Download teaching students how to write chemical formulae using ausubel`s

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.