Question

A number of salts containing the tetrahedral polyatomic anion, \(\mathrm{BF}_{4}^{-}\) , are ionic liquids, whereas salts containing the somewhat larger tetrahedral ion \(\mathrm{SO}_{4}^{2-}\) do not form ionic liquids. Explain this observation.

Short answer

Salts containing the tetrahedral polyatomic anion \(\mathrm{BF}_{4}^{-}\) form ionic liquids due to their lower lattice energy compared to salts containing the tetrahedral ion \(\mathrm{SO}_{4}^{2-}\). The smaller size and lower charge of the \(\mathrm{BF}_{4}^{-}\) anion result in lower lattice energy, favoring ionic liquid formation, while the larger size and higher charge of the \(\mathrm{SO}_{4}^{2-}\) anion lead to higher lattice energy, making it more difficult for ionic liquids to form.

Step-by-step solution

Understand Ionic Liquids

Ionic liquids are salts that have melting points lower than 100 °C and consist of ions. Their unique properties such as non-volatility, thermal stability, and solvation ability make them widely used in multiple applications. Ionic liquids are usually formed by bulky cations and large anions with low lattice energies.

Analyze \(\mathrm{BF}_{4}^{-}\) Anion

The tetrahedral \(\mathrm{BF}_{4}^{-}\) anion consists of a central boron atom surrounded by four fluorine atoms. The ion has a negative charge, and since fluorine is highly electronegative, the electrons are pulled towards the fluorine atoms. The overall ionic radius of \(\mathrm{BF}_{4}^{-}\) is relatively small.

Analyze \(\mathrm{SO}_{4}^{2-}\) Anion

The \(\mathrm{SO}_{4}^{2-}\) anion is another tetrahedral anion made up of a central sulfur atom surrounded by four oxygen atoms. Since sulfur is larger than boron, the ionic radius of \(\mathrm{SO}_{4}^{2-}\) is larger than that of \(\mathrm{BF}_{4}^{-}\). Moreover, it has two negative charges, which makes it more negatively charged than the \(\mathrm{BF}_{4}^{-}\) anion.

Effects on the Lattice Energy

Ionic liquids have low lattice energies, which result in their low melting points. Lattice energy depends on the charge and size of ions. In general, the lattice energy increases when the ion charges increase, and it decreases with increasing ion size. Therefore, for ionic liquids to form, ions with low lattice energies are preferentially formed.

Explain the Difference in Ionic Liquid Formation

The \(\mathrm{BF}_{4}^{-}\) anion's smaller size and lower charge compared to the \(\mathrm{SO}_{4}^{2-}\) anion make the lattice energy lower in salts containing \(\mathrm{BF}_{4}^{-}\). This lower lattice energy favors the formation of ionic liquids when salts containing \(\mathrm{BF}_{4}^{-}\) ions are formed. On the other hand, the greater size and higher charge of the \(\mathrm{SO}_{4}^{2-}\) anion lead to higher lattice energy in salts containing these ions. The higher lattice energy makes it more difficult for ionic liquids to form when salts containing the \(\mathrm{SO}_{4}^{2-}\) anion are produced. In conclusion, salts containing the tetrahedral polyatomic anion \(\mathrm{BF}_{4}^{-}\) form ionic liquids due to their lower lattice energy compared to salts containing the tetrahedral ion \(\mathrm{SO}_{4}^{2-}\).

Key concepts

Lattice Energy

Lattice energy is a key factor in determining the properties of ionic compounds. It refers to the energy required to separate one mole of an ionic solid into its gaseous ions. A higher lattice energy means the ions in the compound are more strongly attracted to each other, making the compound more stable.
Lattice energy depends on two main factors:

• The charge of the ions: Higher charges increase lattice energy because they create a stronger electrostatic force between the ions.
• The size of the ions: Smaller ions produce higher lattice energy as they can pack closely together, enhancing the attraction between them.

In the case of ionic liquids, low lattice energy is preferable, because it allows the compound to maintain a liquid state at a lower temperature. Smaller ion size and lower charges contribute to a decreased lattice energy, facilitating the formation of ionic liquids like those containing the \(\mathrm{BF}_{4}^{-}\) anion.

Tetrahedral Anions

Tetrahedral anions possess a three-dimensional shape where a central atom is surrounded by four other atoms in a symmetrical layout. These anions, such as \(\mathrm{BF}_{4}^{-}\) and \(\mathrm{SO}_{4}^{2-}\), differ in their atomic and electronic arrangements, which influence their properties.
The \(\mathrm{BF}_{4}^{-}\) anion has a boron center bonded to four fluorine atoms. This structure results in a relatively small and singly charged anion. On the other hand, the \(\mathrm{SO}_{4}^{2-}\) anion is larger because it has a sulfur atom at its center bonded to four oxygen atoms, and it carries a higher, double-negative charge.
The variations in size and charge between these anions lead to differences in lattice energies and melting points, impacting whether or not they form ionic liquids. It’s the tetrahedral geometry and the specific constituents of each anion that dictate these chemical behaviors.

Melting Points

Melting points are the temperatures at which a solid turns into a liquid. For ionic substances, melting points are heavily influenced by lattice energy. A lower lattice energy typically results in a lower melting point, allowing the substance to exist as a liquid at lower temperatures.
Ionic liquids have notably low melting points, often below 100°C, which is far below traditional salts. This is tied to their ability to form stable liquid phases due to weaker lattice energies.

• The compact size and lower charge of the \(\mathrm{BF}_{4}^{-}\) anion reduce the lattice energy, resulting in lower melting points—a characteristic feature of ionic liquids.
• In contrast, the larger size and higher charge of the \(\mathrm{SO}_{4}^{2-}\) anion increase lattice energy and, consequently, the melting point, preventing them from forming ionic liquids.

Hence, understanding melting points in relation to lattice energy provides insight into why certain ionic compounds, like those with the \(\mathrm{BF}_{4}^{-}\) ion, more readily form ionic liquids.

Ion Charge and Size

Ion charge and size fundamentally affect the properties of ionic compounds, including lattice energy and solubility.
Charged ions create electrostatic forces between them, which are stronger when the charges are greater. This strong attraction results in higher lattice energies, making the compound more solid at room temperature. Conversely, a smaller charge leads to less attraction, lower lattice energies, and hence, lower melting points.
Ion size plays a significant role, as well. Smaller ions can pack more efficiently and closely together, leading to higher lattice energies. Larger ions result in weaker attractions and thus lower lattice energies.

• For example, the \(\mathrm{BF}_{4}^{-}\) anion, with its smaller size and single negative charge, creates weaker electrostatic interactions, favoring the formation of ionic liquids.
• Comparatively, the \(\mathrm{SO}_{4}^{2-}\) anion's larger size and double-negative charge generate stronger interactions, making it more difficult for them to form ionic liquids.

Understanding these factors is crucial for predicting the behavior of ionic compounds in various environments.