Chemical Energy: A Learning Package Ita C o h e n and Ruth Ben-Zvi Department of Science Teaching. The Weizmann Institute of Science, Rehovot 76100, Israel T h e various asuects of energetics are central to t h e understanding of chemistry. If t h e student has gained insight into t h e meaning a n d role played by energy considerations, t h e structure o r matter, t h e nature of chemical reaction, a n d factors influencing t h e m can he understood better. However, t h e teaching of chemical energy poses many problems. T h e difficulties can be classifed into three categories. 1) Conceptual difficulties. The energy concept is theoretical. The student has to understand its meaning, as well as the link hetween
changes in energy and changes that wcur a t the molecular level. He, for example, has to have a working understanding of the facts that bond dissociation always consumes energy, while hond formation nroduces and that the AH of a reaction is the sum of the e~~~~~~~ it.,~ enerpv of the hond dissociation and bond formation as.....-, rhnnees .~.-~~-.. .~-~~ ~~-~~~ sociated with the given reaction. (Thisunderstanding is made even more difficult by misconceptions which some students bring from their hiology class, where ATP ia still considered as having a "high energy" . phosphate bond.) 1) Colvulatrrm of unlholp) chnnge,~.In many teachingschemesmurh student time is taken up with various kind3 of calculnriunr. Apulications of Heqs's law usually require techniques of ndditim andlor subtraction of chemical equations together with similar calculations involving the appropriate heats of reaction. By using various algorithms, students can also calculate AHo; for example, AHa = XAH, r o (oroduets) - 2AHro(reactants ) (1.2). Thev do . nut alwabs understand the meamng of therr ralculatmns and the cunnectlm hetween the result^ and uhnt has occurred at n molecular level. R.,I ....-. Rrnorimentnl diffieultie.9. mend oart . .. ~ ~ Students ~ , usuallv , -~ . ~ . of~their trme in the laborntory meamring temperature changes in various chemical renetrons and calculating the eorreipunding hcnls of reaction. The equation q = mCAt is introduced and used, very often mechanically. As long as the reactions are simple and routine, the teacher may he unaware of lack of understanding. However, problems soon become apparent when relatively small "complications" are introduced. For example, in the hromination of cyclohexene in a nonaoueous solvent. manv students are at a loss as to uhich valurs are to ire suh\titurcd inw the q u a t i m Oneof the commun mistokes i that instead of using the mays ofthe solwnt, thcy use that crf rhe rycloherme fur thc cnleulation of q . This latter misundeistanding may be caused by the lack of a clear definition of the terms system and surroundings in the context of energy transfer. The bomb calorimeter is usually the only example used to explain the measurement of heats of reaction. Conceptually this i, verysimple because the system and thesurroundings are separated and well defined. Problems begin, however, when the student performs simple reactions; for example, a neutralization reaction, where no visible boundary between the system and the surroundings exists. Therefore, many students consider the entire contents of the beaker as the system. Another problem with the term system is that it may be given different meanings in different cnnteats (l,2). Bent (3) treats this concept in a way that simplifies its use and, in our experience, enables the student to interpret the results of experiments correctly. He defineqthe uniuerse of an event as those objects that might he altered during the event. He then divides the universe into the system and thesurroundings and claims that "this partitioning of the universe in the mind's eye into two parts proves to he very useful in thermodynamic analysis of chemical events." ~~
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package was developed. T h e package was designed to maximize student active involvement in t h e learning process (4, 5,6,7).It is made u p of a sequence of activities, each having a specific objective i n t h e framework of t h e whole scheme. T h e r e are five types of activities which occur i n cycles repeated throughout t h e study of t h e package (see Fig. 1).
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Package The Chemlcal Energy I n a n attempt t o overcome some of the above-mentioned problems, a comprehensive teaching-learning chemical energy ~
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Figure 1. Cycles of activities
Journal of Chemical Education
11 Croup artiui1ie.i hsed on pre.prppared written material. Thr packwe is divided ink, six units. Fnch unit cmun nrutrnd n gruup
artiwty. Gruups of fuur student9 discuss prublpma posed in preprepared written matprial. If the rnajwity fail to answer any of these correctly. they will then he discussed with the wholp class. The level ufdiacussion and the uncr arp detrrmmed rwithin limitv hy the students themselves. ~ ithe l members of a group are encauraeed to oarticioate activelv in the discussion hv raisine questlonr, clarilyinl: pnrhlerm, o r supplying n n s u m . For example. the iirnt group activity deals with exothmnir and endothermic reactions. From a list of processes the members of the group have toidentify those whichevolve energyand those whichabsorbenergy. The definitions of system and surroundings are introduced at this stage, and the memhers of the group have to consider the exchange of energy between the system and its surroundings for various reactions, such as vaporization or combustion. 2) and 3) Laboratory experiments and inquiry questionnaires. The earlv exoeriments are desiened to lead the student eraduallv to understand the connections hetween the exoerimenk data and ment is followed by an inquiry questionnaire consisting of questions which focus student attention on some idea central to the experiment performed. For example, after the first group activity, the student mixes 50 ml of a 1M solution of Na?SOa with 50 ml of 1M solution of BaC12 and measures the temperature change. The following questionnaire is then given. 1) Write down the net ionic equation for the reaction. 2) Identify the system and its surroundings. 3) Can you calculate the energy change in the process? YesINo. 4) If Yes, how? 51 -If -N"~ .*, a. Explain why? h) Suggest asuitable method for obtaining the amount ofenergy involved in the process.
SURROUNDINGS
SYSTEM
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Boio~+soliosl -S~SOIIS~
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ENERGY TRANSFERRED
ENERGY
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Figure 2. Exchange of energy between the system and the swroundings. The su~oundlngsinclude 100 mi of solution which contains aqueous ions, but as long as the concentration of Me ions is low, we can consider the surroundings as 100 mi of water.
In this case, although students had been given the definition of system and surroundings in the written material for the first group activity, for most of them the system consisted of the two solutions, and all the rest (the beaker, the thermometer, the desk, the air, etc.) were the surroundings. I t is clear that on this basis it was impossible for the students to answer questions 3-5 correctly. Thus, typically a student would answer no to question 3, and in a response to 5h we found answers like the following: "I am sure there must be a formula to find the energy change" or "there must he an instrument similar to a thermometer with which one can measure enerev chanee." This examole emohasizes the oroblem inherent in thP'wachi;;g ofdrfimtwn~W ~ I C ~ Wmentionid C above. The student may Iw ahle to qoure a definition hut may not nrce3sarily be ahle tr,~pplyit. After rhissection and the idlowingclass diacuwion, the student is ahle to applv the ilrfinitionx correclly. 4, Trarher-lrd closx dtscussron. As must of the quesrlons in the questionnaires are nor based un previously learned ideas, the student is generally uncertain whether his or her answers are correct, and is therefore eager to get confirmation in the class discussions. The teacher collects the responses to the questionnaires and can use the students' answers (both the correct and incorrect) in the class discussions. The students who answered correctly are intellectually rewarded by the teacher, and those who answered incorrectly are faced with the "conflict" between what they thought and what they learn subsequently. Cogan and Johnson (6)claim that this sort of conflict contributes to the inquiry-learning situation by producing more creative answers, deeper understanding of the prohelm, higher motivation, and higher involvement. Thus, the conflict becomes a positive element in the learning process. The class discussion that follows the questionnaire given might be concerned with the following two aspects: a) The qualitatiue aspect. The definition of system and surroundings is given again and its interpretation for the specific experiment is discussed. Using a "multilayer" transparency the following steps are presented: i) The changes that took plaee'in the reaction-formation of a precipitate and temperature change. ii) Identification of the system-in this case BaZf(.,) and S042-(,, before the reaction and B ~ S O I ~after. ,, iii) Identification of the surroundings which include the water and spectator ions (those which did not participate in the reactiun. namely Ya' .,,and CI-l.,,~. h) Thrquonrtrotir.r o~pert.h m e n t i o n d rhestudents had many difficultieiin surgertinr methuds fur meawring or calculatinp: the amount of energtransferred. Having partitioned th; "universe of the reaction" into system and surroundings, the teacher can.. hv. suitable auestions.. lead the students to undemand that the energy uf the *urroundings itwreases as a rrsulr ofthr rnrrgy trnnsfer from rhesystem. Uou, by the law of conservation of energy, the energy increase of the surroundings must be matched hy a similar decrease in the energy of the system. If we know how to find the energy change in the surro&dings, we know the energy change in th&yste
