We often think about reactions around us as something that "just happens." Science is a study that tries to disprove this statement by explaining what, how, and why (to an extent) using various theories. This is regardless of whether these reactions are visible to human observation. An example of such is why a reaction like ice melting occurs at 0°C. While many associate ice melting as something that just occurs due to nature, others say that it is due to the temperature difference caused by the surroundings to the ice, which gradually causes the ice to melt. However, this ignores evidence that the melting point of ice differs at different pressures. In science, specifically chemistry, an attempt to explain and visualize what is going on is through thermodynamics.
Thermodynamics is a realm of study relating to temperature and energy. It helps determine why all reactions in the world occur. At its core, there are four fundamental laws in thermodynamics:
1. Energy is always conserved within a system as it cannot be created or destroyed.
2. For all processes that occur, the entropy of the Universe must increase.
3. Entropy is 0 at absolute zero.
4. When all objects in a system are at thermal equilibrium, all objects have the same temperature.
Many of these laws are relatively easy to see and understand, like energy being conserved, similar to mass being conserved (on the human scale) according to Lavoisier’s law. Similarly, we can see that when we put multiple objects of different temperatures together, they will eventually reach an equilibrium temperature, as seen in the fourth law (also known as Zeroth’s law). However, in trying to understand the other 2 laws, entropy needs to be explained. So, what is entropy? Entropy at its most basic level is randomness in a system. It can also be seen as how messy something is in a system. It determines whether reactions can occur or not. For a reaction to occur (or to be spontaneous) according to the second law of thermodynamics is for the entropy of said reaction to increase the entropy of the universe. This means that the reaction must make the system more disruptive or disarrayed. An example is a book dropping to the ground. When dropping the book, the air molecules below the book are displaced, and the dust around the group is also displaced. In addition, the potential energy from the fall has decreased as the book is now at a lower height.
In the molecular view, this means the molecules must become less structured. This can be from many factors such as an increase in temperature, having smaller molecules, or the bonds between the molecules becoming looser. An increase in temperature would increase the average kinetic energy of each molecule, thereby making the molecule move more and thus become less arranged. This can be seen in exothermic reactions and why they are more common than endothermic reactions to an extent as they increase the entropy of the system by releasing energy, generally in the form of heat. Having a smaller molecule would mean the molecules in the system are less arranged, thereby increasing the entropy of the universe. This can be seen in how longer molecules such as nitroglycerine are generally more reactive than smaller molecules like nitrogen. However, the most relevant to this article is the molecules becoming looser. As the molecules in a solution become less attracted to each other due to having fewer intermolecular forces, they become easier to manipulate as the molecules become freer to move around. This can be seen in how liquids can move more freely than solids. This, however, also reflects how the molecules in water can move in more possible directions individually. This means the molecules in water become more disorganized, increasing the entropy of the universe.
This helps us understand why ice melts, as ice is rigid and has a lattice structure, meaning it is more organized when compared to water. Thus, to increase the entropy of the universe, ice will melt into water. In addition, due to the difference in temperature between the surroundings and the ice, there is enough energy to break some of the intermolecular forces in the ice molecule. This also explains why the melting point is not constant at different pressures. This is because as pressure increases, there is more force exerted on an area of the molecule, increasing the kinetic energy needed for ice to melt. In contrast, at lower pressures, as there is less force pushing the molecule, less kinetic energy is needed to remove the water molecule from its lattice structure.
In conclusion, the main driving force behind why ice melts is due to its melting increasing the entropy of the universe. Other factors in this are also the temperature difference and the surrounding pressure. I guess one other thing ice has taught us is that disorganization pushes us toward something (maybe not the best things).
Bibliography:
- Drake, Gordon W.F.. "thermodynamics". Encyclopedia Britannica, 4 Dec.
2023, https://www.britannica.com/science/thermodynamics. Accessed 23
January 2024.
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