Question

Draw Lewis structures for the \(\mathrm{AsCl}_{4}^{+}\) and \(\mathrm{AsCl}_{6}^{-}\) ions. What type of reaction (acid-base, oxidation- reduction, or the like) is the following? $$2 \mathrm{AsCl}_{5}(g) \longrightarrow \mathrm{AsCl}_{4} \mathrm{AsCl}_{6}(s)$$

Short answer

The Lewis structures for the AsCl₄⁺ ion include As as the central atom with single bonds to each of the Cl atoms surrounding it, with 3 lone pairs on each Cl atom, and a positive charge on As atom. For the AsCl₆⁻ ion, As is in the center with single bonds to 6 Cl atoms around it, with 3 lone pairs on each Cl atom, and a negative charge on the As atom. The reaction \(2 \mathrm{AsCl}_{5}(g) \longrightarrow \mathrm{AsCl}_{4}^{+}\mathrm{AsCl}_{6}^{-}(s)\) is not an acid-base reaction or an oxidation-reduction reaction since there is no proton exchange or change in oxidation numbers. It can be classified as a ligand exchange reaction.

Step-by-step solution

Draw the Lewis structure for AsCl₄⁺ ion

First, identify the total number of valence electrons in the AsCl₄⁺ ion. As (Arsenic) has 5 valence electrons and Cl (Chlorine) has 7 valence electrons. There are 4 Cl atoms in the ion. So, total valence electrons are: \(5 + 4(7) = 5 + 28 = 33 \) As the ion carries a positive charge, one electron from total valence electrons will be deducted. So, the number of electrons available for sharing is 32. The central atom is As, place it in the center and arrange the 4 Cl atoms around it. Place a single bond between As and each Cl atom. This will consume 8 electrons (1 bond = 2 electrons). Now we still have 24 electrons left. Distribute the remaining electrons as lone pairs among Cl: Each of the 4 Cl atoms gets 3 lone pairs (each pair = 2 electrons), consuming the remaining 24 electrons. Finally, add the positive charge to As atom as it has an expanded octet (more than 8 electrons and able to hold more electrons).

Draw the Lewis structure for AsCl₆⁻ ion

First, identify the total number of valence electrons in the AsCl₆⁻ ion. Similar to step 1, As has 5 valence electrons and Cl has 7 valence electrons. However, this time there are 6 Cl atoms in the ion. So total valence electrons are: \((5 + 6(7))^− = 47^− \) As the ion carries a negative charge, one electron will be added to the total valence electrons, making 48 electrons available for sharing. Place As in the center and arrange the 6 Cl atoms around it. Place a single bond between As and each Cl atom. This will consume 12 electrons. Now we still have 36 electrons left. Distribute the remaining electrons as lone pairs among Cl: Each of the 6 Cl atoms gets 3 lone pairs (each pair = 2 electrons), consuming the remaining 36 electrons. Place the negative charge on As atom as it has an expanded octet (more than 8 electrons and able to hold more electrons).

Determine the type of reaction

Examine the \(\mathrm{AsCl}_5 \rightarrow \mathrm{AsCl}_4$$^{+}\mathrm{AsCl}_6(^{-})\) reaction. As no protons (H⁺ ions) are being exchanged, it is not an acid-base reaction. Moreover, there is no change in oxidation numbers for the As or Cl atoms: As in \(\mathrm{AsCl}_5\) and \(\mathrm{AsCl}_6$$^{-}\) is in the +5 oxidation state, while As in \(\mathrm{AsCl}_4$$^{+}\) is also in the +5 state. Cl in \(\mathrm{AsCl}_5\) and \(\mathrm{AsCl}_4$$^{+}\) is in the -1 oxidation state, and it remains in the -1 state in \(\mathrm{AsCl}_6$$^{-}\) as well. Since there are no changes in oxidation numbers, we can conclude that the reaction is not an oxidation-reduction reaction. The reaction primarily involves the exchange of ligands between the two chemical species, so it can be classified as a ligand exchange reaction.

Key concepts

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. These electrons play a crucial role in chemical bonding and interactions. In compounds like othingscriptexample" othingscriptexample) and othingscriptexample" othingscriptexample), understanding the number of valence electrons helps in drawing accurate Lewis structures.

For instance, arsenic (As) has 5 valence electrons while chlorine (Cl) has 7. When creating a Lewis structure, you sum these electrons based on the number of atoms present. In the othingscriptexample)a total of 33 electrons from As and Cl are involved, but due to the positive charge ( othingscriptexample) this total is reduced by one, giving us 32 electrons for bonding.

The same reasoning applies to othingscriptexample)a formula atom when dealing with anions or additional negative charges, where electrons are added to the total. These valence electrons then determine the bonding pattern and structure in molecules.

Expanded Octet

The concept of an expanded octet refers to certain atoms' ability to hold more than eight electrons in their valence shell. This often involves elements from the third period and beyond, such as phosphorus, sulfur, and arsenic.

In the context of othingscriptexample) has the capacity to accommodate more electrons in its outer shell than the standard octet rule would suggest.

This is possible because these elements have d-orbitals available for bonding. As a result, an element like arsenic can be central in structures bonded with multiple ligands, such as in othingscriptexample)a formula molecule, forming stable compounds through expanded octet configurations.

Oxidation Numbers

Oxidation numbers help track how many electrons an atom gains, loses, or shares when forming compounds. It's a simple way to determine the electron distribution in different types of reactions.

In our referenced reaction involving othingscriptexample), the oxidation number for arsenic does not change; it remains at +5. Chlorine is also consistent at -1 across the structures.

This lack of change in oxidation numbers indicates no transfer of electrons, affirming that a process is not occurring via redox (oxidation-reduction) means. It is a helpful tool in ruling out reactions like acid-base or oxidation-reduction when analyzing chemical equations to identify the type of chemical transformation.

Ligand Exchange Reaction

A ligand exchange reaction involves the swapping of ligands between coordination complexes without any change in their oxidation states. This type of reaction is significant in inorganic chemistry and is observed extensively in coordination compounds.

In the chemical process given: othingscriptexample), we witness a rearrangement forming new complexes: othingscriptexample) and othingscriptexample). Here, ligands (such as the chlorine atoms) are exchanged between the arsenic compounds.

Ligand exchange reactions are characterized by the preservation of oxidation states of the involved atoms and do not result in the formation of a new oxidation state for any atom. Hence, this reaction classification focuses on the exchange aspect rather than electron transfer, making it distinct from more familiar types such as acid-base or redox reactions.