The concept, entropy, is of practical use to chemists and chemical engineers. When a process is being planned, and the proposed reaction is represented by an equation, the entropy can be calculated. If the reaction results in an increase in entropy, it is possible. If the reaction results in a decrease in entropy, it's impossible. One way to describe entropy is to call it disorder. When a reaction results in an increase of disorder, it can mean the increase in motion of molecules in the surroundings, or it can mean the breakdown of a large and complicated molecule into small pieces. The general rule is: the entropy in the universe always increases. Some theoreticians imagine a steady increase in temperature until a limiting temperature is reached. At that point everything is at a standstill because all action depends on a temperature gradient. Perhaps, if they knew about neg-pos, they would change their theory.
To get back to the chemistry laboratory, let us show how it is possible to make the more orderly product from the disorderly one.
If a molecule is made of two atoms, and we want to separate the atoms, we have to put one or both of the atoms in motion. Opposing the motion is the force of attraction between the atoms. If the motion is not sufficient for separation, the atoms will return to each other, and then rebound repeatedly. There is a critical point at which, having passed that point, the atoms escape.
The energy that is required to separate the atoms is called the bond energy. It is energy that the atoms don't have until it is given to them from outside the molecule. It is also called bond strength.
If an atom receives bond energy, it moves back and forth. At two points in its motion it stops very briefly to change direction. At those points, all of the energy is potential. When it moves at maximum speed, the energy is all kinetic. In between it's the sum of its potential and kinetic energy at the moment. The total energy is constant.
If I want to produce hydrogen bromide, I look in the handbook for bond strengths. I find: The total is 10.35 units. Then I find the bond strength of HBr is 6.o8. I need two of HBr, so I get a total of 12.16 units. The total for the products is 1.81 higher than the total for the reactants. That means the reaction is possible. These figures also go into the computation of the entropy increase due to the reaction.
What if I have plenty of hydrogen bromide and no pure bromine? I want some pure bromine, so I explore the possibility of running the reaction backward. I know that it's impossible, but I know that there is a method that exists in all living organisms that can be adapted to this case.
The method is to combine two or more reactions that, together, will result in an increase of entropy.
Suppose I have a supply of hydrogen bromide, and I want to get some bromine liquid. I dissolve hydrogen bromide in water. I bubble chlorine gas into the water. This reaction produces hydrogen chloride and liquid bromine. The water is warmed by the reaction. If necessary, I warm the water still more to cause the bromine to evaporate and become separated from the water. I collect the bromine gas and let it condense into liquid bromine at room temperature. Incidentally, I take care not to come into personal contact with any of these chemicals. They damage living tissue.
The reaction that I have just described is:
This reaction goes in the right direction. It is the sum of two reactions:
|H-Br + H-Br ||----> ||H-H + ||Br-Br |
|H-H + ||Cl-Cl ||----> ||H-Cl + ||HCl |
HBr + HBr + Cl-Cl -----> HCl + HCl + Br-Br
The H-H cancels because it appears on both sides of the equation. The reaction that produces HCl is a right-direction reaction:
The reaction energy for the combined reaction is the difference between the energy of the HCl reaction and the energy of the HBr reaction: 3.12 units - 1.81 units = 1.3 units
I use a right-direction reaction to drive a wrong-direction reaction, when the right-direction reaction has a reaction energy which is greater than the energy of the reaction which goes in the wrong direction. These are called linked reactions.
Linked reactions are very common in living things. In photosynthesis, many linked reactions employ the compound, ATP, adenosine triphosphate. In cells that contract or move, the energy for the motion is transferred by means of linked reactions with ATP.
The molecule contains a chain of three phosphates. The bond that joins the last phosphorus atom to the rest of the molecule is a very weak bond. That makes a phosphate available. The linked reaction involves the combining the phosphate with some other molecule to form a strong bond. The energy of the reaction drives the wrong-direction reaction that the organism needs.
It would not take long to exhaust all of the ATP in a cell. To replenish the supply of ATP, there is another linked reaction. A glucose molecule is reacted wiith oxygen. The energy of the reaction is ample to link several wrong-direction reactions to the glucose-oxygen reaction. The wrong-direction reactions break the strong bonds between phosphate and some other molecule, and reconstruct ATP, with its weak phosphate bond. The ATP with one phosphate missing is ADP, adenosine diphosphate. In the linked reaction with glucose, ADP is joined with phosphate to form ATP.
In the laboratory, the reaction between glucose and oxygen is a small fire. There is no furnace in a living cell. The reaction in a cell is controlled, and it proceeds by a series of steps.
An enzyme makes contact with a glucose molecule. The glucose molecule yields four hydrogen atoms. The bond breaks between two carbon atoms. The glucose, with six carbon atoms, divides into two molecules of pyruvic acid, with three carbons. The pyruvic acid is the fuel that provides the weak bonds for each of the steps. The pyruvic acid molecule, as such, is lost from view in the second step. It is incorporated in the subsequent compounds and provides those compounds with their weakest bonds.