The Lewis Dot Structure for CO2

CO2 Lewis Structure
Created by MakeTheBrainHappy

This is the Lewis Dot Structure for CO2. You could alternatively also draw the structure by including two dots for every bond. That would mean that you would have a total of eight dots around the carbon, thereby filling its octet. The octets of both of the oxygen atoms are also satisfied, since the oxygens have a total of eight electrons around them (thereby filling the valence shell). 

The lewis dot structure of CO2 gives it some unique properties. Since there are no lone pairs on the atom, it is a linear structure which makes the charges cancel it. Therefore it is nonpolar and relatively unreactive. These properties in addition to its small state makes it so that carbon dioxide has a low melting point and is mostly in the gaseous phase at STP (Standard Temperature and Pressure). Even though Carbon Dioxide is quite small, its density is still 1.67 times that of air.

sublimation
A solid block of CO2. The IMF (intermolecular) forces holding this structure together are so weak that the cube is going directly from the solid to the gaseous phase. CO2 is solid at -78.5 ˚C. Source

Do other lewis dot structures for CO2 exist?

Yes, although these would not be the most common structures due to formal charges. You can learn more about what formal charge is in this article. An example of a possible resonance structure would be to make the double bonds in the lewis dot structure single bonds and then give the extra electrons to each of the oxygen atoms. This would give each oxygen atom a -1 formal charge and the carbon atom a +2 formal charge. While this would technically balance out, it would be unlikely due to the covalent nature of the bonds even though the bonds themselves are polar.

Furthermore, other lewis dot structures are highly unlikely due to the increased instability which results from the placement of formal charges. When something has charge it is more likely to interact with the environment in a way that will tend towards a more stable configuration. Consider this to be an extension of the first and second law of thermodynamics; namely, that molecules tend towards lower energy states. Adding charges means that you would need to input energy in order to maintain the charge difference; energy which simply isn't available. 

Does carbon usually form bonds like this?

Yes, carbon combined with another element usually creates non polar structures because carbon can form multiple bonds and rarely has lone pair electrons. It can easily fill its octet requirement with almost any combination of elements and its electronegativity allows it bond with many different types of atoms. The notable exceptions are the compounds CN (cyanide) and CO (carbon monoxide), which the bond is polar due to the unequal sharing of electrons between the two atoms in the structure. The structures do not look like the lewis dot structure for CO2, where the carbon does not have any lone pair electrons on the carbon.

CO Lewis Structure
The Lewis Structure for Carbon Monoxide with lone pair electrons on the carbon. Source

CN Lewis Structure
The Lewis Structure for Cyanide with lone pair electrons on the carbon. Source

If you are interested in learning more about the polarity of CO2 we highly recommend checking out our article regarding this subject. This will discuss trends which are already illuminated above in the lewis dot structure for CO2. Additionally there is a discussion of one of the most important applications of carbon on this planet (hint: the carbon cycle).

What can the lewis structure for CO2 tell us about the solubility of this molecule? 

Generally we can say that the molecule is rather small (it contains only three atoms) and symmetrical based on the lewis structure. Given that water is a polar molecule (due to its bent structure - learn more about water in our Lewis Dot Structure of H2O article or our H2O - polar or nonpolar article) we can already see that the interaction probably won't work. As a reminder there is a typical rule in chemistry known as "like dissolves like." This means that polar solutes (what is being dissolved) will dissolve in polar solvents (what is being dissolved into). A rule of thumb here is to consider the solvent whatever there is more of in the total solution and the solvent what there is less of. 

This hint given by the lewis structure for CO2 is very useful for when we head into the lab in order to measure the solubility. While there are models which can predict the solubility, due to the many different factors involved (temperature, pressure, volume, etc.) they are oftentimes inaccurate and require tweaking based on the experimental results measured. 

CO2 in Water Solubility Curve
Solubility vs. increased pressure for carbon dioxide in pure water at 373.15K (source below)

How should we interpret the graph? 

Well for one we can see solubility increasing with increased pressure. While this may be unexpected (how is solubility related to pressure), we can actually also see that this a phenomena of the lewis dot structure for CO2. For one, the small size predicted (only 44 g/mol when you add up the molar masses of carbon (12 g/mol) and oxygen (16 g/mol * 2)) that this is much more likely to be found as nonpolar gas. In this fact lies the key to our answering the question why pressure impacts the solubility. Normally, if you were to examine the dissolution of a salt into water (take NaCl in water), you would not see drastic effects based on pressure. But examine the diagram below and you see is different when one of the components is in a gaseous phase:

Pressure impacts solubility
How pressure impacts solubility for gaseous phase components. Source

As the pressure increases more gaseous molecules are pressed against the surface of the liquid. This naturally means that more molecules will become dissolved liquid since dissolution rate is directly related to the amount of contact that the gas particles have with the water. In our case we can see that the solubility of CO2 increases for the same reason as pressure increases. 

Solubility for the CO2 as represented by the lewis structure is measured based on the composition in terms of the mole fraction. The mole fraction refers to the ratio between the number of moles of CO2 in solution and the number of moles in total. Even at the highest pressure ratios the mole fraction only reaches 8.00*10^-3 in terms of mole ratio. This means that less than 1% of the water solution is composed of CO2

How does the lewis dot structure for CO2 then help us in interpreting "like-dissolves-like" situations? 

Based on the discussion in the previous few paragraphs it would natural to assume that if you were to examine a less polar solvent that the solubility or mole fraction would go up. For this example I will choose acetone; another very common solvent found within the chemistry lab. While acetone is still considered to be polar (see our article on whether acetone is polar or nonpolar to learn more), in brief acetone has two CH3 groups (hydro-carbon groups considered to be nonpolar) which overall make the molecule much less polar than water. 

Table of Mole Fractions for Dissolution of CO2 into Acetone
Table of Mole Fractions for the dissolution of Carbon Dioxide in Acetone. (Source below)

Above you can see a table of mole fractions and they are far greater than the roughly 1% we saw with CO2 into water. At comparable pressures approx. 50-67% of the CO2 in dissolved in acetone. However the temperatures also have an impact on the solubility as he can be seen in thee table above (generally the values are lower at 333.15K as opposed to 313.15K). 

To help clarify some of the concepts we've been exploring with the lewis dot structure for CO2 we should turn to the relationship between temperature and solubility.

How different solutions are impacted by temperature increases. Source

Something which the lewis dot structure for CO2 unfortunately does not tell us is how exactly the molecule interacts specifically in a whole range of reactions (i.e. whether it is typically endo or exothermic). Endothermic means that the reaction requires energy input in order to reach its final state while exothermic releases energy in the course of the reaction. You can also think about this as the place within the reaction equation where you would place heat. In an endothermic reaction you would place heat with the reactants whereas in an exothermic reaction heat is released with the products. Consider heat to the way in which energy is transferred within these systems. Two conceptual examples are shown below to clarify this explanation:

endothermic reaction: KNO3(s) + ∆(heat) => K+(aq) + NO3-(aq)  

exothermic reaction: Na2SO4(s) => 2Na+(aq) + SO4(2-)(aq) + ∆(heat)

Based on the data points above it seems as though CO2 dissolving in acetone is an exothermic process as it slows down with increased temperature. This "slowing down" occurs due to Le-Chatelier's principle, which states that an increase/excess of a certain piece of the chemical will cause the reaction to more strongly favor the directly that the excess piece is not on. Temperature denotes additional heat in the environment which would slow down the dissolution in our example of Na2SO4(s) or as the data indicates above CO2. 

Is the lewis dot structure for CO2 the only way to represent CO2?

No, the lewis dot structure is not the only way to represent this molecule. One way which is quite popular is the ball-and-stick model which we utilize in our polarity article. There are countless other shortcuts and ways to draw on stick. However, another more complex model called molecular orbital theory tries to incorporate information about the bonding orbitals within the schematic representation. 

Molecular Orbital Diagram for CO2
Molecular Orbital Diagram for CO2. Source

This model tries to incorporate many more observations from the lab which are difficult to explain/glossed over within the simpler models such as the Lewis Dot structure. While we do not need to get into the intricacies, the evolution of our models represents an increased understanding of how these chemical systems function. Nevertheless, we can get incredibly far as we have seen simply by utilizing observations based on the lewis dot structure for CO2.  

6 comments:

  1. Why is the structure of carbon dioxide different from the other oxides of group 4A

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    1. "None of the other elements in Group 4 form double bonds with oxygen, so their oxides adopt completely different structures. When carbon forms bonds with oxygen, it promotes one of its 2s electrons into the empty 2p level. This produces 4 unpaired electrons." - https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_14%3A_The_Carbon_Family/1Group_14%3A_General_Chemistry/Oxides_of_Group_4_Elements

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