THERMODYNAMICS I: EVERYWHERE

Before we understand how thermodynamics affects industrial processes and how their understanding is fundamental in process design, let’s look at some basic day-to-day phenomena in which thermodynamics is present and often goes unnoticed.

That Thermodynamics is everywhere, that every Chemical Engineer already knows, even those who suffer from the matter in college. Thermodynamics is a very vast and dense subject, but it pays to understand some of its main concepts, since they are the basis of many everyday chemical processes and the knowledge of the thermodynamic properties of compounds is essential for the design and optimization of the operation of industrial processes.

But before we talk about the application of thermodynamics to industrial processes, let’s look at some examples of how thermodynamics is responsible for some of the simple phenomena that we observe or experience in our day-to-day life, often without realizing it.

For example, have you ever stopped to think why you feel cold when you get wet from the shower? It is your body losing heat to vaporize the water molecules present. And who is responsible for this? Thermodynamics itself. After all, it is she who governs the interactions between the molecules of different compounds.

Another interesting example is the use of the pressure cooker. Have you ever wondered why foods cook faster in the pressure cooker? As the boiling temperature of the water at atmospheric pressure is 1 atm, when cooking a food in water, the temperature of the water and, consequently, the cooking of the food is limited to 100oC. But by the principles of thermodynamics, observing the variation of the boiling temperature with the pressure, we see that the boiling temperature of the water increases with the pressure. Therefore, when using a pressure cooker, we are raising the boiling temperature of the water under those conditions. Thus, instead of 100oC, water can reach higher temperatures (~ 130oC). So now food is no longer limited to 100oC but to 130oC and therefore cooks more quickly. Interestingly, food will take longer to cook the higher the altitude relative to sea level. This is because the higher the altitude, the lower the atmospheric pressure and, consequently, the lower the boiling temperature of the water, the lower the temperature at which food is limited during cooking.

A parallel can be made in relation to the pressure to explain why it is possible to skate on ice. If we look at the phase diagram of the water, we will find that by increasing the pressure, water in the solid state (ice) can melt and become water in the liquid state. The principle of ice skating is that the blades of the skates exert pressure on the ice and melt it locally. How old were you when you discovered that you never really skated on real ice, but rather a thin film of water?

Figure 1: Phases os water diagram

The phase diagram of water is also different from the phase diagram of most substances, which explains why, as water solidifies, it increases in volume rather than decreasing, contrary to the status quo observed by other substances. In addition, other interesting phenomena can be observed in the water, such as insects that are able to walk on it. In short, she likes to be “different.” But what causes this different behavior? Basically, everything is explained by polarity.

Within a water molecule, the bond between the oxygen atom and the hydrogen atoms is a polar covalent bond. Covalent bond means that electrons are shared between both atoms and polar indicates that atoms are not shared equally. Oxygen “pulls” more electrons than hydrogen, which causes the oxygen atom to negatively and the hydrogen atoms to positively. When interacting with other water molecules, oxygen will attract hydrogens from neighboring molecules and form interactions that are characteristic of very polar molecules, the hydrogen bonds, which are very strong bonds. These bonds do not only form between water molecules but also between water molecules and some other equally polar or ionic compounds. The property of water that determines the ability of its molecules to bind to one another is called cohesion while adhesion concerns the ability of water to bind to molecules of other substances.

Figure 2: Interactions forces between atoms of a water molecule between itself and with molecules of other polar or ionic compounds.

These hydrogen bonds between water molecules explain the phenomenon of some insects walking on water. While the molecules in the interior of the water are attracted equally by the molecules above and below, the molecules of the surface are attracted by those below and by those that are to the side, which causes the resulting force to be down, forming a film which acts as a membrane that is strong enough to allow insects to walk on it. Obviously, it is not strong enough for humans to walk on it.

And why does water in solid form have more volume than water in liquid form? While for most substances the solid form is denser than the liquid form, in the case of water, the hydrogen molecules keep the water molecules more distant from each other when the water is in its solid form than in its form liquid. This is why ice is less dense than liquid water. This explains why ice floats in your glass of water. This also explains why lakes freeze from the surface. When the water begins to solidify, the ice formed, being less dense, remains on the surface, acting as an insulating layer that prevents the bottom of the lake from freezing. This is what allows aquatic beings to survive even when the lake freezes. And does not everything harmonize in nature? Thanks to Thermodynamics!

Now that we have seen how thermodynamics is present in simple day-to-day phenomena, in the next text we will see how it affects industrial processes and how to use it in equipment design and to establish the stages of a process.

This text belongs to the author and should not be reproduced without permission from BetaEQ and the same.

Author of: Clarissa Alves Biscainho

Chemical Engineer – Multinational Company – Germany

Source: IFP School TERG Project – Part 1 – Thermodynamics is everywhere, Ted Ed – How polarity makes water behave strangely.