chemical -principles revisited
edited by
MURIELBOYDBISHOP Clemson University Clemson, SC 29631
Studying Activity Series of Metals Using Deep-Learning Strategies Tien-Ghun Hoon, Ngoh-Khang Goh, and Lian-Sai Chia National Institute of Education, Nanyang Technological University, Republic of Singapore 1025 The difficulty that many high school students have with chemistry is a familiar story, Students see no connection between the numerous facts, and they are unable to visualize the abstract concepts of chemistry. Surface-level processing strategies, such a s rote memorization of a s much information a s needed to pass a n examination, and indiscriminant reading of the chapters, are the main learning tools acquired in most high school chemistry classes. The result is that high school students see chemistry a s a subject made up of many different, unconnected topics. A better way of teaching the inter-relationship between chemical concepts is through the use of deep-level processing strategies, where new information is linked to previously learned materials. The result of using these strategies is enhanced comprehension and skills, and better retention of knowledge. Because high school teachers teach the activity series of metals, this topic is used to illustrate the linking together of numerous chemical concepts involving the activity of metals-quantitative analysis, corrosion, electrolysisthrough the use of deep-level processing strategies. Example 1illustrates a problem in which the students must link their knowledge of the chemical reactivity of the metals to the chemical nature of the compounds formed. Rote memory would not help the students solve this problem. Example 1. Explain why calcium reacts readily with dilute sulfuricacid initially hut stops reacting after about 20 s. Answer 1. Although calcium is a very reactive metal, the reason it does not react readily with dilute sulfuric acid after a period of time is that the product, calcium sulfate, is insoluble in water and coats the metal, preventing contact of metal and acid. If a student only knows that an active metal can react with dilute acids, but without knowing the nature of the metal andlor the compound formed, he or she may have difficulty in answering this question. I t is hoped that, by integrating the topic of Activity Series of Metals with other topics, students will be able to change their perceptions toward chemistry and also to apply the right learning strategies in their studies. We have found the following strategies to be effective in increasing students' comprehension, skills, and confidence. Introducing the Activity Series The first treatment of the activity series of metals comes early in most texts, usually after the introduction of classification and grouping of the elements in the Periodic Table. Students can be taught to associate the order of activity of
metals with the Periodic Table. The activity series should be recalled each time a new topic related to metals and their compounds is introduced: reactions of acids and bases with metals, salt formation, electrolysis, etc. Deriving the Activity Series Students will accept the activity series if they have been shown how to establish the order of the metals in the series. The teacher can demonstrate the reactions of some of the common metals with wateristeam, dilute non-oxidizing acids, and aqueous ions and oxides of some other common metals. A simple test of the effectiveness of this method of teaching is given in Example 2. Example 2. Why is hydrogen placed between iron and copper in the activity series of metals? Answer 2. Hydrogen can be displaced from dilute oon-oxi-
dizing acids (e.g., hydrochloric acid) by iron, but not by capper. Examples 3 and 4 are tests of deeper comprehension of the activity series. 3. Why is aluminium apparently unreaetive toward E-ple steam and acids? Answer 3. This is mainly because of the protective oxide layer on aluminium. Example 4. Explain why potassium, sodium, and calcium metals do not react with the metal ions of other metals in aqueous solution to give the free metal (displacethe other metal), even though they are placed high in the activity series of metals.
Table 1. Activity Series, Periodic Table Electronic Configuration Metal
Group
Period
Electronic Configuration
4 2,8,8,1 I potassium 3 2,8,1 sodium I 4 2, 8,8,2 calcium II 3 2,8.2 magnesium II 3 2,8. 3 aluminum Ill Transition Metals zincf Transition Metals iron Transition Metals copper By definition, zinc is not a transition metal. But il can be classified as a transition metal in high school chemistryfar simplicity.
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Answer 4. Pntasaium, sodium, and calcium metals react better with water rto produce hydmgen, than they react with the metal iuns ofother metals to g v e the free metal.
Remembering the Order of the Series Table 1 illustrates a techniaue bv which students can use the periodic table to relate the order of metals in the activitv series to the electron confirmrations of elements in the ~&iodsand Groups. 1. The decrease in activity of metals across a period from left to right can be explained as being dependent uDon two factors: increased positive nuclear charpe and deout that creased radius of the atom.-Teachersshould electrons adding at essentially the same energy level (3s and 3p in period 3) in a given period are drawn closer to the nucleus by the increased nuclear charge. The outcome is a decrease in activity of the metal as the size of the atom decreases; the outer electrons cannot be lost as readily from a hieher nuclear charee. 2. ~heyncreasein activig of metals down a group is ex~ l a i n e das also beine deoeudent uDon nuclear charee and Padius of the atom. within a group'the energy of the-outermost electron increases significantly from one major energy level to the next; a n electron in a 4s orbital (potassium) is higher than the electron in a 3s orbital (sodium), which, in turn, is higher than the outermost 2s electron in lithium because as the outermost electrons increase in energy from one main energy level to the next, the nuclear charge becomes less effective at attracting these electrons. A significant increase in the radius of the atom is seen within the group from top to bottom and the activity (loss of the outermost electron) increases in the same direction. (Note: lithium and most Period 1 elements do not obey these eeneral trends). 3. %ansition metals are considered to be less active than the first three mouDs of metals because the increase in nuclear charge results in a stronger binding of the oub ermost electrons that are adding into the same energy level, i.e., 3d in the third period. At the same time that this new material is being introduced, a review of previously learned topics-atomic structure, periodic table, and descriptive chemistry-should be made. Repetition and mastery of concepts provides the student with the opportunity to incorporate new knowledge into previously learned materials. This leads to better comprehension of both old and new materials.
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-
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Predicting the Position of an Unknown Metal in the Series
Examples 5 and 6 illustrate the technique for using relevant information in the Periodic Table to place an unknown metal in its correct order in the activity series. Example 5. Metal X reads slowly with steam. There is no visible change observed when it is placed in iron(I1) sulfate solution. Suggest the position of metal X in the activitv -.--.series of metals. -." .Answer 5. Because there is no visible reaction between X and iron(I1)sulfate solution, X is not higher than Fe in the activity series of metals. However, because X reacts with steam slowly, it must be above hydrogen in the activity series. Hence, the position of X is below Fe hut above H. Examole 6. Lithium is the first member of Grouo I alkali metals. From this infomation alone..suezest the oosition -~~ of lithium in the activity series of metals. Answer 6. Lithium is the first member of Group I alkali metals, so it must be less reactive than any other Group I 52
Metal potassium sodium
Group
Period
I I
lithium
I
4 3 2
calcium magnesium etc.
II II
4 3
etc.
etc.
Table 3. Methods of Extraction and Group Numbers
Metal potassium sodium calcium magnesium aluminum zinc iron comer J
Group
~
Journal of Chemical Education
--
Extraction method
I I
II II 111
Electrolysis
Transition metals
Reduction by coke
Table 4. Thermal Stability of Carbonates and Nitrates
Metal
Carbonate
potassium sodium calcium magnesium
not decomposed
zinc iron copper
Nitrate
decomposed to give nitrite and oxygen (less readily decomposed)
&
Ao~lvina . . . the Activltv Serles to Other ConceDts The stage of application is important because it helps students to reflect, polish, and consolidate what they have learned. Explicit examples of application are illustrated below.
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Table 2. Deducing Order of Activity fromperiodic Table
and carbon dioxide
decomposed to oxide, oxygen, and nitrogen dioxide
(most easily decomposed)
metals. However, because Group I metals are more reactive than metals fmm any other groups, lithium will be more reactive than calcium. Hence, the position of lithium is below potassium and sodium hut above calcium in the activity series (Table 2). Predicting a Method of Extraction of a Metal
Teachers should point out to students that the "cause", the chemical activity of metals, is directly related to the "effect", the formation of a stable compound-the more active the metal, the more stable the compound formed. Therefore, oxides and other compounds of the more active metals, such as sodium and aluminum, are more difficult to decompose into their elements: the reversal of the formation of that compound. Hence, methods such as electrolysis have to be ised to decompose these compounds. Oxides and other com~oundsof the less active metals. such as zinc, iron, mercury, and silver, are relatively easily decomposed. Silver oxide may explode because it is so unstable. The industrial extraction of metals from compounds found in nature depends upon the reduction of the metal compound to the free metal. A cost-effectivemethod for extracting il leas active metal from its natural oxide source is the reduction of its oxide with coke. Table 3 shows how the methods of extraction can be associated with the arrangement of metals in the Periodic Table.
Table 5. Properties of Metals Arranged In Alphabetical Order of the Names of Compounds
Cation
Color of -~ aqueous ~
~
Table 7. Color of Compounds and Activity Series of Metals
Action of Heat On Metal Carbonates
Cation
decomposed to oxide and carbon dioxide decomposed, when strongly heated, to oxide and carbon dioxide readily decomposed to oxide and carbon dioxide etc.
potassium sodium calcium
aqdeous solution
snlutinn . . .. ... ..
aluminum
colorless
calcium
colorless
copper
blue
etc.
etc.
Table 6. Properties of Metals Classified in Alphabetical Order of Color of Compounds
Color
Substances
black
CuO, FeO, MnOn, CuS, PbS, etc.
blue
hydrated copper (11). anhydrous co2+compounds
brown
iron (Ill) compounds, Pb02, 12 solution
green
cu2+. ~e'+,~
etc.
etc.
Predicting the Thermal Stability of a Salt of a Less Common Metal
The activity series provides a link to the ease of decomposition of salts ofmetals, such as nitrates and carbonates, by heating (thermal stability). Table 4 shows the similarity of the decomnosition reactions of the nitrates and carbonates of metals. Note that potassium and sodium compounds act differently from other metal compounds. Finding the Identity of a Metal Ion (Qualitative Analysis)
Tables listing the colors, solubilities, action of heat and individual tests for the cations of metals, as shown in Table 5 and 6, are a burden to students. Information in tables of this W e cannot be linked to previous knowledge. Again, teachin(: the properties and riactions ofcom~oundsas mverned by the position in the actlvlty series of th;metals inthese com~ounds,is more effective. Tahle 7 shows the student how to link certain chemical properties of compounds with their appearance-eolor of ions (color is associated with nartiallv filled d orbitals). color bf salts, solubility of precipitates. k a result, skills of deduction can be enhanced in qualitative analysis. Understanding Electrolysis
High school students benefit from certain generalizations. By noting a similarity between the activity series and the electrochemical series, they can predict which metals are formed preferably from their ions during electrolvsis. Because the more active metals. such as notassium, sodium, and calcium, form positive ions (by the loss of electrons) with ease, the metal ions of these metals will have less tendency to form the metal (by the gain of electrons) a t the cathode during electrolysis. Electrolysis of soluble metal compounds oecurs more easily where the ions of a less active metal, such as copper, tin or silver, forms the metal a t the cathode. Hence, compounds of more active metals require a higher voltage to electrolyze, and the active metals are not formed as easily from their metal ~
~.~
colorless
color of AqueoJs sodt~m common salts hydroxlae test
white
aluminum zinc iron(ll)
pale green pale green
imn(lll)
yellow 1 red-brown light brown blue blue or dark green
copper(l1)
i "
coor of
no ppt. no ppt. white ppt. insoluble in excess white ppt. soluble in excess white ppt. soluble in excess dirty green ppt. insoluble in excess reddish-bmwn ppt. insoluble in excess blue ppt. insoluble in excess
Table 8. Reaction between Metals and Water and Use of Metals as Electrodes in Simple Cells
Metal
Suitability as Electrodes
potassium sodium calcium
not suitable as electrodes as they react with water
-
Voltage
hlaher I maonesium 7 alumlnum sultable as electrodes as voitage they do not react wlth zlnc iron water copper
J
I
I
ions a t the cathode during electrolysis. If the voltage is high enough, water electrolyzes rather than the metal ion. Understanding Simple Voltaic Cells
In addition to the relationship of the activity series and the electrochemical series, two other factors will aid students in the understanding of simple voltaic cells. First, higher-order thinking skills are required to recognize that a metal that reacts with water (activity series) cannot serve as an electrode (electrochemical series), because the metal has become a soluble ion. The ion cannot spontaneously gain electrons in a voltaic cell. Second, the relative voltages listed in the electrochemical series can be used to predict which metal will act best as the anode and will form its ions easier. Table 8 groups metals according to their "suitability as electrodes" and their "relative voltage". For example, potassium, sodium, and calcium metals are not suitable as electrodes because they react with water. Magnesium, aluminium, zinc, iron, and copper metals may be used as electrodes because they do not react with water. The voltage difference between two metallic electrodes governs the amount of voltage capable of being produced by a cell operated as a battery. The further apart the two metals in the series, the higher the voltage. Thus, magnesium and copper used as the electrodes of a cell give higher voltage than zinc and copper would. Volume 72 Number 1 January 1995
53
waterlsteam
minium boat? Why should copper wire and aluminium wire not he connected toeether to carry electricity? The outcome of usine dee~-learninestrategies as demonstrated hire, c i s hopedywill result in changes in the wav students ~erceive chemistry, and changes-in their learning strategies.
-
dilute acids
is derived fmm
t
Conclusion The figure summarizes the relationships between Activity Series of Metals and some other chemistry topics. The series is derived from t h e reactions of metals with waterlsteam and dilute acids. The topics on Periodic Table and Electronic Structure exlain the order of metals in the series. BeBides these, the series also can be applied to enhance the understandine of manv other topics. It is hoped that this paper will kncourage our students to view chemistry from a Relationships between activity series of metals and other chemistry topics. wider perspective and associate the related topics in the curriculum. Understanding the Process of Corrosion of Metals In addition, training of science process skills also will occur, when we make clear links among those related conMetals high in the activity series are more easily torcepts, as indicated in the above concept map. Hence, roded in the atmosphere because they are more active toing explicit links in the process of teaching chemistry can ward oxygen, water, and acid-forminggases (e.g., sulfur dilead effectively to meaningful learning. oxide, carbon dioxide). The teacher can discuss the relationship between the activity series and the corrosion ~ ~ k ~ ~ ~ l ~ d ~ ~ process. Methods of prevention of corrosion by coating with We would like to thank heartily Muriel B. Bishop of a metal higher in the activity series-galvanizing iron with Clemson University, whose advice, suggestions, and assiszinc, and sacrificial protection of iron by the more active tance have led to improvements in this paper. metals, such as magnesium or zinc-can be related to the activity series. The students may be challenged by the ~ i tcited ~ ~ ~ t ~ ~ ~ question of whichmetals would increase the rate of rusting Chang,R, Chemidry, 4*thd ;NervJeraey:McCra w.Hi 1991. z. c a m a c h o , ~ . ; ~ o o d . ~ .sci ~ ~ he sh. . 198s,z6(3),251-272. the iron. Why would copper screws not be used in an alu-
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Journal of Chemical Education
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