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TEACHING STUDENTS HOW TO WRITE CHEMICAL FORMULAE USING AUSUBEL’S EXPOSITORY TEACHING METHOD. BY MATTHIAS U. EKERE DEPARTMENT OF CHEMISTRY FEDERAL COLLEGE OF EDUCATION, EHA-AMUFU ABSTRACT Writing of chemical equation form the bedrock upon which chemistry education is built, yet it is reported as a difficult concept in chemistry. This informs the need for the concept to be taught well using a teaching method that will facilitate learning by students. The teaching method that is recommended in this paper is Ausubel’s expository Teaching method. According to Ausubel, learning occurs mainly through reception rather than through discovery. Many students need teachers for school learning. Ausubel observed that information prepackaged by a teacher is organized and can be easily connected with the students’ existing knowledge. He emphasized prior knowledge as the key to understanding new material or information. Example, in writing of chemical equations, the teacher teaches the students chemical symbols, atomicity, valences of elements and radicals followed by the rules guiding, the writing of formulae and the rules guiding the writing of chemical equation, Otherwise, no meaningful learning can take place. TEACHING STUDENTS HOW TO WRITE CHEMICAL FORMULAE USING AUSUBEL’S EXPOSITORY TEACHING METHOD. Introduction Writing of chemical equation had been reported as a difficult concept in chemistry. The problem of writing of chemical equation appears to be persistent up till now. The West African Examinations Council (WAEC) Chief Examiners report had consistently been reporting candidates’ poor performance in writing of chemical equation. In WAEC Chief Examiners report (WAEC 2003) , it was stated that candidates performed poorly in the area of writing of formulae and correct chemical equations. The same report on poor performance in writing of chemical equations were repeated in WAEC chief examination report in (WAEC, 2005; 2007; 2010) respectively. Eze (1995), Nworji (2004) and Ekere (2014) reported of the candidates poor knowledge in writing of chemical equations. Writing of chemical equations form the bed rock upon which chemistry education is built. Gagne (1977) in his theories of learning hierarchy and learning prerequisite, posited that students can obtain the prerequisite knowledge of concepts and principles only if they have acquired the underlying capabilities (in this case, rules guiding the writing of chemical equations) which are needed to practice and understand science. In support of this learning theory, Nworji (2004), opined that students have difficulty in writing chemical formula of compounds because they exhibit poor knowledge of the rules guiding the writing of chemical formulae of compounds. If the students are properly grounded on the rules guiding writing of chemical formulae, writing of equations ( which is the language of chemistry ) will not pose any difficulty to them. David Ausubel’s theory of reception learning / expository teaching is Ekere (2014) another theory that gives support to this study. According to Ausubel, learning occurs mainly through reception rather than through discovery. Many students need teachers for school learning. Concepts and principles are better learned when they are presented and understood but are not discovered. Ausubel called this method expository teaching with emphasis on meaningful reception learning. Ausubel observed that information prepackaged by a teacher is organized and can be easily connected with the students’ existing knowledge. He emphasized prior knowledge as the key to understanding new materials or information. To facilitate reception learning, the teacher structures the learning situation so that meaningful learning can take place. Example, in writing of chemical equations, the teacher teaches the students chemical symbols, atomicity, valencies of elements and radicals followed by the rules guiding the writing of formulae and the rules guiding the writing of chemical equations otherwise, no meaningful learning can take place. The aim of this paper is to discuss the use of Ausubels’ expository method to teach students how to write chemical formulae. Ausubels’ expository teaching has three phases. 1. Presentation of advance organizer which is an introductory statement broad enough to encompass the information to be presented and to relate it to the students’ existing knowledge. Advance organizer gives the students the information they will need to make sense of the upcoming lesson and to help them remember and use information they already have but which they may not realize is relevant to the lesson (Woolfolk, 1998). 2. Presentation of learning task or material by presenting the subordinate content or learning material and 3. Strengthening cognitive organization. This final phase involves students putting together what has been taught. All information must be integrated into what students already know if it is to be remembered. Phase 1: Presentation of Advance Organizer Chemical symbols. Chemists have always sought shorthand for describing substances and the changes which they undergo. John Dalton and Antoine Lavoisier made little pictures to represent the atoms. Berzelius, a Swedish scientist who lived at the same times as Dalton, introduced the modern symbols of representing elements. In the modern system of representing elements, certain principles are applied ( Bajah,Teibo, Onwu, and Obikwere, 1999). a. The first principle uses the first letter of the name of an element as its symbol. Examples include. Element Symbol Hydrogen H Boron B Carbon C Nitrogen N Oxygen O Fluorine F Phosphorus P Sulphur S Iodine b. I The second principle uses the fact that should the names of two elements begin with the same alphabet, the first two letters, or in some cases the first letter and one other are used. Example Element Symbol Helium He Lithium Li Berylium Be Neon Ne Magnesuim Mg Aluminium Al Silicon Si Chlorine Cl Argon Ar Calcium Ca Chromium Cr Manganese Mn Zinc Zn c. Bromme Br Barium Ba The third principle states that in some other cases, especially where the names of several elements begin with the same letters, the Latin (Bajah, et al 1999) names of elements are used. As in (a) and (b) above, either the first letter of the Latin name or the first letter and one other, are used. examples include: Element Latin name Symbol Sodium Natrium Na Potassium Kalium K Iron Ferrum Fe Copper Cuprum Cu Silver Argentum Ag Tin Stannum Sn Gold Aurum Au Mercury Hydrargyrum Hg Lead Plumbum Pb. Writing atomic symbols correctly is the basic foundation that should be given to the students to enable them write chemical formulae correctly and the above principles should be learnt properly. It would be observed that when an atomic symbol is represented by one alphabet, it must be written in capital letter and when it is represented by two alphabets the first letter is written in capital while the second is in small letter, example ‘H’ for hydrogen and ‘Mg’ for magnesium and not ‘h’ for hydrogen and ‘’mG’ for magnesium. When the students learn how to write these symbols that represent the atoms correctly, writing the chemical formulae will not pose any difficulty, (Nworji 2004). Chemical Formulae A symbol represents an atom whereas a formula stands for a molecule. If a molecule has only one atom, the symbol and the formula are the same. When a molecule contains two or more atoms of the same kind, a small subscript is used to indicate the number of atoms present. (Ababio 2007; Zumdahl and Zumdahl 2003), A molecule of Hydrogen containing two atoms of hydrogen is written as H2; Nitrogen as N2; while Ozone which contains three atoms of oxygen is written as 03. Formulae for compounds contain the symbols for the different elements that are combined chemically to form the compound. The two or more atoms that are joined together are represented by writing their symbols side by side. Thus, a calcium oxide molecule is CaO, Iron (II) sulphide is FeS. There may be few exceptions for example in H2O2 where oxygen is – 1. If more than one atom of an element is present in a compound, a numeral is placed after and below the symbol to indicate the number of atoms of that element. Thus FeCl2 means that a molecule of iron (II) chloride contains one Fe (iron) atom and two Cl ( chlorine) atoms. The formula HCl for hydrogen chloride indicates that I atom of hydrogen and 1 atom of chlorine are present in the molecule. Also, the number 1 is understood before the entire formula unless another numeral is used. Three molecules of hydrogen chloride are thus represented by 3HCl. Sometimes parentheses are used in the formulae of some compounds, such as Mg (NO3)2, Fe2 (SO4)3 etc. In such case, the numerals outside the parentheses are multipliers. They show the number of times that the irons within the parentheses are to be taken. Thus the compound magnesium trioxonitrate (V), Mg(NO3)2 contains one magnesium atom two nitrogen atoms and six oxygen atoms. There are rules, which when followed by students will enable them to write correct chemical formulae. These are shown below. Valences of Elements and Radicals To enable us write chemical formulae, a system of small whole numbers, related to the combining ratios of elements, has been developed on the basis of arbitrary rules. Such numbers are called oxidation numbers or oxidations states (Ababio, 2007), Four of the arbitrary rules are: 1. An uncombined element has an oxidation state of zero examples are 02, Fe, Na and H2. 2. In a compound, the more electronegative elements are assigned negative oxidation states, and the more electropositive (or less electronegative elements) are assigned positive states. Example, in a compound KCl, K is more electropositive than Cl and is thus assigned an oxidation state of +1. Cl is therefore assigned - 1. 3. In a compound, the sum of the positive oxidation states and negative oxidation states is zero. 4. In an ion, the sum of the oxidation number is equal to the charge on the ion. An ion can be an atom that carries a charge or a group of atoms that carries a charge. Such groups that carry either a positive or a negative charge are called radicals. An acid radical is thus a small group or cluster of atoms carrying a negative charge that keeps its identity, or keeps together, in many reactions. Here are some examples with their oxidation numbers, (Bajah 1999; Chang. 2002) Oxidation no -1 -2 -3 -N02 -C03 -P03 -N03 -S03 -P04 -Cl03 -S04 -Mn04 -Cr04 -0H -HC03 -HS04. Here are also some examples of elements with their oxidation numbers. Elements Oxidation numbers Cl -1 F -1 Br -1 I -1 O -2 (-1 in peroxides H N +1 (but -1 in metal hydrides. -3 Na- +1 K +1 Ca +2 Mg +2 Al +3 Cu +1 or +2 Fe +2 or +3 When the symbols for elements and the oxidation numbers assigned to them are known, one is more equipped to write the correct formulae for chemical compounds. The following rules will help, (Bajah, 1999; Onasanya, and Orowale, 2007). Phase 2: Presentation of Learning Task or Material by Presenting the Subordinate Ccontent or Learning Material. Writing Chemical Formulae Formulae of compounds can be deduced from the valences of the component elements or radicals. The valences help in constructing the formulae of compounds and the following rules apply in writing these chemical formulae (Bajah, Teibo, Onwn, and Obikwere, 1999). (i) (ii) (iii) Write the symbols for the element and radicals Write the Oxidation numbers above and to the right of the symbols. Now write the symbols again, reversing the oxidation numbers, omitting the sign, and writing the numbers ( but not 1) below and to the right of the symbols. (iv) If a radical occurs more than once it must be enclosed within parentheses the sum of positive oxidation numbers of a compound must be equal to the sum of its negative oxidation numbers. Example: Write the chemical formulae of the following compounds. (i) Potassuim chloride (ii) Sodium hydroxide (iii) Sodium Oxide (iv) Aluminium Oxide (v) Calcium hydroxide (vi) Magnesuim tetraoxo Sulphate (vi) (vii) Aluminium tetraoxo Sulphate (vi) (viii) Aluminium tetraoxo phosphate (v) (i) Solution: Step 1: Potassuim – K chlorine Cl Step II: III 2. K+ Cl – K1CL1 x KCL Soduim hydroxide Step 1: - Na OH II - Na+1 OH-1 III- NaOH 3. Sodium Oxide Step 1: - Na O II - Na+1 O-2 III - Na2O 4. Aluminium oxide Step1 - AL O II - AL3+ O-2 III - AL2O3 5. Calcuim hydroxide Step1 - Ca OH II - Ca+2 OH- III - Ca (OH)2 6. Magnessuim tetraoxo sulphate (vi) Step 1 II III 7. Mg Mg+2 SO4 SO4-2 MgSO4 Step 1 - AL II - AL+3 SO4 SO4-2 III - AL2(SO4)3. 8. Aluminium tetraoxo phosphate (v) Step 1- AL PO4 II - AL3+ PO4-3 III – ALPO4 9. Calcuim nitride Step 1 - Ca N II - Ca2+ N-3 III - Ca3 N2. Phase 3: Strengthening Cognitive Organization. 1. Write the chemical symbols of the following elements or radicals. (i) Potacsium (ii) Phosphorus (iii) Lead (iv) Iron (v) Magnesium (vi) Sulphur (vii) Sodium (viii) Trioxo carbonate (iv) ion (ix) Hydroxide (x) ammonium . (Bajah et al 1999; Ababio 2013; Zumdahl and Zumdahl 2003) 2(a) What is the formula of compound formed by two elements x and y whose valences are +2 and -3 respectively. b. Write the chemical formulae of the following compounds. (i) ammonia (ii) copper(ii) hydroxide (iii) Tetraoxo phosphate (vi)acid (iv) Sodium trioxonitrate(v) (v) calcium trioxocarbonate (v) 3. Write the out the reactants and the products in the following (i) Hydrogen gas reacts with oxygen gas (ii) Action of water on calcium oxide (iii) The combustion of sulpur to form sulphur (vi) oxide. The Ausubel’s learning theory constitute a good frame work on which the learning of writing of chemical equations is based. If the writing of chemical equations which is a brief representation of chemical reactions by only formulae and symbols is well handled there will be no difficulties in students understanding the concepts. The writing of chemical equation is the language of chemistry and if the students are not well grounded in it, understanding chemistry generally will be difficult. REFERENCES Ababio, O.Y (2013 New School Chemistry for senior secondary school (6th Ed). Onitsha: Africana First publishers Plc. Bajah, S.T. Teibo, B.O, Onwu G, Obikwere, A. (1999) Senior Secondary Chemistry 1, Lagos: Longman. Chang, R (2002) Williams College Chemistry (7th Ed), New York: McGraw – Hill Higher Education. Ekere, M.U (2014) Analysis and classification of students learning difficulties in the writing and balancing of chemical equations. Unpublished M.Ed thesis. UNN Eze C.U (1995). secondary school Student’s difficulties in the application of current IUPAC system of nomenclature in organic chemistry. Unpublished MED Thesis, Nsukka. University of Nigeria. Gagne, R.M (1997) The Condition of Learning (3rd ed ) New York: Holt, Rinehart and Winston. Nworji, I (2004) Difficulties encountered by senior secondary school chemistry students in the understanding of mole concept. Unpublished M.Ed Thesis. UNN. West African examination Council, (2003; 2005; 2007; and 2010) Chief Examiners Reports in Chemistry. Yaba, Lagos: Amao press Ltd. Woolfolk, A. E (1998) Educational Psychology (7th Ed) Boston: Allyn & Bacon. Zumdahl, S.S, and Zumdahl, A.Z (2003) Chemistry (6th Ed) New York: Houghnton company.