Chemical Reaction Analogies

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.

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.

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!

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".

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.

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".

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.

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.

CHEMICAL REACTION ANALOGIES

... from ScienceAnalogies.com