Kansas Science Teacher Vol. 16, Spring

Transcription

1 "Seeing" Ionic Formulas in Chemistry Dr. Malonne Davies 1, Dr. Arthur Landis 2, and Linda Landis 3 1 Department of Physical Sciences 2 Department of Physical Sciences 3 Science and Mathematics Education Center Emporia State University Emporia, KS Introduction Learning chemistry begins with the prospect of learning the chemical language. The first step is learning the symbols and names for the elements, i.e., the chemical alphabet. The extended alphabet includes formulas and names for the ions (electrically charged particles). The next step in the language of chemistry is to understand what chemical formulas mean; then how to construct them using the ions. The formulas are the equivalence of chemical words. These words can be used in chemical sentences called equations. Teachers of science are always looking for ways to make abstract ideas more concrete for their students. Few processes are more rewarding in allowing students to understand and work in chemistry-related topics than chemical formula writing. However, this task is very abstract especially for beginning practitioners. This activity illustrates a method to allow middle and high school students to visualize chemical formulas. Students manufacture special ion cards that permit visualization of the makeup of compounds. The authors have mentored teachers who have then been quite successful in teaching 5 th grade students to construct correct formulas using this method. Two additional advantages are part of the method. One advantage is that students can practice formula formation with a visual check of correctness without the need for the teacher to view each formula. The second advantage is the extended use of the produced cards as flash cards for studying ion formulas and names. Monatomic cations (pronounced cat i ons, positively charged ions) are named by adding the word ion after the element name. So the positively charged species of sodium is called the sodium ion, Na +. Monatomic anion (pronounced an i on, negatively charged ion) names are based on the root name of the element with the suffix ide. Thus, the chlorine atom is electrically neutral while the chloride ion has a charge. The students should be aware that chlorine and chloride represent different forms of the element, each having different properties. How the names of all cations and anions are determined is a topic that would require another article. Likewise, determination of the charge on a particular ion is beyond the scope of this article. Thus, it is easier for beginners to just memorize the symbols and charges, along with the ion names, as chemical vocabulary words. Given the chemical formula Na 2 O, what does it mean? (See additional material about names and charge labeling that comes later). Let us dissect the formula so that we can understand the process rules for writing formulas based on the formula. 1) Na 2 O means that the cation is Na +1 because in scientific use, the cation is placed first in a formula. The O stands for the oxide anion, O -2, which is placed second. Notice that no charges appear in the formula. 2) The subscript 2, the small number 2 that is placed below and to the right of the Na symbol, means that two of the sodium ions are present in the formula unit. Kansas Science Teacher Vol. 16, Spring

2 3) No subscript on an ion symbol means that an understood 1 is there, even though it is not written. So one O -2 ion is present in one formula unit of Na 2 O. 4) Why 2 Na +1 and only 1 O -2? Because another rule is that formulas of compounds are electrically neutral. This means that the total positive charge and the total negative charge must be equal. (The total positive charge is the positive ion s charge times the number of that ion present in the correct formula. The total negative charge is the negative ion s charge times the number of that ion present in the correct formula). Teachers may notice that the ions are shown with the sign of the charge first followed by the magnitude (Ca +2 ). Textbooks will most likely show the magnitude then the sign (Ca 2+ ). We want the students to concentrate on the two different types of charges (the sign) first, then on the magnitude so we have used the older convention for charge notation. Although the current convention is not to print the numeral 1, but leave it understood, we have included it here since we feel it helps students as they begin writing formulas. After working with the ion symbols, the students are ready to tackle the task of correctly writing chemical formulas. Writing chemical formulas is a daunting task for beginners. Students must remember not only the ion formulas, including charge, but also the process rules for putting together a correct formula. Much practice is mandatory for students to become comfortable with the process. The learning process entails a considerable investment of time if the teacher must scan each formula, either indicating the formula as correct or giving counsel on the errors. Using the ion card method students can check their own work because every correct formula will consist of matched lengths of cation and anion cards. The materials needed for each student are white 3 inch by 5 inch index cards, two colors of markers, a pencil, a ruler, and scissors. Ion card production is easy, reasonably fast, and inexpensive. Teachers may be tempted to provide ready-made cards for students to increase time efficiency or to use two colors of cards. However, we consider this a mistake. Much of the effectiveness of this activity is lost if students do not construct the cards for themselves. Some rote learning is required and the repetition of writing the symbols, charges, and names helps this process. Cards of one color (we recommend white but any one color is fine) should be used so that students are not clued into the knowledge of which cards are cations and which cards are anions based upon card color alone. Additionally, color stripes that come together in the center of the formula help ensure that students place the cations first and the anions second in formulas and names. Matching stripes in the center keeps students from joining cation to cation or anion to anion. Tables 1 and 2 give some cations and anions commonly encountered in middle and high school texts. The teacher should provide the students with a similar list of the cations and anions they are expected to learn. Kansas Science Teacher Vol. 16, Spring

3 Table 1 Cations Table 2 Anions Cations Anions Formula Name Formula Name Li +1 lithium ion Cl chloride ion K +1 potassium ion Br bromide ion Na +1 sodium ion I iodide ion +1 NH 4 ammonium ion OH hydroxide ion Cu +1 copper(i) ion nitrate ion Ca +2 calcium ion O -2 oxide ion Co +2 cobalt(ii) ion S -2 sulfide ion Mg +2-2 magnesium ion CO 3 carbonate ion Zn +2-2 zinc ion SO 4 sulfate ion Fe +2 iron(ii) ion Cu +2 copper(ii) ion P phosphide ion Pb +2 lead(ii) ion PO 4 phosphate ion Al +3 Co +3 Fe +3 aluminum ion cobalt(iii) ion iron(iii) ion Note that the Roman numeral in some ion names is important because it is an integral part of the name. It allows one to distinguish between compounds containing Cu +1 and Cu +2 ions. The Roman numeral tells you the magnitude of the charge on the ion [Cu +1 is copper(i) ion and Cu +2 is copper(ii) ion]. The compounds formed by these ions are very different. Copper(I) chloride, CuCl, is a white compound that melts at 430 C while copper(ii) chloride, CuCl 2, is a bright yellow compound that melts at 620 C. In the tables most of the listed ions are monatomic, i.e., they have only one atom in the ion formula like Li +1 or Cl. Some have more than one atom, like NH 4 +1 and OH. These are polyatomic ions that can proceed through many chemical reactions unscathed. The polyatomic ions given in the table are quite common. When a polyatomic ion is contained more than once in a formula, a parenthesis is required in the formula, e.g. Mg 3 (PO 4 ) 2 so that we know that the subscript 2 after the parenthesis applies to the whole PO 4. In the example, there are 3 Mg, 2 P, and 8 O. Without the parenthesis, the formula would seem to be Mg 3 PO 42 representing 3 Mg, 1 P and 42 O atoms. We will start with the sodium ion as an example of how to make cation cards. In construction of the ion cards, the important part of the index card is the 5-inch side. The system allows one inch of card for each unit of charge on the ion. Thus, a +1 ion would have an ion card that is 1 inch long while a +3 ion would have one that is 3 inches long. Similarly, a -2 anion s card will be 2 inches long. Making a Cation Card [using Na +1 as the example] 1) Place an index card on a piece of paper (to help in not smearing the desktop) with the long edge vertical. Make a nice colored stripe along its right edge from top to bottom. Kansas Science Teacher Vol. 16, Spring

4 Card with stripe on right 2) Since we are making a card for a +1 ion, the Na +1 ion, measure 1 inch down along the colored edge from the top and make a line horizontally across the card. Measure another inch down and put another line. Continue until there are four lines are across the card making five one-inch sections. Card with stripe divided into 5 sections 3) Cut along the four drawn lines, making five individual 1-inch cation cards. 4) Place a cation card with the colored stripe on the right side. Now label it as Na +1. Label the other four cation cards the same. 5) Turn the cards over and write sodium ion on the reverse of each. Na +1 sodium cation card front sodium ion reverse side Anion cards for the negative ions can be constructed in a like manner. Let us use the chloride ion as the example. Kansas Science Teacher Vol. 16, Spring

5 Making an Anion Card [using Cl as the anion] 1) Place a white card vertically on a piece of paper, making a nice colored stripe [using a different color than the cation card stripe] along its left edge. Card with stripe on left 2) Again allowing one inch for each unit of charge, make a horizontal line along the colored edge one inch from the top since chloride ion has a charge. 3) Continue until four lines have been drawn across the card, making five one-inch sections. Card with stripe and 5 sections 4) Cut along the four drawn lines to separate the five individual 1-inch anion cards. 5) Place an anion card with the colored stripe on the left side. Label it and the four other anion cards as Cl. 6) Turn the cards over and write chloride ion on the reverse of each. Cl chloride anion card front chloride ion reverse side Writing the correct formula for a compound formed between Na +1 and Cl 1) Place the cation card for sodium ion with the colored stripe in a vertical position (on the right side of the card). 2) Place the color stripe on the anion card along the colored stripe on the cation card. Kansas Science Teacher Vol. 16, Spring

6 Placing the 2 color stripes together in the center ensures that the student places the cation of the left and the anion on the right. The cation first orientation is mandated because in American scientific use, the cation is always first in a formula and name (This is a rule for students to remember). 3) Note that the length of colored stripe on the sodium cation card and on the chloride anion card match exactly. For each correct formula, the total length of colored line for the cation will exactly match the total length of the colored line for the anion. This placement mimics the neutrality of compounds, i.e. the total positive charge must equal the total negative charge. When this exact match of length of colored stripes is attained, the graphic representation of the formula under consideration is correct. Na +1 Cl Sodium and chloride cards matched 4) To write the chemical formula simply put the formulas for the ions (without the charges) in the numbers given by the graphical representation as subscripts. Here the numbers are one sodium ion and one chloride ion. 5) The correct chemical formula is: Na 1 Cl 1. (The scientific connection is that when a subscript is 1, it is not written but understood). So we would write the formula NaCl. However, beginners may write the ones until they become more comfortable with the task). 6) The correct chemical name is simply the cation name (without the word ion) then the anion name (without the word ion): sodium chloride Practice making other cards and formulas Making calcium cation cards and carbonate anion cards. 1) for the calcium cation cards: a) Make a cation colored stripe vertically down the right side of another index card. b) Measure two inches and make a horizontal line (remember +2 = 2 inches) across the card. Measure another 2 inches and put a second line. Note that you now have the ability to make two 2-inch cards and one 1-inch. [You can use the leftover inch to make another +1 cation card]. c) Cut the cation cards apart. On the 2-inch ones, put Ca +2 with calcium ion on the reverse side. 2) for the carbonate anion cards: a) Make an anion colored stripe on the left side vertical of another index card. b) Measure two inches and make a horizontal line (remember -2 = 2 inches) across the card. Measure another 2 inches and put a second line. Note that you now have the ability to make two 2-inch cards and one 1-inch. [You can use the leftover inch to make another anion card]. c) Cut the anion cards apart. On the 2-inch ones, put CO 3-2 with carbonate ion on the reverse. Kansas Science Teacher Vol. 16, Spring

7 Formulas can now be represented for 3 more compounds 1) Ca +2 and CO 3-2 a) Put the calcium cation card first with its colored stripe on the right side. b) Put the carbonate anion card next to it on the right side with its colored stripe against the colored stripe on the calcium card. c) They match exactly so the chemical formula contains one calcium ion and one carbonate ion; it is written CaCO 3 and the name is calcium carbonate. d) Ca +2 CO 3-2 Calcium and carbonate cards matched 2) Na +1 and CO 3-2 : a) Put the sodium cation card first with its colored stripe on the right side. b) Put the carbonate anion card next to it on the right side with its colored stripe against the colored stripe on the sodium card. c) They do not match exactly. Further action is necessary. Na +1 CO 3-2 d) Since the carbonate card is longer than the sodium card, put another sodium card below the first, with its horizontal edge touching the sodium ion card above it and its striped edge against the carbonate card stripe. e) Now the striped edges of the cation cards and the anion card match exactly, so the correct graphic representation is given. A formula unit of sodium carbonate has two sodium ions and one carbonate ion so it is written Na 2 CO 3 and the name is sodium carbonate Na +1 CO 3-2 Na +1 Two sodium match one carbonate. 3) Ca +2 and Cl ions a) Put the calcium ion card first with its colored stripe on the right side b) Put the chloride card next to it on the right side with its colored stripe against the colored stripe on the calcium card. c) They do not match exactly. Further action is necessary. Kansas Science Teacher Vol. 16, Spring

8 Ca +2 Cl d) Since the calcium card is longer than the chloride card, put another chloride card below the first, with its horizontal edge touching the chloride ion card above it and its striped edge against the calcium card stripe. e) Now the striped edges of the cation card and the anion cards match exactly, so the correct graphic representation is given. A formula of calcium chloride has one calcium ion and two chloride ions. It is CaCl 2 and the name is calcium chloride. Ca +2 Cl Cl One calcium matches two chloride Now we will try a slightly more complex formula -- that formed between the Mg +2 and PO 4 ions. Make 4 Mg +2 cation cards and 4 PO 4 anion cards in a manner similar to the earlier cards. When done they should look like this: Mg +2 PO 4 Magnesium cation card Phosphate anion card Place one Mg +2 cation card and one PO 4 anion card with their colored stripes in the center. Note that the PO 4 anion card is longer than the Mg +2 cation card. See below. Mg +2 PO 4 Place another Mg +2 cation card below the first one with its colored stripe touching the colored stripe of the PO 4 anion card. Now the Mg +2 cation cards are too long to be matched by the single PO 4 anion card. Kansas Science Teacher Vol. 16, Spring

9 Mg +2 PO 4 Mg +2 Place another PO 4 anion cards below the first one with its colored stripe touching the colored stripe of the Mg +2 cation cards. Note that the two PO 4 anion cards are longer than the two Mg +2 cation card. Mg +2 PO 4 Mg +2 PO 4 Place a third Mg +2 cation card below the first two with its colored stripe touching the colored stripe of the lower PO 4 anion card. Now the lengths match perfectly! Mg +2 PO 4 Mg +2 Mg +2 PO 4 Three magnesium match two phosphate We have three Mg +2 cation cards and two PO 4 anion cards to make their lengths match so the formula is Mg 3 (PO 4 ) 2 and the name is magnesium phosphate. Here are two final examples. These use the two forms of cobalt ions, Co +2 and Co +3. Kansas Science Teacher Vol. 16, Spring

10 Co +3 and Make the Co +3 and ion cards in a similar manner to the cation and anion cards in the earlier examples. Place a Co +3 cation card first and then place a anion card next. They do not match. Co +3 Place another anion card next since the cation card is longer. Co +3 The cation card is still longer so place a third anion card below the first two. The match is now achieved. Co +3 One cobalt(iii) matches three nitrates The correct formula is one Co +3 and three, written as Co( ) 3. Remember the parenthesis is not optional. The name is cobalt(iii) nitrate. Notice that there is no space between the element name and Roman numeral. Second is Co +2 and. Make the Co +2 and ion cards in a similar manner to the cation and anion cards in the earlier examples. Place a Co +2 cation card first and then place a anion card next. They do not match. Co +2 Kansas Science Teacher Vol. 16, Spring

11 Place another NO 3 anion card next since the cation card is longer. The match is now exact. Co +2 One cobalt(ii) matches two nitrates The correct formula is one Co +2 and two, written as Co( ) 2. Remember the parenthesis is not optional. The name is cobalt(ii) nitrate Summary In the beginning, students are more comfortable working with cards that are labeled with specific ion symbols and names. Eventually, students will understand that the important part of formula writing depends on charge not on the individual species. When this happens, the ion cards have served their usefulness because the student has made the conceptual leap to generalization of formula writing and no longer needs the cards for this purpose. Putting the names, as well as the symbols, on the cards permits the ion cards to be used as flashcards for learning symbols, charges, and names Kansas Science Teacher Vol. 16, Spring

12 Supplemental Information The following suggested beginner set of ion cards will allow students to work with 72 different possible compounds. Cation name # of cation cards Anion name # of anion cards sodium ion 5 chloride ion 5 potassium ion 5 nitrate ion 5 ammonium ion 5 hydroxide ion 5 copper(i) ion 3 oxide ion 5 calcium ion 4 carbonate ion 4 magnesium ion 4 sulfate ion 4 iron(ii) ion 3 phosphate ion 4 cobalt(ii) ion 3 copper(ii) ion 3 aluminum 3 cobalt (III) 3 iron(iii) 3 The beginner set can be constructed from 28 index cards following the layouts shown below. The cation set requires 17 index cards while the anion set uses 11 index cards. Above each layout is the number of index cards to be prepared using the given layout. Layouts for the cation cards 1 card 2 cards 2 cards 3 cards 3 cards 1 card 1 card 1 card 3 cards Na +1 Ca +2 Mg +2 Fe +3 Al +3 Cu +2 Cu +2 Cu +1 Na +1 Cu +1 Co +3 Na Ca +2 Mg +2 Cu +2 NH 4 Cu +1 Na Fe +2 Zn +2 NH 4 NH 4 Co +2 Na +1 K +1 K +1 K NH 4 NH 4 Layouts for the anion cards 1 card 1 card 2 cards 1 card 2 cards 4 cards Cl O -2 O -2-2 CO 3 Cl PO 4 Cl O -2 OH -2 CO 3 Cl OH -2 SO 4 Cl OH OH OH Names and formulas for the combinations possible with this beginners set are shown on the next page. Kansas Science Teacher Vol. 16, Spring

13 While the combinations of each cation with each anion result in 91 different combinations, only 82 of the combinations result in compounds that actually exist. The combinations that do not exist as compounds are the copper(i) ion with the nitrate, hydroxide and phosphate anions; aluminum ion with carbonate ion; and ammonium ion with oxide ion; and cobalt(iii) ion with carbonate, nitrate, and phosphate. Below is the list of the actual compounds for which formulas can be made with the beginner set of ion cards. Compound name Formula Compound name Formula aluminum chloride AlCl 3 iron(ii) chloride FeCl 2 aluminum hydroxide Al(OH) 3 iron II) hydroxide Fe(OH) 2 aluminum nitrate Al( ) 3 iron(ii) nitrate Fe( ) 2 aluminum oxide Al 2 O 3 iron(ii) oxide FeO aluminum phosphate AlPO 4 iron(ii) phosphate Fe 3 (PO 4 ) 2 aluminum sulfate Al 2 (SO 4 ) 3 iron(ii) sulfate FeSO 4 ammonium carbonate (NH 4 ) 2 CO 3 iron(iii) carbonate Fe 2 (CO 3 ) 2 ammonium chloride NH 4 Cl iron(iii) chloride FeCl 3 ammonium hydroxide NH 4 OH iron(iii) hydroxide Fe(OH) 3 ammonium nitrate NH 4 iron(iii) nitrate Fe( ) 3 ammonium phosphate (NH 4 ) 3 PO 4 iron(iii) oxide Fe 2 O 3 ammonium sulfate (NH 4 ) 2 SO 4 iron(iii) phosphate FePO 4 calcium carbonate CaCO 3 Iron(III) sulfate Fe 2 (SO 4 ) 3 calcium chloride CaCl 2 magnesium carbonate MgCO 3 calcium hydroxide Ca(OH) 2 magnesium chloride MgCl 2 calcium nitrate Ca( ) 2 magnesium hydroxide Mg(OH) 2 calcium oxide CaO magnesium nitrate Mg( ) 2 calcium phosphate Ca 3 (PO 4 ) magnesium oxide MgO calcium sulfate CaSO 4 magnesium phosphate Mg 3 (PO 4 ) 2 cobalt(ii) carbonate CoCO 3 magnesium sulfate MgSO 4 cobalt(ii) chloride CoCl 2 potassium carbonate K 2 CO 3 cobalt(ii) hydroxide Co(OH) 2 potassium chloride KCl cobalt(ii) nitrate Co( ) 2 potassium hydroxide KOH cobalt(ii) oxide CoO potassium nitrate K cobalt(ii) phosphate Co 3 (PO 4 ) 2 potassium oxide K 2 O cobalt(ii) sulfate CoSO 4 potassium phosphate K 3 PO 4 cobalt(iii) chloride CoCl 3 potassium sulfate K 2 SO 4 cobalt(iii) hydroxide Co(OH) 3 sodium carbonate Na 2 CO 3 cobalt(iii) oxide Co 2 O 3 sodium chloride NaCl cobalt(iii) sulfate Co 2 (SO 4 ) 3 sodium hydroxide NaOH copper(i) carbonate CuCO 3 sodium nitrate Na copper(i) chloride CuCl sodium oxide Na 2 O copper(i) hydroxide CuOH sodium sulfate Na 2 SO 4 copper(ii) carbonate CuCO 3 sodium phosphate Na 3 PO 4 copper(ii) chloride CuCl 2 zinc carbonate ZnCO 3 copper(ii) hydroxide Cu(OH) 2 zinc chloride ZnCl 2 copper(ii) nitrate Cu( ) 2 zinc hydroxide Zn(OH) 2 copper(ii) oxide CuO zinc nitrate Zn( ) 2 copper(ii) phosphate Cu 3 (PO 4 ) 2 zinc oxide ZnO copper(ii) sulfate CuSO 4 zinc phosphate Zn 3 (PO 4 ) 2 iron(ii) carbonate FeCO 3 zinc sulfate ZnSO 4 Kansas Science Teacher Vol. 16, Spring