CHEMICAL REACTION ANALOGIES
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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
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
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
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
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!
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".
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.
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".
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.
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.
From this Analogies page you can go either to the
other analogy pages:
[ Atomic Structure | Biology
| Chemical Bonding | Chemical
[Electrolytes | Equilibrium
| Matter ]
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