CHEMICAL REACTION ANALOGIES
… from Science Analogies.Info
- Activated Complex is like a Window Sill
- Catalyst Behaviour is like a High Jump Competition
- Catalyst is like a Minister
- Collision Geometry is like a Piece of a Jigsaw Puzzle
- Endothermic and Exothermic Reactions are like a Sponge and Water
- Limiting Reactant is like Meals at a Cafeteria
- Rate of Reaction is like Rate of Production at a Factory
- Wrestling Analogy for the Collision Theory
Activated Complex is like a Window Sill
When enough activation energy is added, the reacting molecules
interact very closely with each other to produce the activated complex.
It is an aggregate of particles which has a higher potential energy than
either reactants or products, and represents an intermediate stage or
situation that must be reached before the reaction will proceed to
produce the products.
A fire fighting analogy can be used to illustrate this idea. If
firemen wish to access a fire on the second floor of a building, they
can’t enter if they just climb up the ladder so their feet are at the
level of the second floor. They must climb a little higher, to a higher
potential energy location, so that their feet are at the level of the
window sill; then they can climb in and drop down to floor level to
complete their entry.
In reverse, the firemen can’t go from the second floor directly to
the ground; they must first climb up to the window sill then go down
their ladder to the ground. In both cases the window sill, just like the
activated complex, represents a specific higher energy stage that must
be reached in order to move from the original to the final situation.
Source: Licata, Kenneth P. Chemistry Is Like a … Science Teacher 1988, 55(8), 41
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Catalyst Behaviour is like a High Jump Competition
In a high jump competition … when the bar is very high, only a
small per cent of athletes will be able to successfully get over. This
is analogous to a chemical reaction which has a very large activation
energy so that only a small percent of molecules have sufficient energy
to reach the activated complex stage and react.
When the bar is set quite low, it takes less energy for the jumper
to get up to the necessary height to clear the bar, resulting in a
greater per cent of successful jumps. This is analogous to the action of
a catalyst …. it provides a reaction mechanism which decreases the
activation energy required to reach the activated complex stage. This
produces a greater fraction of successful collisions and a faster
reaction rate.
Source: Original
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Catalyst is like a Minister
A catalyst is a substance which stimulates or causes a chemical
reaction to take place, but is not itself permanently changed as a
result of the reaction. A catalyst is like a minister at a wedding
ceremony … the minister causes the ceremony to take place, plays a role
in determining how fast the ceremony takes place, and is not himself
permanently changed as a result of the ceremony … unlike the couple
getting married, who are permanently changed as a result of the
ceremony!
Source: Original
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Collision Geometry is like a Piece of a Jigsaw Puzzle
Just because two molecules collide does not necessarily mean that
they will react with each other. A successful collision may require that
the two molecules or species must collide with the correct collision
geometry, that is, be oriented in just the correct fashion so that
certain atoms will encounter each other during the collision. It is
similar to saying that when putting a piece into a jigsaw puzzle, you
can’t just put it in any way you want …. the piece will only fit
successfully if it is the right side up and is turned with the correct
orientation so that the projections and indentations match up …. that
is, it must have the correct “collision geometry”.
Source: Original
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Endothermic and Exothermic Reactions are like a Sponge and Water
An endothermic reaction is one which absorbs heat energy, so the
products have a higher enthalpy or heat content than the reactants. This
is like taking a sponge and pouring 50 ml of water into it … the sponge
ends up with a greater water content than before the “endothermic”
reaction. Furthermore, the amount of water added (50 ml) corresponds to
the heat of reaction.
In an exothermic reaction, heat energy is given off to the
surroundings so the product molecules have a lower heat content than the
reactants. This is like taking a wet sponge and squeezing it into a
funnel … the sponge ends up with a lower water content than before the
“exothermic” reaction. The amount of water which is squeezed into the
funnel and collected, is a good concrete way to visualize the heat of
reaction.
Source: Original
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Limiting Reactant is like Meals at a Cafeteria
The action of a limiting reactant to determine the extent to which a
reaction takes place is like producing meals at a cafeteria. When you
run out of chicken pieces, no more chicken dinners can be produced, even
though there might be a good supply of mashed potatoes and vegetables …
the other “reactants”.
Source: Original
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Rate of Reaction is like Rate of Production at a Factory
A chemical reaction can be imagined as analogous to an automobile
manufacturing plant. Raw materials like iron, plastic and glass are the
reactants and cars are the finished product which roll off the assembly
line at the other end of the factory. To measure the rate of this
reaction, you could measure the rate at which a product is produced (eg.
#cars/day) or measure the rate at which a single reactant is used up (
eg. #tons of iron used up/day). This is the same idea as measuring the
rate of a chemical reaction.
Source: Original
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Wrestling Analogy for the Collision Theory
The most basic statement of the Collision Theory is that molecules
must collide with each other in order to react. This is similar to
saying that wrestlers must actually contact one another in order to
wrestle.
Source: Original
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ELECTROLYTE ANALOGIES
… from Science Analogies.Info
Analogies in this section deal with the properties of solutions and electrolytes of all types, and related concepts.Electrolytes are like Nuts and Bolts
If a bolt represents a positive ion, a nut analogous to a negative
ion and assembled units represent undissociated molecules of the
electrolyte, these units can be used to represent various types of
electrolytes:
A strong electrolyte would be represented by an equal no. of separate nuts and bolts in a box (complete dissociation).
A weak electrolyte would be represented by assemblying most nuts
and bolts into pairs, and having only a few disconnected (small per cent
ionization).
A non-electrolyte would be represented by assemblying all the nuts
and bolts into pairs so that none were disconnected (no ionization).
A more concentrated solution of a weak electrolyte would be
represented by showing a greater number of assembled nuts and bolts, but
a smaller per cent of them dissociated into separate pieces.
Source: Fortman, John J. Pictorial Analogies X: Solutions of Electrolytes Journal of Chemical Education January 1994, 71(1), 27
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Hydronium Ion Concentration is like a Seesaw
Imagine a seesaw or teeter totter with hydronium ion concentration
at one end and hydroxide ion concentration located at the other end. As
the seesaw operates, it will show the required inverse relationship
between these factors …. as the hydronium ion concentration goes up, the
hydroxide ion concentration goes down, and vice versa.
Similarly, a seesaw with pH at one end and hydronium ion
concentration at the other, shows the relationship these factors have ….
as hydronium ion concentration goes up, the pH goes down, and vice
versa.
Source: Fortman, John S. Pictorial Analogies XI: Concentrations and Acidity of Solutions Journal of Chemical Education May 1994, 71(5), 431
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EQUILIBRIUM ANALOGIES
… from Science Analogies.Info
Entropy is like Marbles in a Box
Entropy is an important factor in discussing equilibrium. Either
the forward or reverse reaction will be favoured by the entropy factor …
that is, allowing atoms and molecules to move from a state of lesser to
greater disorder.
Imagine taking a small box and into it packing marbles so that all
the marbles in each horizontal layer are the same color, or show some
other definite pattern. When the box is shaken for a while then the
contents examined, you will see that the marbles are now quite randomly
arranged … they have spontaneously moved to a state of greater disorder
(higher entropy).
Source: Smoot, R.C., Price, J. and Smith, R.G. Chemistry A Modern Course Don Mills: Maxwell Macmillan Canada, 1987, p.386
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Equilibrium is like Substituting Players in a Sports Game
Equilibrium is a situation attained when the rates of forward and
reverse reaction are equal, so that there is no change in the
concentrations of the reactants and products. Equilibrium can be reached
at various points; the concentrations of reactants and products need
not be equal, only their rate of exchange. The equilibrium point also
shifts in response to stresses placed upon the system.
In a sports game like soccer or basketball, for every new player
substituted onto the field, an old player must leave – thus the rate of
these opposing reactions are equal. There is no change in the number of
players on the field, even though their identities are different. There
is no requirement that the number of players on the field and on the
bench be equal (and usually they are not equal); the only thing that
must be equal is the rate of exchange between these two groups.
In a game like hockey, a penalty would be like a stress … it
increases the number of players leaving the ice, compared to the number
of players going onto the ice. At this new equilibrium point the rate of
player exchange is again equal for the duration of the penalty. When
the penalty is over, the equilibrium point shifts back to the original
position.
Source: Licata, Kenneth P. Chemistry Is Like A … The Science Teacher 1988, 55(8), 43.
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PROPERTIES of MATTER ANALOGIES
… from Science Analogies.Info
- Graham’s Law of Diffusion is like Running a Race
- Mean Free Path is like Bumper Cars
- Phases of Matter are like Students at School
Graham’s Law of Diffusion is like Running a Race
According to Graham’s Law, the velocity or rate of diffusion of a
gas is inversely related to the square root of the molecular mass. Thus,
molecules of small mass travel more rapidly and molecules of larger
mass travel more slowly. This idea can be easily remembered by
considering the following analogy:
Consider 10 runners who were so closely matched that they had
essentially equal times for running a 1000 metre race. Now suppose half
of the runners were required to carry an extra 10 kg mass that was
attached to them by a belt or backpack. Likely the 5 who had to carry
the extra mass would now lag behind the rest of the runners.
Source: Based on Toon, E.R. and Ellis, G.L. Foundations of Chemistry New York: Holt, Rinehart and Winston, 1973 p. 136
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Mean Free Path is like Bumper Cars
The mean free path is the average distance which a molecule travels
between collisions with neighboring molecules. This is like riding in
the bumper cars at a carnival … you only can travel a short distance, on
the average, before being involved in a collision with another bumper
car.
Source: Original
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Phases of Matter are like Students at School
Students in a classroom are analogous to the particles of a solid,
since they have a regular arrangement and a limited freedom of movement.
Students shifting or turning in their seats represent the vibrational
motion of solid particles.
During breaks between classes, students have a wider range of
motion. They now also have limited translational motion which allows
them to move among one another to the doorway and through the halls, but
are still confined to the volume of the school. This is analogous to
the behaviour of liquid particles.
At the end of the day, students are like gas particles since they
have unrestricted and primarily translational motion which causes them
to escape from their school building and diffuse throughout the
community.
Source: Licata, Kenneth P. Chemistry Is Like A … The Science Teacher 1988, 55(8), 42.
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