☰ Contents · Chemistry (General chemistry)

Degree of dissociation. Ionic equations

Lessons 10 · 1 lessons · S. Masharipov, A. Mutalibov, E. Murodov, H. Islomova. General chemistry, Grade 11, 1st edition. G‘afur G‘ulom Publishing and Printing House, Tashkent, 2018
10

Degree of dissociation. Complete and net ionic equations

Textbook: pp. 54–58
GoalCalculate the degree of dissociation and write molecular, complete ionic and net ionic equations for ion-exchange reactions.
New words
degree of dissociation · dissotsiatsiyalanish darajasiion-exchange reaction · ionlarning almashinish reaksiyasicomplete ionic equation · to‘liq ionli tenglamanet ionic equation · qisqa ionli tenglama
Explanation

The degree of dissociation α is the ratio of the number of molecules that have split into ions to the total number of molecules in the solution: α = n/N; as a percentage α % = α·100 %. In the book an electrolyte is conventionally weak if α % < 3 %, moderate for 3–30 % and strong above 30 %. It depends on the nature of the substance and the solvent, on concentration (it grows on dilution) and on temperature. Reactions between electrolyte solutions are exchanges of ions and go to completion only if a precipitate, a gas or a poorly dissociating substance (such as water) forms. In an ionic equation strong electrolytes are written as ions, while precipitates, gases and weak electrolytes keep their molecular formulas. First the molecular equation is written, then the complete ionic equation, and the ions that appear unchanged on both sides (spectator ions) are cancelled to give the net ionic equation. In an ionic equation both the number of atoms and the sum of charges must be equal on the two sides.

Worked examples
BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl. Complete ionic: Ba²⁺ + 2Cl⁻ + 2Na⁺ + SO₄²⁻ → BaSO₄↓ + 2Na⁺ + 2Cl⁻. Net ionic: Ba²⁺ + SO₄²⁻ → BaSO₄↓ (Na⁺ and Cl⁻ are spectator ions).
Of 400 molecules of a weak acid, 10 have dissociated. α = 10/400 = 0.025, that is α % = 2.5 % — a weak electrolyte by the book’s criterion.
Class activity

«Ionic chess» in pairs: one student names two electrolyte solutions, the other says whether a reaction occurs (a precipitate, gas or water forms) and writes the net ionic equation on the board.

Practice
1
Of 300 HNO₂ molecules, 24 have dissociated. What is α in percent?
2
In a solution with α = 60 %, how many of 500 KOH molecules have dissociated?
3
Write the net ionic equation for CaCO₃ + 2HCl → CaCl₂ + CO₂↑ + H₂O.
4
Write the net ionic equation for KOH + HNO₃ and explain why the reaction goes to completion.