Chemistry Education II
Chem 673
Misconceptions in chemical
education/chemistry education.
Misconceptions
• Atomic structure
• Bonding
• Chemical equilibria
• Chemical and physical change
• Acid–Base Theories
• Chemical reaction
Public misconceptions of chemistry
• Natural and synthetic
• Green gas effect
• Green chemistry
 Perceptions of Ancient Scientists
Students’ conceptions, through identical observations, parallels have
been noted between the beliefs of today’s youth and many of the ancient
scientists
It makes sense to study the development of some historic theoretical
themes and examine how they are deep-rooted in science:
• theory of basic matter by the Greek philosophers,
• transformation concepts of the alchemists,
• the Phlogiston theory,
• historic acid–base theories,
• ‘‘horror vacui’’ and particle concept,
• atoms and the structure of matter, etc.
Theory of basic matter by the Greek philosophers
Matter is the substrate from which physical existence is derived,
The word "matter" is derived from the Latin word Materia, meaning
"wood", or “timber”, in the sense "material", as distinct from "mind" or
"form". The image of wood came to Latin as a calque from the ancient
Greek philosophical usage of hyle (ὕλη).
• Transformation concepts of the alchemists,
Alchemy (from Arabic: al-kīmiyā; from Ancient Greek: khumeía)
Alchemists attempted to purify, mature, and perfect certain materials.
Common aims were chrysopoeia, (artificial production of gold,) the
transmutation of "base metals" (e.g., lead) into "noble metals"
(particularly gold)
• The phlogiston theory
The phlogiston theory is a superseded scientific theory that postulated
the existence of a fire-like element called phlogiston contained within
combustible bodies and released during combustion. The name comes
from the Ancient Greek φλογιστόν phlogistón, from φλόξ phlóx
• historic acid–base theories,
Over century the definition of acid and base has changed
First defined by its observable physical properties such as taste and
how it looks like, sour taste
The different historic acid–base concepts are briefly described in the
following periods of time:
BOYLE. In 1663, Robert Boyle characterized all acids by using the plant
colouring, litmus: a red litmus colour shows acidic solutions, a blue colour
Basic solutions. Boyle became the creator of today’s indicator paper
LAVOISIER. After the fall of the Phlogiston Theory and the discovery of
oxygen, Lavoisier studied the combustion of carbon, sulphur & phosphorus in
1777. By dissolving the resulting non-metallic oxides in water, he found that
all these solutions show acidic effects.
DAVY. The discovery of the element chlorine by Davy in the year 1810, resulted
in the finding of the gaseous compound, hydrogen chloride (HCl), and its watery
solution, hydrochloric acid. With the realization that hydrogen chloride is
essentially an oxygen-free compound, the search went on for a method of
describing acid solutions in a new manner.
LIEBIG. Through the analysis of many organic acids and the knowledge of
reactions of these solutions with non-noble metals to produce hydrogen, Liebig
pragmatically stated in 1838: ‘‘Acids are substances that contain hydrogen
which can be replaced by metals’’.
ARRHENIUS. Upon examination of the electrical conductivity of many
solutions, the term ‘‘electrolyte’’ for conducting substances was assigned.
The acidic solutions also conducted electricity, and therefore belonged to
the group of electrolytes., that acids are substances that dissociate in
water to yield electrically charged atoms or molecules, called ions, one of
which is a hydrogen ion (H+) and that bases ionize in water to yield
hydroxide ions (OH−).
BROENSTED. After verifying the structure of atoms and ions by
different models of nucleus and shell, hydrogen ions were classified as
protons which do not exist freely and which connect with water
molecules forming hydronium ions H3O+(aq).
any compound that can transfer a proton to any other compound is an
acid, and the compound that…
• horror vacui and particle concept,
reflects Aristotle's idea that "nature abhors an empty space.“
later his idea was criticized by the atomism of Epicurus and Lucretius,
that nature contains no vacuums because the denser surrounding
material continuum would immediately fill the rarity of an incipient
void
‘‘nature avoids empty space without any material, nature shows a horror
vacui, a fear of empty spaces’’
• Atoms and the Structure of Matter
The old Greek philosophy offered at least two famous schools of
thought. Some followers of Democritus and Leukipp were convinced
that continual separation of a portion of matter must be finite and that
matter contains atoms (gr.: Atomos, indivisible).
• in 1649 Gassendi rehearsed Democritus’ idea of ‘‘atoms and empty
space as the only principles of nature, apart from the complete full and
complete empty space nothing else can be considered’’
• One should perhaps consider and use historic concepts to analyze
historical conceptual changes, develop today’s concepts of education
and compare with those changes of the past. Moreover, the historical
changes may be included in the teaching–learning strategies and
materials; the students should talk about and realize that ‘‘their
problems are similar to those of scientists of the past’’
• If the teacher compares and contrasts the historical misconceptions
with the current explanation, students may be convinced to discard
their limited or inappropriate propositions and replace them with
modern scientific ones, they use similar explanations and approaches
of the ancient scientists, and are led by teachers to the ways of
scientific thinking of today.
Students’ Misconceptions and How to Overcome Them
• Misconceptions are not only to be observed in today’s children or
students even scientists and philosophers developed and lived with
many misconceptions in the past.
• Just like early scientists did students develop their own concepts by
similar observations e.g., in regard to combustion.
• Ideas that are developed without having any prior knowledge of the
subject are not necessarily wrong but can be described as alternative,
original or preconcepts
• Every science teacher should know these preconcepts for his or her
lessons – this is why many empirical researchers are working all over
the world.
• Researchers are also finding chemical misconceptions in advanced
courses.
• Because they cannot be only attributed to the students but mainly
caused by inappropriate teaching methods and materials, they can be
called school-made misconceptions.
• They are clearly different from preconcepts that tend to be unavoidable.
• Inappropriate teaching methods can be stopped by keeping teachers
up-to-date in their subject through advanced education.
• To make suggestions of instructional strategies to improve lessons,
 Students’ Preconcepts
 Self-developed concepts made by students do not often match up with
today’s scientific concepts
 Young folks have often, through observation, come up with their own
mostly intelligent ideas of the world.
 When students talk about combustion, saying that ‘‘something’’
disappears & observe that the remaining ash is lighter than the original
portion of fuel, then, they have done their observation well and have
come up with logical conclusions.
This is why we cannot describe their conclusions as incorrect but rather as:
 original or pre-scientific ideas,
 students preconceptions or alternative ideas,
 preconcepts.
It is common to come across several preconcepts at the beginning stages of
scientific learning at the elementary, middle and high school levels of
chemistry, biology and physics.
 School-Made Misconceptions
• When students get involved in a subject matter that is more difficult, a
different type of problem arises: school-made misconceptions.
• Due to their complexity, it is not often possible to address certain
themes in a cut-and-dry manner
• Occasionally questions remain open and problems are not really solved
for a full understanding: school-made misconceptions develop.
 Examples to illustrate school-made misconceptions:
Composition of Salts. A famous example of school-made misconceptions
of our students arises from the Dissociation Theory of Arrhenius.
In 1884, he postulated that ‘‘salt molecules are found in solid salts as the
smallest particles and decompose into ions by dissolving in water’’
Today, experts recognize that there are no salt molecules, that ions exist
all the time – even in the solid salt. By dissolving the solid salt, water
molecules surround the ions, and hydrated ions are not connected, they
move freely in the salt solution.
 Students’ Concepts and Scientific Language
 Concepts regarding life in general, which have been sustained over
several years, are more deeply rooted than new concepts
 Many school-made misconceptions occur because there are
problems with the specific terminology and the scientific language,
specially involved substances, particles and chemical symbols are
not clearly differentiated.
 If the neutralization is purely described through the usual equation
H
C
l
+
N
a
O
H
N
a
C
l
+
H
O
2
Then, the students have no chance to develop an acceptable mental
model that uses ions as smallest particles.
 Effective Strategies for Teaching and Learning
• All teaching should begin with children’s experiences – each new
experience made by children in a classroom is organized with the aid of
existing concepts’’
• Without explicitly abolishing misconceptions it is not possible to come
up with scientific sustainable concepts .
• Lessons should not merely proceed from ignorance to knowledge but
should rather have one set of knowledge replace another.
• Chemical education should be a bridge between students’ preconcepts
and today’s scientific concepts’’
• It quite obvious that teachers should not assume their students enter
their classroom with no knowledge or ideas what so ever.
• A lesson, which does not take into account that students have
existing concepts, usually enables them to barely (ላመል፤እንደ
ምንም) following the lecture until the next quiz or exam.
• After that, newly acquired information will gradually be forgotten:
students tend to return to their old and trusted concepts.
• Nowadays, teachers and pedagogy experts agree that one should be
aware of student’s ideas before the ‘‘bridge can be successfully
made between the preconcepts and the scientific ones
• An important goal is to allow students to express their own preconcepts
during a lesson or, in the attempt to introduce new subject matter in a
lesson, to let them be aware of inconsistencies regarding their ideas and
the up-to-date scientific explanation.
• In this way, they can be motivated to overcome these discrepancies
• For the teaching process, it is therefore important to take students’
developmental stages into account according to:
student’s existing discrepancy (ልዩነት) within their own explanations,
– inconsistencies(የሚጻረር) between preconcepts and scientific
concepts,
– discrepancies between preliminary(ቀዳሚ )and correct explanations
of
experimental phenomena,
– possibilities of removing misconceptions,
Misconceptions in chemical education.
• Misconceptions are erroneous(የተሳሳተ) perceptions (ግንዛቤ) of what is
universally accepted as physical laws that have been experimentally
tested to date.
• Misconceptions are not only to be observed in today’s children or
students –even scientists and philosophers developed and lived with
many misconceptions in the past
• Many of this misconception during the teaching and learning process
transferred by the teacher to the students which will eventually affect
their perception toward the actual concept
• Teachers may carry with them wrong chemistry concepts and may never
realize it.
• School-made misconceptions.
• An elephant is like a wall!” exclaimed the blind man feeling the
body of an elephant. “No, No,” cried another blind man pulling
the tail. “An elephant is like a rope!” “You are all wrong, an
elephant is like a fan!” said yet another stroking the ear of the
elephant.
• Such are the misconceptions of things that we cannot see.
• Our understanding of Chemistry is not very much different from
the misconceptions of the blind men.
• We are not able to “see” atoms and electrons, hence, we have to
conceptualize them using mathematical representations and
models which are often erroneous
• teachers had only to decide how to plan a lecture in order to
transmit scientific ideas to their pupils, perhaps incorporating
laboratory experiments or new technology-based methods.
• However, research has found otherwise.
• Latest studies in science education show that children and
adolescents have many images and ideas about nature and their
own surroundings
Misconceptions of Atomic Structure
• Why do misconceptions of atomic structure need to be
addressed?
• Atomic structure is the basis of all other topics in chemistry
• Organic chemistry etc
•
If misconceptions are formed of the atomic structure, then
students will struggle to grasp concepts of the other areas of
chemistry.
Model of the atom
• Use of models to explain the structure of the atom are a main
way to support meaningful teaching and learning processes
• But, certain models of the atom are the basis of many
misconceptions that are formed of the structure of an atom.
Five main misconceptions
 An orbital is the same as an orbit”
 “air exists between particles in atoms”
 “there is an edge, or boundary, to an atom”
 “each orbit exists alone in space”
 “electrons orbit the nucleus like the planets around the sun
An orbital is the same as an orbit
• Atomic orbitals are the regions of space within an atom
where electrons are found.
• At secondary or tertiary level chemistry students are starting
to learn about the s, p, d, and f orbitals.
• An orbit is the course which electrons follow within an
atomic orbital.
Strategy to avoid this
• To avoid students making the misconception that orbits and
orbitals are the same, teachers could draw orbits : like figure
bellow
Figure 1: Atomic orbitals and orbit
• By doing this, students will understand that the orbit is the
dashed line in the centre and the orbital is the area encased by
the two outside dashed lines
Air exists between particles in atoms
• A few pictures of atoms show the space between the nucleus
and the orbitals, like in Figure 2.
• This model of the atom is called the “Bohr” model.
•
Many students see this space as containing air because air fills
up most space where something does not exist.
• What students need to understand is that air is made up of gas
atoms such as oxygen and helium, so therefore an atom cannot
have other atoms inside it.
• The space inside an atom is empty space.
Strategy to avoid this
• Discuss with the students what they think makes up an atom
apart from electrons, protons and neutrons.
•
If any students suggest “air”, ask them what air is made up of.
• When they come to the conclusion that air is made up of atoms
then explain how atoms cannot contain air.
There is an edge, or boundary, to an atom
• Many pictures of atoms are drawn in a way that gives the idea
that atoms have an edge or boundary. Figure 3 is a good
example of this as many students would view this picture and
interpret it as atoms having a shell with the electrons and
nucleus on the inside.
Strategy to avoid this
• When drawing any models of the atom, do not draw
the atomic orbitals as solid lines. Instead, use dashed
lines or dotted lines so the students understand that
the orbitals are not solid shells around the nucleus
Each orbit exists alone in space
 The different orbitals in an atom, s, p, d, and f, each have
different energy levels.
 1s2, 2s2, 2p6, etc. The different orbitals which have the same
energy level, such as 2s2 and 2p6 exist in the same space
Strategy to avoid this
• When teaching the concept of the s, p, d and f orbitals, show the
students a diagram like in Figure 4:
• The different colors of each energy level will get students to
understand that orbitals of the same energy level are found in the
same space within an atom Figure 4 Atomic Orbitals (Clark, 2000)
Electrons orbit the nucleus like the planets
around the sun
• Electrons orbit the nucleus in a very different pattern
to planets around the sun. Planets orbit the sun in a flat
circular pattern, and students make this misconception
from the “Bohr” model back in Figure 2. Electrons
orbit the nucleus in a round sphere pattern, as seen in
figure 5.
Strategy to avoid this e- orbit the nucleus like the planets
• When drawing models of the atom, do not draw the atomic
orbitals on a flat plane. Try to draw the orbitals like the planetary
model (Figure 5). When you have to draw the atom like the Bohr
model to explain another concept, make sure that the students
understand that you are only drawing it this way to teach the
particular concept.

Why this Misconceptions
• Chemistry is a very conceptual subject, and many of its concepts are
rather abstract
Some origins of misconceptions may be broadly categorized into the
following:
i) Present understanding of chemical knowledge is inadequate to
explain concepts.
ii) Over-simplifications of concepts to facilitate understanding.
iii) Bad chemistry
iv) Vernacular misinterpretations of concepts.
Present understanding of chemical knowledge is inadequate
to explain concepts
• Democritus (460–370 B.C.) proposed that matter was made of discrete
indivisible particles, which he called atomos, meaning "cannot be cut,
• his ideas were largely ignored until the scientific revolution of the
16th, 17th, and 18th centuries
• Similarly many theories students find confusing
E.g.
• bandings, the valence bond theory, the crystal field theory, the ligand
field theory, and the molecular orbital theory etc .
• They are taught that electrons revolve around orbitals and at the same
time they can be found anywhere near the nucleus.
Over-simplification of concepts to facilitate
understanding
• It is often difficult to explain something which is not visible and has
little or no accurate resemblance to reality
• E.g. In attempting to illustrate a chemical bond between two atoms,
spheres are erroneously connected together by a line which is
supposed to represent a bond.
• There are many examples of ‘misrepresentations’ of chemical ideas in
secondary text books which are often introduced as analogies to
explain certain concepts. In the process, students are often led to
develop wrong impressions.
• E.g. Electrons are neatly arranged in spheres representing shells and
sub-shells with ‘magic number’ of 2, 8, 18, 32 electrons
• Electron density surfaces are represented by spherical, dumbbell
shape, and clover-leaf shape orbitals (s, p, d orbital)
• Many students believe that electrons really occupy such shapes
BAD CHEMISTRY
• This arises mainly from teachers who do not have a good
understanding of chemical principles, or the teacher himself is
unaware of the misconceptions.
• Teachers may carry with them wrong chemistry concepts and may
never realize it.
Vernacular misinterpretations of concepts
• Due to the diversity in culture and language, perceptions can differ
quite significantly among students
• result of misinterpretation of text, beliefs or vernacular
translations; the latter is relevant to countries where English is not
the mother tongue and having a more diverse cultural background
compared to the western culture
Misconceptions regarding the chemical structure and bonding
The role of models
Chemistry as a discipline is dominated by the use of models
 scientific models used by chemists to understand chemical
bonding is one factor that contributes to students finding this
topic difficult
 Student think that models are toys or small incomplete copies of
actual objects,
 They do not look for ideas or seek purposes in the model’s form
 teachers themselves may have misconceptions regarding
scientific concepts and models
 Some teachers conceive scientific models in mechanical terms
and believe that models are true pictures of non-observable
phenomena and ideas
 Models are not “right answers”;
 scientists’ and teachers’ attempts to represent difficult and
abstract phenomena in everyday terms
 If students fail to understand the limitations of the models it
can reinforce or lead to further misconceptions.
The relations between internal and external representations
• Chemical structure and bonding is a topic in which understanding is
developed through diverse models
• Matter can be represented on three levels, as represented. In fig.
Frequently these are referred to as the macroscopic (physical
phenomena), microscopic (particles), and the symbolic levels
(chemical language and mathematical models).
• Robinson (2003) has suggested that students must first
thoroughly (ጥብቅ) understand how to convert a symbol into the
meaningful information it represents
 The macro and tangible,
 The sub micro( atomic and molecular) and
 The representational (use of symbols and mathematics)
 It is psychological folly (ስህተት) to introduce learners to ideas at all
three levels simultaneously. Herein, lay the origins of many
misconceptions. The trained chemist can keep these three’s in
balance
Students’ misconceptions and how to overcome them
• It’s a good idea to start chemistry lessons by demonstrating lots of
interesting phenomena and surprising experiments.
• Students should see appropriate 3-D models and draw related 2-D
model drawings in their notebook – so they will construct their
own mental models.
Particle Model of Matter
Experiments on Particle Model of Matter
• Growing of Alum Crystals
• Prepare a saturated sol of alum (KAl (SO4)2 x12 H2O). Filter the
and take a little portion of it into the crystallizing bowl. Let it
stand for two days until some octa hedron shaped crystals are
formed. Attach the best crystal to a thread and hang it from the
glass rod into the saturated solution, The crystal grows to a fist
size over weeks and months; it has the form of an octahedron.
Alum crystals in saturated solution
Close-Packing Model for the Alum Crystal
• Glue together a layer of 5x5 spheres in a square shape, in
addition glue together further layers with 4x4 spheres, with
3x3 and with 2x2 spheres.
• Place these layers upon each other, Finish the arrangement
placing one single sphere on top, and another sphere at the
bottom
• Observation: Octahedral shapes of alum crystals and close-
packing models are identical
Electrostatic Forces for a Bonding Model
• The students are probably capable of visualizing and accepting
the close-packing of glued-together spheres
• However, they are bound to ask what attractive force keeps the
particles together in the original crystal
• Rub a plastic rod on wool and use it to pick up little pieces of
paper from the table. These attracting forces are known as
electric forces which are responsible for bonding of particles in
a crystal.
Misconceptions about Chemical equilibria
• Chemical equilibrium is one of the basic subjects in the chemistry as
this subject is related to other areas of chemistry like solubility,
electro-chemistry, and acid-base.
• If a student has misconceptions about chemical equilibrium, these
misconceptions can interfere with subsequent learning.
 The most frequently encountered misconceptions about Chemical
equilibria are
• No reaction occurs at equilibrium.
• The rate of the forward reaction is greater than the reverse reaction
at equilibrium.
• Concentrations of the reactants are equal to the concentrations of the
products at equilibrium.
• When one of the reactant is added, equilibrium always shifts to the
products’ side.
• When one of the reactant is added to the equilibrium mixture, only
the concentration of products changes.
• If the amount of a reactant is increased, its concentration remains
the same.
• When a solid substance is added to heterogeneous equilibrium
systems, equilibrium is disturbed.
• The numerical value of Keq changes with the amounts of reactants
or products.
• Concentration of the products or reactants change with the addition
of a catalyzer.
Teaching and Learning Suggestions
• Solubility Equilibrium In saturated solution of NaCl
NaCl(S, white)
Na+(aq)
+ Cl-(aq)
• To show chemical equilibria are not static but rather dynamic:
back and forth reactions are constantly happening at an equal
rate.
• using a magnifying glass or by taking photographs over a long
period:
• several crystals constantly increase in size, whereas others get
smaller.
The misconceptions regarding the amount of solid materials in
equilibrium and the dynamic aspect
• If one observes a saturated NaCl, solution together with solid
sodium chloride, and adds an additional portion of solid NaCl to it,
this portion sinks down without dissolving
• If one measures the density of the saturated solution before and
after the addition of salt portions, one gets the same measurements
• The concentration of the saturated solution does not depend on
how much solid residue is present; equilibrium sets in between the
saturated solution and arbitrary amounts of solid residue
• If concentrated HCl acid is added in a clear saturated NaCl
solution, then white NaCl precipitates as fine crystal
• The drastic increase in concentration of Cl( aq) ions causes a
disturbance in equilibrium, and so much NaCl precipitates
until a new equilibrium is established
Mental model on reaction of saturated salt solution with hydrochloric acid
Acid–Base Reactions and the Proton Transfer
• The term, acid, was at first used by Boyle in the 17th century
• acids are materials that change the color of certain plant extracts and
that dissolve limestone
• Bronsted was the first to develop an acid–base concept that was no
longer related to substances, but rather to the function of particles.
Acids are proton donors and are capable with suitable reaction
partners to donate protons to base particles or proton acceptors
• from Arrhenius’ point of view, acids are substances but, from the
view of Broensted, acids are small particles.
Misconceptions about acid
 Some misconceptions related to acids and bases, specifically
on the differences between
 pure acids and acidic solutions,
 On neutralization, and
 On differences between strong and weak acids.
 Acids & bases are attributed an ‘‘aggressive effect’’
 Acids eat away, acids destroy, and acetic acid is a destructive
and dangerous substance, not used in normal everyday life
 When they think of acids they often think of destruction:
‘‘everything is decomposed or destroyed by acids’
Strong and Weak Acids
• acid strength is solely=ብቻ based on the pH value
Misconception about Pure Acids and Acidic Solutions.
schematically drawing of pure sulfuric acid and the 0.1 molar
solution, by two students
The term ‘‘dissociation’’ appears, however, to be totally misunderstood
by these two students.
pH value
‘‘pH value of pure acid is less; pH values are different for acids and
acidic solutions
Teaching and Learning Suggestions
• It is important to teach the necessary functions of acids, i.e. as
preservation methods, as spices or as stomach acid in the
digestive system
• One should show the aggressive properties by pointing out the
reaction products emphasizing more of a chemical process than
of the complete destruction of material
E.g. 1. The reaction of sugar with concentrated sulfuric acid
2. Limestone Deposit Removers – Acidic Household Cleaners
‘‘removal of calcium deposit’’ is a chemical reaction which
forms new substances
(acidic household cleaners); check the type of acid from its
label and interpret the information, show the rxn
Drain Cleaner – an Alkaline Household Chemical
• hydroxide solutions are also aggressive substances which
should not be placed near the skin but especially not near the
eyes.
• The properties are mainly used to dissolve and remove
‘‘organic leftovers’’ from the kitchen or bathroom
• Cover several pieces of sodium hydroxide with a few ml of
water in a beaker and stir with the thermometer. Add wool and
paper pieces to the solution and stir well. Repeat the test with
drain cleaner, and interpret the label on the bottle.
• Observation: Sodium hydroxide (NaHO) dissolves and the
mixture gets very hot, wool and paper pieces dissolve in the
concentrated solution. The same happens in the drain cleaner
solution.
pH Values of Several Bathroom and Kitchen Chemicals
• using universal indicator paper determine the pH of kitchen,
bathroom and laboratory;
Electrical Conductivity of Solutions of Acids, Bases and Salts
• Students often only see formulas of acids and bases but cannot
really imagine the ions in their solutions and hence develop
misconceptions of molecules in solutions
• In order to point out the existence of ions in acidic and basic
solutions, one should test the electrical conductivity and
compare the results with salt solutions.
Hydrochloric Acid – from Solid Sodium Chloride
pH Values – by Dilution of Hydrochloric Acid
Misconceptions about Chemical Reactions
• Chemistry as a branch of science that the contents of facts, concepts,
laws, theories obtained through the process and scientific analysis
relating to the nature
• Research studies showed that students, even at university level, were
found to have misconceptions about energy in chemical reactions




heat and temperature,
endothermic and exothermic reactions,
Combustion reactions,
bond energy,
Some studies showed that most students thought heat as a substance
rather than energy where as they described temperature as intensity
of heat
Endothermic and Exothermic reactions
• They classified burning of copper as an endothermic reaction since
only heating of copper caused formation of copper oxide
• endothermic reactions cannot be spontaneous. In addition, students
thought that;all reactions occurring naturally without application of heat are
exothermic
• Some students thought that heat was always needed for chemical
reactions to occur
Bond energy
• Bond energy is another concept about which students were found
to have misconceptions. In terms of overall energy change
• Bond breaking as an energy release process where as they thought
that energy is required for bond making.
• The notion that both processes of bond breaking and bond making
require input of energy was common
• Students generally have a misconception that bond formation is
endothermic and bond breaking is exothermic. They believed that
to form something, we must make an effort and so energy should
be used up.
• They couldn’t imagine the possibility of chemical reactions to be
spontaneous.
Combustion reactions,
• Combustion isn't a reaction; it is a release of heat which destroys
things
• Heat is in the fuel being burned and is not formed during
combustion.
• Combustion is a change of state of matter – solid or liquid to
gaseous.
• Mass is lost in combustion
• Oxygen aids combustion but does not participate
Physical Change vs Chemical Change
Misconception:
Physical changes are reversible while chemical changes are
not.
Truth: Both physical changes and chemical changes are
reversible. Not all reactions are reversed easily, but it may
occur.
Why the misconception:
Often the differences between chemical and physical changes
are taught incorrectly.
To make a clear distinction between chemical and physical
changes, it is often taught that physical changes are reversible
while chemical changes are not.
Easy demonstration:
Most students understand how physical changes are reversible,
but often students do not observe a simple reversed chemical
reaction.
An Easy demonstration dissolve KOH is in water and then
dextrose is dissolved in the solution methylene blue is added
the solution to turn blue. Over time the blue color fades and
the solution becomes colorless, but if the flask is shaken then
the blue color will reappear.
The color change results from the reversible oxidation-reduction reaction of the
methylene blue indicator. In alkaline solutions, glucose is oxidized to Dgluconic acid
HOCH2(CHOH)4CHO + 3 OH - ---> HOCH2(CHOH)4CO2 + 2 H2O + 2 e -
methylene blue is reduced from the blue (oxidized) form to the colorless (reduced)
form
shaking the flask dissolves O2 in the solution, which oxidizes the indicator back
to the blue (oxidized) form.
common misconceptions about chemical reactions
Misconception
A chemical reaction will produce a temperature change, a gas, a color
change, and a precipitate
Chemical reactions always produce a gas.
Chemical reactions are always irreversible.
Chemical reactions require heat to begin.
All chemical reactions produce heat.
Matter can disappear in a chemical reaction, especially in a reaction that
produces a gas.
Products result from matter being created not rearrangement of bonds in
reactants
• Chemical reactions are caused by mixing of
substances.
• Chemical reactions between gasses are simply
mixing.
• Chemical reactions are caused by active agents
acting on passive agents.
• Chemical reactions must be driven by external
intervention, e.g. heat.
• Rusting is something the nail draws out of the air.
• Reactions are caused by atoms trying to fill shells.
• Substance is not conserved in reactions.
Chemical Changes – Common Misconceptions
• Conservation of particles does not occur during a chemical
change
endothermic reaction
Sodium bicarbonate, citric acid, 2l jug, food colouring, water,
washing bowl
• Mix 200 ml of the sodium hydrogen carbonate and 100 ml of the
citric acid into the 2l plastic jug
• Put in washing bowl containing foam
• Result : The reaction is endothermic, taking heat in form the
surroundings and so the foam feels cold to the touch.
•
THE END