How Do Ion Pairs Form? Definitions, Examples & Uses
In Physics and Chemistry, an ion pair is a duplex of charged particles (typically charged atoms or molecules), one positive and the other negative. For the physicist, an ion pair is the positively charged particle (positive ion) and the negatively charged particle (negative ion) produced simultaneously by adding enough energy to a neutral atom or molecule to cause it to dissociate into oppositely charged fragments.
Thus, an energetic electron passing near or through an oxygen molecule, $${{O}_{2}}$$, may force one of the molecule's electrons out. As a result, an ion pair is formed consisting of the positive oxygen ion, $${{O}^{2+}}$$, and the negative detached electron, $${{e}^{-}}$$.
Ion Pair Chromatography Principle
Ion chromatography principle is a technique for separating hydrophilic or charged analytes on columns using stationary phases that are reversed phase or “neutral”. It entails changing the polarity of the charged analytes via interaction with an ion-pairing reagent added to the mobile phase. These reagent molecules have charges that are diametrically opposed to those of the analyte ions with which they can form electrostatic bonds. The analyte-reagent ion pairs behave like neutral, hydrophobic moieties that can be separated on C18 or C8 columns. IPC is used to separate polar organic acids, bases, zwitterions, and inorganic ions.
Ion pairing reagents can also be referred to as ion pairing additives or hetaerons. These molecules resemble soap because they have a polar head group and hydrophobic hydrocarbon chains. As a result, when Göran Schill introduced this technique in 1973, it was dubbed "soap chromatography". It is also known as ion interaction chromatography because the reagent ion interacts with the stationary phase to control the retention of ions in the sample.
Ionic Compounds Definition
Ions of opposite charge are neatly packed together to form crystalline solids. Nonmetals and metals react to form an ionic compound. In other words, ionic compounds are those that are held together by ionic bonds. Elements can lose or gain electrons to achieve their closest noble gas configuration. The formation of ions for the completion of the octet (either by gaining or losing electrons), will give them stability.
Metals generally lose electrons to complete their octet in a reaction with nonmetals, while nonmetals gain electrons to complete their octet. Ionic compounds are formed when nonmetals and metals react.
Ionic Bond Formation
Ionic bonding can occur as a result of a redox reaction in which atoms of an element (usually metal) with a low ionisation energy give up some of their electrons in order to achieve a stable electron configuration. Cations are formed as a result. An atom of another element (usually a nonmetal) with a higher electron affinity accepts one or more electrons to achieve a stable electron configuration, and the atom becomes an anion after accepting electrons. For elements in the s-block and p-block, the stable electron configuration is typically one of the noble gases, with specific stable electron configurations for d-block and f-block elements.
The electrostatic attraction between anions and cations causes the formation of a solid with a crystallographic lattice in which the ions alternately stack. Since it is usually impossible to distinguish discrete molecular units in such a lattice, the compounds formed are not molecular in nature. The ions themselves, on the other hand, can be complex and form molecular ions such as the acetate anion or the ammonium cation.
Ionic Bond Examples
Below are some examples of Ionic bonds:
Lithium Chloride (LiCl)
Sodium Chloride (NaCl)
Potassium Chloride (KCl)
Cesium Chloride (CsCl)
Lithium Hydroxide (LiOH)
Silver Iodide (AgI)
Silver Hydroxide (AgOH)
Zinc Sulphide (ZnS)
Important Questions
1.What are the properties of ionic bonds?
Ans. The following properties are observed in ionic bonded molecules due to the presence of a strong force of attraction between cations and anions:
Ionic bond is the most powerful of all bonds.
Since ionic bonds have charge separation, they are the most reactive of all the bonds in the appropriate medium.
The boiling and melting points of ionic bonded molecules are extremely high.
Ionic bonded molecules in aqueous solutions or molten state are excellent conductors of electricity. This is because of the presence of ions, which act as charge carriers.
2. What are the solvents used in chromatography?
In most cases, liquid solvents are used in chromatography. Common liquid solvents used in fast protein liquid chromatography (FPLC), high-performance liquid chromatography (HPLC), and liquid chromatography-mass spectrometry include water, methanol, isopropanol, acetonitrile, and formic acid (LC-MS). These chromatography solvents extract, dissolve, and move samples without altering their chemical structure permanently, making them an essential component of standard separation techniques. Solvents are frequently used in conjunction with water or another solvent. However, some solvents are not miscible and must be used as a pure reagent. Only other polar solvents will dissolve water and other polar solvents.
Key Features
An ion pair is a duplex of charged particles (typically charged atoms or molecules), one positive and the other negative.
Ion chromatography (IPC) is a technique for separating hydrophilic or charged analytes on columns using reversed phase or "neutral" stationary phases that do not carry charges.
Ions of opposite charge are neatly packed together to form crystalline solids. Nonmetals and metals typically react to form ionic compounds. In other words, ionic compounds are those that are held together by ionic bonds.
Multiple Choice Questions
1.Which is true for ion pairs?
(a) Bigger organic anion and smaller cation
(b) Close association of cation and anion
(c) Non polar solvents
(d) All of the above
Answer: (b)
2.Which of the following types of chromatography involves the movement of the mobile phase through the stationary phase via gravity or capillary action?
(a) Column Chromatography
(b) High-Pressure Liquid Chromatography
(c) Gas Chromatography
(d) Paper Chromatography
Answer: (a)
FAQs on Ion Pair: Meaning, Types, and Examples
1. What is an ion pair in chemistry?
An ion pair is a chemical species that consists of a cation and an anion held together by electrostatic attraction, without forming a true covalent or ionic bond. This association is most common in solutions where the ions are not fully separated or 'dissociated'. Unlike the rigid structure of an ionic crystal, an ion pair is a temporary and distinct entity moving together within a solvent.
2. How are ion pairs formed in a solution?
Ion pairs form when the electrostatic attraction between oppositely charged ions is strong enough to overcome the separating effect of the solvent molecules. This process is highly dependent on the dielectric constant of the solvent. In solvents with a low dielectric constant (i.e., less polar solvents), the electrical force between ions is stronger, leading to a higher tendency to form ion pairs. Increased concentration of the electrolyte also promotes their formation as ions are closer to each other.
3. What are the main types of ion pairs?
Based on the extent of solvent interaction between the ions, ion pairs are primarily classified into three types:
- Contact (or Tight) Ion Pairs: The cation and anion are in direct contact with each other, with no solvent molecules between them.
- Solvent-Shared Ion Pairs: The cation and anion are separated by a single, shared solvent molecule.
- Solvent-Separated Ion Pairs: The cation and anion each retain their own primary solvation shell, but are still electrostatically associated as a single unit.
4. Can you give a real-world example of an ion pair?
A classic example is a concentrated solution of sodium chloride (NaCl). While in dilute water, Na⁺ and Cl⁻ ions are fully solvated and move independently, in a more concentrated solution or in a less polar solvent, some Na⁺ and Cl⁻ ions will associate to form a [Na⁺Cl⁻] ion pair. This affects the solution's conductivity and colligative properties, as the effective number of independent particles is reduced.
5. How does an ion pair differ from a stable ionic bond or a fully dissociated salt?
The key difference lies in the state and environment:
- An ionic bond typically refers to the strong, ordered electrostatic forces holding ions together in a solid crystal lattice (e.g., solid salt).
- A fully dissociated salt exists in a highly polar solvent like water, where ions are completely separated and individually surrounded by solvent molecules (solvated).
- An ion pair is an intermediate state found in solution, where a cation and anion are associated but not locked into a lattice, acting as a single, neutral entity for a short time.
6. What factors influence the formation and stability of an ion pair?
Several factors determine the likelihood of ion pair formation:
- Solvent Polarity: Low polarity (low dielectric constant) solvents strongly promote ion pairing.
- Ionic Charge: Higher charges on the cation and anion (e.g., Mg²⁺ and SO₄²⁻) lead to stronger electrostatic attraction and more stable ion pairs.
- Ionic Size: Smaller ions with high charge density tend to form more stable and tighter ion pairs.
- Concentration: Higher electrolyte concentrations increase the probability of ions encountering each other and forming pairs.
- Temperature: The effect of temperature can be complex, but it generally influences solvent properties and ionic motion, thereby affecting ion pair equilibrium.
7. What is the importance of ion-pairing agents in analytical chemistry?
Ion-pairing agents are crucial in a technique called ion-pair chromatography. These agents are large organic ions (like tetrabutylammonium or hexane sulfonate) added to the mobile phase. They form neutral ion pairs with charged analyte molecules that would otherwise not be retained on a non-polar stationary phase (like in reverse-phase HPLC). This allows for the effective separation and analysis of ionic or highly polar compounds such as pharmaceuticals, peptides, and organic acids.
