February 6, 1966
Are there questions.
If you recall, last time we were discussing a reaction in which oxygen from the air combined with glucose an aqueous solution of glucose to give us water and some other oxidation products. And we did this by means of certain substances that changed color as the various parts of the reaction took place.
By observing the changes in color, one was able to determine the rates at which the reaction proceeded; and we found that these rates were dependent upon the concentrations of the various substances in the solution.
As it turns out, the rate is proportional to the concentration of methylene blue times the concentration of glucose times the concentration of hydroxide ion; so we have this sort of relation.
Now. The equation for the reaction is glucose plus oxygen, gaseous, equals water plus oxidation products.
Ah! Ah. But in our expression for the rate of reaction, we have two substances [underlines MB and OH-] that don't even appear in the net reaction. Why not? Because they're catalysts.
They aren't used up in the reaction, but they're recovered and can be used over again. If I start out with one mole of hydroxyl ions, I have one mole left when I get done. The hydroxyl isn't used up.
But its presence does make a difference.
If I have two moles of hydroxyl ion instead of one, then the concentration of OH- is doubled, so the rate is doubled.
If I have half a mole, the concentration is half of the original and the rate is only half as great.
If I don't have any, the concentration is zero, so this product [the right half of the rate expression] is zero, and the rate has to be zero, which means the reaction won't take place.
Now that doesn't mean that this reaction [indicating the second equation, glucose plus oxygen equals water plus oxidation products] won't take place. It's still possible to oxidize glucose by other methods, by using a different sort of mechanism, such as is used for oxidizing glucose in the body.
But the particular mechanism, the set of reactions, we've been discussing requires the presence of hydroxyl ion and methylene blue, also, by the way. If these two species aren't present, the reaction won't take place.
And the reason is that these two species actually take part in the reaction. They're changed. And then later on, they're converted back into their original forms, so that the net effect is zero.
Now, most of you, I imagine, have been taught in high school that a catalyst is something that affects the rate of a reaction but doesn't take part in it. I'm afraid you're going to get into trouble if you think of catalysts merely as cheerleaders. They don't just stand by on the sidelines encouraging a reaction. They actually enter into the reaction. But it happens that, later on in the reaction, they're converted into their original forms again.
So catalysts are very important. If they're included in a process, the process can't take place without them even though they aren't mentioned in the expression for the net reaction.
That is all.