Ion association

Key takeaways

Ion association belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Ion association to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Ion association from memory before moving on to harder problems.

Reference excerpt

In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each other, as ion pairs, ion triplets, etc. Intimate ion pairs are also classified according to the nature of the interaction as contact, solvent-shared or solvent-separated. The most important factor to determine the extent of ion association is the dielectric constant of the solvent. Ion associates have been characterized by means of vibrational spectroscopy, as introduced by Niels Bjerrum, and dielectric-loss spectroscopy.

Classification of ion pairs

Ion pairs are formed when a cation and anion, which are present in a solution of an ionizable substance, come together to form a discrete chemical species. There are three distinct types of ion pairs, depending on the extent of solvation of the two ions. For example, magnesium sulfate exists as both contact and solvent-shared ion-pairs in seawater.

Mg ( aq ) 2 +

+ SO 4 ( aq ) 2 − ↽ − − ⇀ MgSO 4 ( aq ) {\displaystyle {\ce {Mg^2+_{(aq)}{}+ SO4^{2-}_{(aq)}<=> MgSO4_{(aq)}}}}

In the schematic representation above, the circles represent spheres. The sizes are arbitrary and not necessarily similar as illustrated. The cation is coloured red and the anion is coloured blue. The green area represents solvent molecules in a primary solvation shell; secondary solvation is ignored. When both ions have a complete primary solvation sphere, the ion pair may be termed fully solvated (separated ion pair, SIP). When there is about one solvent molecule between cation and anion, the ion pair may be termed solvent-shared. Lastly, when the ions are in contact with each other, the ion pair is termed a contact ion pair (CIP). Even in a contact ion pair, however, the ions retain most of their solvation shell. The nature of this solvation shell is generally not known with any certainty. In aqueous solution and in other donor solvents, metal cations are surrounded by between 4 and 9 solvent molecules in the primary solvation shell, An alternative name for a solvent-shared ion pair is an outer-sphere complex. This usage is common in coordination chemistry and denotes a complex between a solvated metal cation and an anion. Similarly, a contact ion pair may be termed an inner-sphere complex. The essential difference between the three types is the closeness with which the ions approach each other: fully solvated > solvent-shared > contact. With fully solvated and solvent-shared ion pairs the interaction is primarily electrostatic, but in a contact ion pair some covalent character in the bond between cation and anion is also present. An ion triplet may be formed from one cation and two anions or from one anion and two cations. Higher aggregates, such as a tetramer (AB)4, may be formed. Ternary ion associates involve the association of three species. Another type, named intrusion ion pair, has also been characterized.

Theory Ions of opposite charge are naturally attracted to each other by the electrostatic force. This is described by Coulomb's law:

F = q 1 q 2 ε r 2 {\displaystyle F={\frac {q_{1}q_{2}}{\varepsilon r^{2}}}}

where F is the force of attraction, q1 and q2 are the magnitudes of the electrical charges, ε is the dielectric constant of the medium and r is the distance between the ions. For ions in solution this is an approximation because the ions exert a polarizing effect on the solvent molecules that surround them, which attenuates the electric field somewhat. Nevertheless, some general conclusions can be inferred.

Ion association will increase as: the magnitude(s) of the electrical charge(s) q1 and q2 increase, the magnitude of the dielectric constant ε decreases, the size of the ions decreases so that the distance r between cation and anion decreases. The equilibrium constant K for ion-pair formation, like all equilibrium constants, is related to the standard free-energy change:

Δ G ⊖ = − R T ln ⁡ K , {\displaystyle \Delta G^{\ominus }=-RT\ln K,}

where R is the gas constant and T is the absolute temperature. Free energy is made up of an enthalpy term and an entropy term:

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Ion association

Start with the simplest possible case. Write down what Ion association claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Ion association before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Ion association ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Ion association

In research: Ion association appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Ion association in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Ion association is common in secondary-school and first-year university syllabi. It links to neighbouring topics Equilibrium chemistry, so understanding it makes those chapters shorter.
In everyday life: Look for Ion association outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Ion association in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Ion association means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Ion association out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Ion association in simple terms?

In chemistry, ion association is a chemical reaction whereby ions of opposite electric charge come together in solution to form a distinct chemical entity. Ion associates are classified, according to the number of ions that associate with each other, as ion pairs, ion triplets, etc.

Why does Ion association matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Ion association?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Ion association.