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The role of energy
Energy also has a role to play in the entropy or randomness of a chemical system, by which we mean a quantity of substance or substances (such as a reaction mixture). Energy exists in ‘packets’ called quanta. This means that it is similar to molecules. You cannot have half a molecule and, similarly, you can have any whole number of quanta of energy but not 1½ or 3.25 quanta. Quanta are like marbles rather than plasticine. Like the distribution of atoms in space, the distribution of quanta of energy between molecules is also random because, like molecules, energy quanta do not ‘know’ where they are supposed to be – ‘energy doesn’t care’. We can simulate this distribution.
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The distribution of energy |
We can see that the more quanta of energy and the more molecules there are in a system, the more ways there are to distribute them.
This can be shown simply by the following examples but can be shown more generally by the simulation. Tables 4, 5 and 6 show the possible arrangements of four quanta between two molecules, five quanta between two molecules and three quanta between three molecules respectively.
| Molecule 1 | Molecule 2 |
| 0 | 4 |
| 1 | 3 |
| 2 | 2 |
| 3 | 1 |
| 4 | 0 |
Table 4 Two molecules, four quanta – five arrangements
| Molecule 1 | Molecule 2 |
| 0 | 5 |
| 1 | 4 |
| 2 | 3 |
| 3 | 2 |
| 4 | 1 |
| 5 | 0 |
Table 5 Two molecules, five quanta – six arrangements
| Molecule 1 | Molecule 2 | Molecule3 |
| 3 | 0 | 0 |
| 0 | 0 | 3 |
| 0 | 3 | 0 |
| 1 | 1 | 1 |
| 1 | 2 | 0 |
| 2 | 1 | 0 |
| 1 | 0 | 2 |
| 2 | 0 | 1 |
| 0 | 1 | 2 |
| 0 | 2 | 1 |
Table 6 Three molecules, three quanta – 10 arrangements
The distribution of energy quanta also contributes to entropy because of the relationship S = k lnW. The more heat energy we put into anything, the more its entropy increases because there are more quanta and thus more ways, W, to distribute them. Most chemical reactions involve a change of heat energy (enthalpy), either given out from the reactants to the surroundings or taken in from the surroundings into the products. So we must also take this into account when we are considering the entropy change of a chemical reaction. It is not just the chemical reaction that matters but the surroundings as well.
| The role of energy | |
