Entropy

Entropy is all the fault of the second law of thermodynamics – essentially, all processes (chemical reactions included) will be spontaneous or feasible when there is an increase the entropy.

But what is entropy?

Entropy is a thermodynamic property like temperature, pressure or volume and it is best thought of as a measure of how random or disordered the molecules, and their energy, are.

If we take an ice cube and let it melt, the entropy of our system (the water molecules) increases because there are more ways of arranging molecules in a liquid than in a solid lattice. And liquid water molecules have more energy, so there is more energy to distribute amongst our more random molecules.

Similarly, if we thermally decompose solid calcium carbonate to form calcium oxide and carbon dioxide, there is an increase in the entropy because we have moved from one reactant particle to two products particles, and one of the products is a gas. In fact the reason this reaction is possible at a high enough temperature, despite it being highly endothermic, is because of the positive entropy change.

The very best introduction to the concept of entropy is to watch Brian Cox and his sandcastle.