Queensland Branch Schools' Chemistry Lecture 1991 Energy and Chemistry

     Introduction

    Some years ago as an 11th or 12th grade student puzzling over a particularly difficult physics or chemistry problem, I used to wonder whether the study of science was so important. I guess many of you ask yourselves the same question regularly. The decision I made is obvious, but I believe that the question is even more critical to you in the early 1990s. In the world we live in, study of the central sciences such as chemistry, physics and mathematics is of crucial importance. And I affirm that statement, regardless of whether those studies finish at the end of matriculation, or they continue for additional years at university in specific areas of science or in applied areas such as engineering or medicine. By way of illustration, I want to take a (rather light-hearted) look at aspects of a topic often dealt with in the media, and about which you probably feel you have some knowledge. Let me give you a rather different perspective.

    World's energy resources

    Figure 1 gives some predictions (1) as to the production and consumption of a few of our energy resources over the next millennium. In such a long time scale there are naturally many uncertainties involved, but the overall picture is probably reasonable. For example, a resource such as petroleum (oil) is clearly non-renewable - a certain amount is relatively easy to access, but ultimately the cost will rise in response to scarcity, the cost of exploration and the increasing difficulty of access. In this particular case, it is certain that during your lifetimes we will reach the point where our use of this resource rapidly declines and it is replaced as an energy source. We are often told that coal will provide much of the replacement - that is probably true in the short term, but ultimately the same scenario holds for coal as it did for oil, and while the peak in the curve is perhaps two centuries away it may come sooner if the environmental consequences of burning coal (namely, the production of carbon dioxide and sulphur oxides) become unacceptable.

    Three points might be made from the Figure 1:

    • The total human consumption of energy resources is marked on the graph. Clearly in the short term there is no "shortage" of energy and our choice is more to which is the most appropriate to use from economical and environmental viewpoints. As the population and the energy use rises, in the medium and longer term decisions will need to be made on the basis of resource availability.
    • Nuclear fusion, the process that goes on in the Sun and in the hydrogen bomb, is the subject of intense research by physicists. If we are able to confine that reaction it would provide us with a clean and plentiful supply of energy - while there have been some very promising recent advances, realistically the technology may be some three decades away. The extraordinary debate that went on two years ago about "cold fusion" gave some indication of the importance of fusion (2); if that claim were true it would arguably be the most important scientific discovery this century, but so far it is unsubstantiated.
    • The amount of solar energy hitting the earth's surface is enormous. In Figure 1 , a level corresponding to only 10% of that resource is shown, and it is approximately 100x greater than our current total usage of energy (the vertical scale is logarithmic).

    The nuclear fusion and solar options are our only long term alternatives as providers of energy resources. The fusion technology is not yet accessible. Despite the fact that the solar energy is available, it has not been extensively used. There are reasons for that. Firstly, the supply is diffuse - in Queensland, a square kilometre of the earth's surface receives radiation with the energy equivalent of ca. 2000 tonnes of coal in one average 24 hour day. The coal, however, is relatively compact and can be moved to wherever it is needed - clearly the solar energy is spread over the wide area and cannot be transported in the same way. Moreover, it is spasmodic because of the day/night sequence and changes in weather. Secondly, the location of the majority of the world's population is to be found in temperate regions of the earth whereas the greatest solar irradiation occurs between the tropical zones. A solution to that problem would be the conversion of solar energy to an appropriate fuel, allowing the energy to be stored and transported as required to where it was needed. And that is the proposition I want to examine - what are the chemical principles involves in the concept of harvesting the sun's energy?


    Energy and chemistry - 'Hot' reactions

    There is an intimate relationship between energy and chemistry. Each substance has its own energy content, and so any chemical reaction in which two or more substances are combined to form a product will take place with either a surplus or shortage of energy. We call these endothermic and exothermic reactions, where (respectively) energy will either be absorbed from the environment to make up the energy balance or energy will be released into the surroundings, as shown in Figure 2.

     A dramatic example of an endothermic process is the reaction of the two solids barium hydroxide octahydrate Ba(OH)2.8H20 and ammonium thiocyanate NH4NCS (3a). The chemical equation is

    NH4+ + OH- + energy → NH3 + H2O

    and sufficient energy is absorbed from the immediate surroundings that the temperature in the reaction vessel drops by ca. 40°C in less than a minute. If a flask containing the reaction mixture is placed on a wooden block wetted with a few drops of water, the flask freezes to the wood and the two can be lifted together.
    An example of an exothermic process is the burning (or combustion) of magnesium in oxygen to produce magnesium oxide (3b,4).

    2Mg + O2 → 2MgO + energy

    The excess energy is seen as an intense flash of light, and this reaction was used extensively in the old replaceable flash bulbs and is still used in flares and pyrotechnics. Another spectacular example comes from a series of reactions known as the "thermite" processes (3c): the most common example is the reaction of powdered ferric (iron) oxide and aluminium, which react to produce aluminium oxide ("alumina") and iron. The reaction releases considerable energy so that the temperature rises within seconds to ca. 2000°C and the iron that is formed is actually molten. The reaction was used by railway crews in isolated areas as a means of welding railroad tracks.

    Fe2O3 + 2Al → Al2O3 + 2Fe + energy

    'Cold' reactions

    Those of you who watched the recent TV series by Sir David Attenborough entitled "Trials of Life" will no doubt have been fascinated by the program on animals that emit light (such as glow-worms, fire-flies and luminescent fish). Quite clearly these organisms rely on chemistry to produce that light, but equally as obvious is the fact that the respective reactions occur at biological temperatures and are not subject to the wild fluctuations characteristic of the reactions in the previous section. There must be another link between chemistry and energy....

    To give an idea of what is happening, let me introduce you to a character held in great affection by all chemists, whom I will call "Mr Molecule". In the cartoon shown in Figure 3, we can follow the sequence from 1, where our relaxed character is in his "ground state". In our story we give him energy in the form of a hot potato. On receiving it he is clearly no longer relaxed and is said to be in an "excited state" 2. Since he would obviously prefer to be in the ground state, let us examine the means by which he can achieve that. Of course he can immediately drop the hot potato. However, let us suppose that he has slow reflexes or is a particularly thoughtful character and survives the initial panic of the first excited state to reach a more rational state 3, in which he is still excited but is at a slightly lower energy than the original excited state since some loss of energy occurs in the time it took reaching the second excited state. He now has three distinctive choices:

    ​a. He can now throw away the hot potato - he then goes to the ground state, and importantly the potato he discards is not as hot as it was when he received it because of the loss of heat that occurred in going from the first to second excited states;
    b. He can dissipate the energy into the surroundings by juggling the potato; and
    c. He can pass the energy on to something else - for example, if there is a bucket of water handy and he put the hot potato into it, he would return to the ground state and the water in the bucket would have become hot.


    Three choices - throw the energy away, dissipate the energy into the immediate environment, or pass the energy on to something else.

    The cartoon series is of somewhat more than passing significance - it turns out that a real molecule absorbing light energy has a similar set of options. Following absorption of the light, the "ground state" molecule goes to the excited state, 2, from where it can immediately release the absorbed energy. However, it may rapidly convert to a lower energy but longer-lived excited state, 3, from which it can undergo one or more of three processes:

    Luminescence - this is equivalent to (a) in Figure 3 where the cartoon character throws away the (slightly cooled) hot potato, and in an analogous way the radiated light (luminescence) is always lower in energy than the absorbed light.
    *M → M + light , (* represents an excited state)

    You will be familiar with luminescence (phosphorescence and fluorescence are sub-categories of the general phenomenon): eg. if we shine a beam of blue laser light (5) through a solution of the red dye Rhodamine we see an intense yellow fluorescence. The same light on a solution of the orange-coloured metal compound [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) produces a red luminescence (3d). You will also be familiar with the fluorescent materials put in soap powders so that on absorption of ultraviolet light they luminesce white light - the glow of a white shirt or dress under the "black light" (UV) at a disco is quite spectacular. And if someone at the disco has a glass of gin and tonic which develops an eerie blue glow under the same "black light", it's not the effect of the gin - the quinine in the tonic water is luminescing.

    Non-radiative decay - this is equivalent to (b) in Figure 3 where the cartoon character dissipates the absorbed energy into his own movement and into the surrounding atmosphere. An example of this is the reaction between the gases hydrogen and chlorine. Chlorine is yellow-green in colour due to absorption of light in the blue-UV region: on such absorption, the energy promotes vibration of the Cl-Cl bond to the extent that dissociation takes place to produces chlorine atoms, whereupon the following sequence occurs

    so that the cycle becomes more and more rapid and a violent explosion occurs (3e).

    Quenching: this is equivalent to (c) in Figure 3 where the cartoon character passes the absorbed energy on to something else. In the case of quenching, the absorbed energy is used to drive a chemical reaction of the excited state. As an example, the intense red luminescence of [Ru(bpy)3]2+ obtained on irradiation by the blue light (see earlier) is totally stopped by the addition of Fe(III) to the solution, due to the sequence shown below in which an electron is transferred from the excited state to the "quencher" (in this case ferric ion):
    [Ru(bpy)3]2+ → *[Ru(bpy)3]2+

    *[Ru(bpy)3]2+ + Fe(III) → *[Ru(bpy)3]3+ + Fe(II)

    The question may be asked as to whether one can manipulate this sequence. For example, in terms of the cartoon character, can "Mr Molecule" pick up the hot potato from the bucket of hot water, go to an excited state, and throw the hot potato away? Or in real terms, can a chemical reaction occur in which a molecule is raised to the excited state from which it releases the energy in the form of radiation (i.e. luminescence)? The answer is yes, and the phenomenon is known as "chemiluminescence".
    chemical reaction → *M → M + light

    Again you will be familiar with examples of chemiluminescence - it explains the biological luminescence of fireflies, glow-worms, luminous fish (which generally arise from chemical reactivity with oxygen within bacteria) but without the temperature changes that were a feature of some of the reactions I showed earlier. You may also be familiar with the emergency torches used in diving and boating in which two chemicals are mixed to form an excited state of a dye contained in the mixture which then luminesces brightly: the reactions may be summarized as follows:

     where the dye 9,10-bis(phenylethynyl)anthracene is used for a yellow chemiluminescence (3f).


    Using the relationship of light and chemistry

    This interrelationship between light and chemistry means that chemistry may be used to harvest light energy. A graphic example of this is an experiment where one shines light on a solution containing a mixture of [Ru(bpy)3]2+ and [Co(NH3)5Cl]2+ : the ruthenium complex goes to the excited state (as we saw earlier) but undergoes quenching by the cobalt species and oxidises to form [Ru(bpy)3]3+ - the cobalt species falls apart after the reaction and takes no further part. As a result of absorbing the light energy and undergoing oxidation, the ruthenium species changes in colour from orange to green. By doing a chemiluminescence experiment (in this case adding sodium borohydride solution) the green ruthenium species goes to the excited state and luminesces red light, demonstrating that the absorbed light energy had been stored within the chemical system (3d).

    [Ru(bpy)3]2+ → *[Ru(bpy)3]2+
    *[Ru(bpy)3]2+ + [Co(NH3)5Cl]2+ → [Ru(bpy)3]3+ + [Co(NH3)5Cl]+
    [Ru(bpy)3]3+ + BH4- → *[Ru(bpy)3]2+ + BH3 + (1/2)H2
    *[Ru(bpy)3]2+ → [Ru(bpy)3]2+ + light

    One of the interesting things about science is that often, when you think you have a good idea, you find nature has already developed it to a very sophisticated level on an enormous scale. In the present case, the use of chemistry to harvest solar energy is done in nature by a process known as "photosynthesis". Many of us think of photosynthesis as the conversion of carbon dioxide and water to carbohydrates (sugars, starch and cellulose) and oxygen by plants using energy from the sun. Well, let us look in a little more detail at that process...

    In the cartoon of Figure 4, we see that sunlight impinges on the plant and is absorbed within the organism by a molecule (chlorophyll) which goes to the excited state. When a molecule does this, an electron is relocated from one energy level to a higher energy level. In the absence of anything else happening, the excited electron will decay back to its initial state - but in the photosynthetic scheme the excited electron is rapidly moved from the site where the light was absorbed to somewhere else in the organism. The net result is called charge separation and is manifested in the organism having a region with an electron excess and a physically separated region with an electron deficiency; you will recognise that these centres will be potential sites for chemical reactions called reductions and oxidations, respectively. In the case of photosynthesis, the reduction is of carbon dioxide to carbohydrates and the oxidation is of water to oxygen.

     The photosynthesis process is indeed the harvesting of sunlight, the conversion of light energy to a chemical fuel. If you are in doubt about that consider that all our coal and oil reserves are derived from decayed plant matter whose origin is in photosynthesis! Despite the immensity of the scale however, the€ efficiency of this conversion is relatively low, being of the order of 1% even for the most efficient plant systems (eg. sugar cane). The question one may ask is whether modern chemistry and physics can provide more efficient schemes for conversion of light to chemical fuels.

    Let me give one simple example. As shown in Figure 5, if a p-type semiconductor is placed in a beaker of water containing a little acid, and connected via an external circuit to a metal or carbon electrode immersed in the same solution, then on shining light on the semiconductor bubbles of gas will be evolved at both electrodes; nitrogen at the semiconductor and oxygen at the inert electrode. In this case, the semiconductor is acting as the "Mr Molecule", absorbing the light energy and directing excited electrons to its surface where they reduce water to hydrogen. The other electrode is left with a positive potential and it oxidises water to oxygen. This simple example shows the conversion of light energy to a fuel, in this case hydrogen.

    Importantly, the conditions of the experiment may be maximised so that the efficiency of light conversion to the fuel is in excess of 15%.

    The use of a solar panel to generate a potential difference between two electrodes is really no different in principle, except that the light absorbing medium is external from the solution. The solar panel fills the role of absorbing the light and allowing the 'charge separation' of the excited electrons which are used in promoting chemistry in an electrolysis cell. Among a number of examples that demonstrate this is the electrolysis (using carbon electrodes) of a solution containing potassium iodide and the pH-indicator phenolphthalein (6). While the light is shining on the panel, at the electrode to which the electrons are directed (cathode) the reaction

    2H2O + 2e- → 2OH- + H2

    occurs and the gas hydrogen is evolved, while the region around the electrode goes pink because the hydroxide ions produced make the solution alkaline and the indicator turns pink. At the other electrode (anode), iodide ions are oxidised to iodine which is seen as a brown colour developing at the electrode.

    2I- → I2 + 2e-

    Quite clearly, the principle of conversion of light energy to a usable fuel is achievable using the relationship between light and chemistry. If we were to take the earlier cartoon of photosynthesis (Figure 4) and modify it, we might envisage some sort of "artificial photosynthesis" in which some "Mr Molecule", yet to be identified or developed, is used to absorb light within a system designed to separate the excited electrons to a site where they might reduce carbon dioxide to the fuels methanol or methane (natural gas) or reduce water to hydrogen (see Figure 6). At the oxidising centre, as in photosynthesis, water might by oxidised to oxygen.

     As we have seen from these discussions, this is a workable hypothesis. And the challenge to modern science? The identity of the "Mr Molecule" in such a scheme, and research proceeds in many chemistry and physics laboratories around the world to design better molecules for this purpose.

    The future?

    We have looked at perceived problem of modern society - the availability of energy sources to generate electrical power to supply our industries and to maintain and develop our way of life. Undoubtedly science will find solutions, as it will to other problems such as the control of pollution, the protection of our environment, the production of food, the development of medicines and many others.

    But why is the particular scientific discipline of chemistry so important in that context? The answer is related to the central position of chemistry among the sciences. Chemistry impinges on so much of our lives - energy, medicine, mining, agriculture, food, the environment, industry (for example textiles, soaps and detergents, paint, cement, semiconductors, cosmetics), forensic science, etc...

    The community must come to an increasing understanding that many of the problems I mentioned above have a scientific basis and so the answers are to be found in science. Unfortunately, there is often a tendency to look on science as their cause, rather than the means to their solution - arguably many of the problems that exist have been caused by commercial rather than scientific decisions on alternative choices. Nevertheless, increasingly the community needs to be informed to be able to understand the basis of these issues. And it is there that the knowledge of science in the general community is important. But in this day and age of the 30-second sound-bite on the TV and the ready acceptance of a trendy phrase we often have a problem with the level of scientific insight required; the possession of a little knowledge can be dangerous.

    Advances in science will obviously come from those with particular scientific training, but the public and those in positions of responsibility will increasingly need to be discerning about what is science and what is populist phraseology. I asked you earlier whether your studies in science at secondary school were important - my answer is that more than ever we will need people trained and informed in science. I offer you every encouragement to continue with your studies - they are important for you, for the society in which we live, and for the future of this country.

    References

    1. McMullan, J.T., Morgan, R.,_and Murray, R.B., "Energy, Resources and Supply", Wiley (London), 1976, Figure 1 is adapted from Fig. 1.3 of this reference (p.6).
    2. Clark, R.W., J Chem Ed., 1991 , 68, 272-279.
    3. Adapted from Shakhashiri, B.Z., "Chemicar Demonstrations - A Handbook for Teachers of Chemistry", Volume 1, University of Wisconsin Press, 1983: (a) p.10-12, (b) p.38-39, (c) p.85-89, (d) p.194-199 , (e)p.121-123, (f)p.146-152.
    4. Adapted from Shakhashiri, B.Z., "Chemicar Demonstrations - A Handbook for Teachers of Chemistry", Volume 2, University of Wisconsin Press, 1985: p.162.
    5. In the lecture series an argon laser unit, designed for use in forensic science, was loaned to the author by COHERENT SCIENTIFIC PTY. LTD. (Adelaide) for these demonstrations. The company is thanked for its generosity.
    6. Adapted from Summerlin, L.R., Borgford, C.L., and Ealy, J.B., "Chemical Demonstrations - A source book for Teachers", Volume 2, 2nd Edition, American Chemical Society (Washington), 1988; p.201.

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