Problem 103 - General Chemistry: Principles and Modern Applications

Question

A compound consists of \(47.5 \%\) S and \(52.5 \%\) Cl, by mass. Write a Lewis structure based on the empirical formula of this compound, and comment on its deficiencies. Write a more plausible structure with the same ratio of \(\mathrm{S}\) to \(\mathrm{Cl}\).

Step-by-Step Solution

Answer

The empirical formula for the compound is SCl but the molecule is inadequately represented by this as Sulphur only has 7 electrons. A more plausible Lewis structure of the compound would be SCl2 where Sulphur (S) shares an electron with each Chlorine (Cl) atom, forming single bonds. This structure adheres to the octet rule.

Step 1: Determine the Empirical Formula

As there are no absolute masses provided, it can be deducted that 100 g of the compound is being referred to. This would mean there are \(47.5 g\) of Sulphur and \(52.5 g\) of Chlorine. By dividing these masses with their respective atomic weights (\(32.07 g / mol\) for Sulphur and \(35.45 g / mol\) for Chlorine), the number of moles for each can be found. Hence, the number of moles of Sulphur is \((47.5/32.07) = 1.48\) and Chlorine is \((52.5/35.45) = 1.48\). Therefore, the empirical formula of the compound is SCl.

Step 2: Draw the Lewis Structure Based on the Empirical Formula

Using the empirical formula, a Lewis structure can be drawn. Sulphur is in the center as it is less electronegative. According to its valence electrons, Sulphur (S) has 6 electrons while Chlorine (Cl) has 7. An electron from Sulphur can be shared with Chlorine forming a single bond between them. In this structure, Chlorine will have an octet of electrons but Sulphur will only have 7.

Step 3: Commenting on the deficiences

The produced Lewis structure has Sulphur (S) with only 7 electrons which does not follow the octet rule. Therefore, this structure does not accurately represent the correct geometry of the molecule.

Step 4: Provide a Plausible Lewis Structure

To form a structure where each atom adheres to the octet rule, a second compound of Chlorine should be added. Sulphur can share one of its remaining electrons with another Chlorine atom forming another bond. So, the new Lewis structure will consist of Sulphur single bonded with two Chlorine atoms, making the molecule SCl2.

Key Concepts

Empirical Formula

The concept of an empirical formula is pivotal when it comes to understanding the basic composition of chemical compounds. The empirical formula represents the simplest whole-number ratio of atoms in a compound. This is very different from the molecular formula, which shows the actual number of atoms of each element in a molecule. For the compound given in the original exercise, with 47.5% sulfur (S) and 52.5% chlorine (Cl), no specific quantities are stated. Instead, we're asked to imagine working with 100 grams of the compound.

• Given 47.5 grams of S and 52.5 grams of Cl, to find the empirical formula, we convert these masses to moles.
• Divide the mass of each element by its respective atomic weight: for sulfur at 32.07 g/mol, and chlorine at 35.45 g/mol.
• Both elements result in approximately 1.48 moles, leading to the empirical formula of SCl, as both have the same number of moles.

This empirical formula represents the basic 1:1 ratio of sulfur atoms to chlorine atoms in this compound.

Octet Rule

The octet rule is a key principle in chemistry that describes how atoms bond. It states that atoms tend to form bonds in such a way that each atom has eight electrons in its valence shell, resembling the electron configuration of a noble gas. In the context of sulfur and chlorine, here's how the octet rule applies:

• Sulfur has six valence electrons and plays a central role in forming bonds with chlorine.
• Chlorine has seven valence electrons and needs one more to complete its octet.

In the initial Lewis structure, sulfur shares one of its electrons with chlorine, allowing Cl to reach its full octet. However, this arrangement leaves sulfur with only seven valence electrons, which does not satisfy the octet rule. To address this deficiency, we introduce another chlorine atom, allowing sulfur to achieve a complete set of eight electrons by forming two bonds. This adjustment results in a more stable SCl2 molecule, adhering to the octet rule for sulfur.

Chemical Bonding

Chemical bonding is the way atoms combine to form compounds and molecules. Bonds form due to the attractive interactions between atoms, leading to varying types of chemical structures. Sulfur and chlorine form chemical bonds to create stable structures. Let's explore the bonding process:

• Atoms bond to achieve stability, often by fulfilling the octet rule.
• When sulfur bonds with chlorine, they form a single covalent bond as they share electrons.

The importance of bonds and their strengths can't be overlooked. In the exercise, the initial SCl structure formed shows a simple single bond. However, due to the failure to complete sulfur's octet, this bond implies instability. By creating the SCl2 structure where sulfur bonds with two chlorine atoms, the molecule becomes more plausible under the octet rule. Multiple bonds enhance the internal structure stability, allowing the molecules to effectively interact and align with nature's camouflaged symmetry.