Writing the Concentration Equilibrium Constant Expression for the Reaction Involving 2 CuI
When dealing with sparingly soluble salts, the solubility product (K<sub>sp</sub>) is the key equilibrium constant that describes the balance between the solid phase and its dissolved ions. Copper(I) iodide (CuI) is a classic example of such a salt. In many textbooks and exam questions the reaction is presented as the dissolution of two formula units of CuI:
[ 2,\text{CuI (s)} ;\rightleftharpoons; 2,\text{Cu}^{+}(aq) + 2,\text{I}^{-}(aq) ]
The task is to write the concentration equilibrium constant expression (often called K<sub>c</sub>) for this reaction. So below you will find a step‑by‑step guide, the scientific background that justifies each step, a collection of frequently asked questions, and a concise conclusion that reinforces the main ideas. The article is organized with clear subheadings, bolded key terms, and bullet lists to improve readability and SEO relevance.
1. Understanding the Reaction and Its Stoichiometry
Before writing any equilibrium expression, it is essential to confirm that the chemical equation is balanced and that all phases are correctly indicated That's the part that actually makes a difference..
- Solid phase (s) – CuI remains in its crystalline form and does not appear in the expression for K<sub>c</sub>.
- Aqueous ions (aq) – The dissolved copper(I) cations (Cu⁺) and iodide anions (I⁻) are the only species that contribute to the equilibrium constant.
The balanced reaction for two formula units is:
[ \boxed{2,\text{CuI (s)} ;\rightleftharpoons; 2,\text{Cu}^{+}(aq) + 2,\text{I}^{-}(aq)} ]
Why the coefficient 2?
When we multiply the elementary dissolution reaction CuI (s) ⇌ Cu⁺ (aq) + I⁻ (aq) by 2, we simply consider the simultaneous dissolution of two independent crystals. This does not change the type of equilibrium, but it does affect the numerical value of K<sub>c</sub> because the exponents in the expression are tied to the stoichiometric coefficients Turns out it matters..
2. General Rules for Constructing K<sub>c</sub>
- Include only gaseous or aqueous species. Pure solids and liquids have an activity of
