Catalysts are physical or chemical facilitators for those reactions which seem as though they ought to go but don't. For example, together hydrogen and oxygen gases are highly reactive, yet they will not react unless a catalyst is added to their container. That catalyst could be a spark, or a piece of platinum. Then instantly a huge amount of energy is released as water is formed. And as soon as we start noting energy changes we have gotten ourselves into the realm of thermodynamics. Now that is a big word, but one that is simple enough for a five year old child to understand!
The reason that thermodynamics is so vague in most peoples' minds is that chemists have grabbed this term and have tried to make it there own. Yet, because the thermodynamic laws are "universal laws," they must apply to all things in the universe. So let us move our considerations of "thermo" to the realm of comprehensible physical objects with which we are acquainted.
Let us get romantic and look at Niagra Falls! The water comes down the river and falls off the cliff and crashes to the bottom. Energy could be captured from that falling water: it could be used to run turbines that make electricity. So, today almost 80% of the river's water goes through turbines rather than over the falls.
My bright readers, you should have no trouble now relating this graph of the chemists to Niagra Falls. It is obvious! The amount of energy gained here is depicted as G as the water in the upper river falls off the cliff to flow away in the lower river.
Many rivers are dammed. The water backs up behind the dam. If there have been storms in the uplands, those torrents lift the water level behind the dam to such heights as to overflow the dam, and, again, that energy can be tapped. Unfortunately, power generation in this case depends on storms, and usually the power generators would sit idle.
Taking back another of the stolen ideas of thermodynamics, we see our dam with the water level raised behind it. Only if the water level is raised to the top of the dam (or the water mysteriously decides to leap over the top) will power be able to be harnessed. In this figure, the amount of energy available (but not obtained) is again designated as G. And if the water molecules behind the dam did decide to jump over the dam, the amount of energy needed to give them that boost is shown as E*, otherwise known as "activation energy".
So how do we get continuous power generation?
Happily the dam engineers had the forsight to put adjustable gates below the top of the dam. Now the water doesn't need to jump over the top of the dam. It only needs to go through the gates, and then spill down to run the turbines. And the gates, being adjustable, can be continuously lowered to keep allowing water to flow through the turbines. The gates would, of course be raised when storms raise the lake behind the dam. Then the whole process of slowly lowering the gates would start all over again.
If all this is still as clear as the Gibbs-Helmholtz Equation,
then consider two other common activities, and see if you can figure out what the G and "activation energy" values are:
- Syphoning water out of an aquarium into a drain on the floor, and
- Taking a ride on a roller coaster.
Now let's look at some biochemical reactions! We know that many reactions need heat to keep them going, and other reactions (once they've been "activated") give off heat. To boil water, you must continue to add heat. To move yourself too quickly across a room all you need to do is stick a platinum pin into a large balloon containing a 2 to 1 mix of hydrogen and oxygen gases: you needed only to expend a small amount of energy to stick the pin in the balloon, and you get a lot of heat given off by the instantaneous reaction forming a cup of water. These were examples of an endergonic and an exergonic reaction, respectively.
But let us not get off track: we're supposed to be focusing on biochemical reactions! Here is an example of a reaction that gives off heat (although only a trace, really). Notice, for schematic reasons, the arrow has a downward tilt to indicate that power is available to be harnessed (exergonic).
And, looking a little ahead in the course, here is one that you have to keep "cooking" to get it to go (endergonic):
What if we were able to harness the power in the first reaction and then use that energy to drive the second one? These are what are called "coupled reactions."
Much of the food we eat is run through "catabolic" exergonic reactions so that energy can be harnessed to drive the "anabolic" (synthesizing) endergonic reactions. Most of our food goes to CO2, and only a small amount is converted into the stuff we are made of. Can you name some classes of those biochemicals? (Hint: polysaccharides, and...)
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