Understanding the geometry of CO₂ or carbon dioxide is of great importance for understanding its current utility, as well as being necessary to know how to use it to achieve good energy efficiency.
It is worth mentioning that it is a colorless and odorless gas, slightly acidic but non-flammable, which occurs naturally in the atmosphere from the circulation of carbon through the oceans, plants, the earth, and also animals.
That said, molecules are made up of atoms, and when we talk about molecular geometry, we are referring to the three-dimensional shape of the atoms that compose it, determining the properties a molecule may have, such as polarity, phase, reactivity, color, and biological activity, among others.
Sources of CO₂
There are natural sources of CO₂ such as hot springs, volcanoes, and geysers, and it is also released when carbonate rocks are dissolved in water. Furthermore, as it is water-soluble, it can be found naturally in glaciers, groundwater, rivers, seas, lakes, and ice fields, as well as in natural gas and oil deposits.
Likewise, CO₂ is produced through the respiration of all aerobic organisms.
All the air that enters the lungs of humans and animals is expelled as CO₂; in the case of fish, it is what leaves through the gills and enters the water. A similar process occurs during plant respiration.
It can also be produced when we burn wood to make fire and when bread, beer, or wine is produced through the fermentation of sugars.
What is the geometry of CO₂?
Due to the presence of 2 electrons, the geometry of CO₂ is linear, having a bond angle of 180°.
Each carbon dioxide molecule is composed of a pair of oxygen atoms bonded to a carbon atom via double covalent bonds.
In more detail, we can state the following:
- In the geometry of CO₂, we have a carbon atom in the middle of two oxygen atoms, forming a type of sigma bond. With three of them, an octet is completed, joining the one at the beginning, so there are no lone pairs of electrons.
- Because in CO₂ geometry the bonding angles are 180 degrees and the electrons are distributed in a symmetrical way, the geometry of CO₂ presents a linear form.
In other words, in a CO₂ molecule, there are two pairs of valence electrons surrounding the carbon center, and the lone pairs of oxygen repel each other.
In this way, there is repulsion on both sides of the central carbon atom, which in turn is double-bonded to each oxygen atom, so no lone pair will be found.
To determine the CO₂ geometry of molecules and their shape, one can use the VSEPR hypothesis, which states that both the electronic and molecular geometry of a molecule is linear when it contains no lone pairs, or the AXE method.
Uses of CO₂
As CO₂ is an inert gas, it can be used in various processes.
Such as welding, as an anesthetic, a solvent, to make carbonated drinks, to harden concrete, to carbonate soda, and as a raw material in some chemical products and fuels; however, one of the most important uses is for HVAC or refrigeration systems.
Since CO₂ is a natural refrigerant that can change from a liquid to a solid state by varying its temperature or pressure, it can be used in industrial or commercial installations as a refrigerant.
That said, in HVAC systems, CO₂ is used in the subcritical and transcritical phases:
- Subcritical phase: this is when the gas in refrigeration systems is below its critical point (31°C / 73 bar). At this point, its behavior is similar to other refrigerants: it absorbs heat by evaporating, then it is compressed and subsequently condenses (changing from gas to liquid as it loses heat).
- Transcritical phase: this is when the refrigerant gas in a refrigeration system exceeds its critical point. In this case, CO₂ does not condense (the transition from gas to liquid does not occur), so specific controls are needed due to high discharges, and pipes with a design pressure of 120 bar are required.
The pressure in this fluid, which has been compressed and cooled, must be reduced so that it can then be condensed into liquid form to feed the evaporator of the refrigeration equipment.
How is CO₂ currently used?
The use of CO₂ today has been increasing in various applications and in large transcritical HVAC systems. In fact, many manufacturers are creating equipment that operates with natural refrigerants.
In HVAC systems, one of the basic components is the heat exchanger, which is a module whose function is to transfer heat between two liquids separated by a compact barrier. They are generally used for energy production, chemical processing, or refrigeration systems.
These new heat exchangers are being designed to suit more demanding refrigeration systems of different sizes.
Among the uses of the heat exchanger are:
- Raising the temperature of one fluid using another that is hotter.
- Cooling a fluid using another with a lower temperature.
- Allowing a fluid to reach its boiling point using another that has a higher temperature.
- Condensing certain gases by using cold fluids.
- Allowing a fluid to reach its boiling point while another hotter gaseous fluid condenses.
Advantages of CO₂ gas as a refrigerant
By using CO₂ as a refrigerant, the following benefits are obtained:
- CO₂ is considered one of the most environmentally friendly options for use in greenhouses. When used in these refrigeration systems, a 99.99% purity level is achieved, demonstrating that it is one of the best choices for the environment.
- Being an inert gas, it is non-toxic and non-flammable, and is considered excellent for heat exchange in condensers, evaporators and gas coolers.
- When CO₂ is used in refrigeration, another major advantage is that its cost is much lower than that of other refrigerant gases.
- Compressors used with CO₂ prove to be more efficient and also provide excellent heat transfer.