Intermediate Topic 487 of 1102 Kinetics & Equilibrium
Degree of Dissociation
The fraction of a substance that dissociates at equilibrium.
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In-depth explanation of Degree of Dissociation
The degree of dissociation alpha is the fraction of a substance that has dissociated into products at equilibrium, ranging from zero to one. For a reaction A <-> B + C that starts with concentration c and no products, the equilibrium concentrations are c(1 - alpha) for A and c alpha for each product, so K = c alpha^2/(1 - alpha). Because alpha appears squared, the degree of dissociation changes significantly with concentration. Dilution increases alpha, which is known as the Ostwald dilution law, since the equilibrium shifts toward more particles as the total amount per volume decreases. This concept is especially important for weak electrolytes and gaseous dissociations.
Going deeper: Degree of Dissociation
Key ideas: For A <-> B + C from c0, K = c0 alpha^2/(1 - alpha); for small alpha, alpha is about sqrt(K/c0). For gases starting with 1 mol: total moles = 1 + alpha, so molar mass of the mixture = M0/(1 + alpha), giving alpha = (D - d)/((n - 1)d) from vapour densities. The van't Hoff factor is i = 1 + (n - 1) alpha.
Common mistakes and connections: Students use alpha as a percentage in formulas without converting, or forget that the 1 - alpha approximation fails when alpha is above about 0.05. Another error is using the same expression for A <-> 2B, whose formula includes 4 alpha^2. Also, density measurements of dissociating gases give alpha only if the stoichiometry is known. Ties to colligative properties and Ostwald dilution.
Visualization of Degree of Dissociation
For N2O4 to 2NO2 at 298 K dilution raises alpha (Ostwald dilution law); near 1 atm, c = 0.034 M, alpha is about 0.19. The interactive lesson shows this as a live animation or chart.
Worked example and use case
N2O4 is roughly 18 to 20 percent dissociated into NO2 at 25 degrees C and 1 atm.
Frequently asked questions about Degree of Dissociation
What is Degree of Dissociation?
The fraction of a substance that dissociates at equilibrium.
Is Degree of Dissociation a beginner, intermediate or advanced topic?
Degree of Dissociation is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Degree of Dissociation?
Study these earlier topics in Kinetics & Equilibrium first: Complex Ion Equilibria, Effect of Complexation on Solubility, Equilibrium in Gas Phase.
What comes after Degree of Dissociation?
The next topics in Kinetics & Equilibrium are Relationship of Rate Constants and K and Principle of Microscopic Reversibility.
Can I practise Degree of Dissociation with quizzes?
Yes. The interactive lesson includes a self-check question on Degree of Dissociation, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Degree of Dissociation fits in Kinetics & Equilibrium
Degree of Dissociation is topic 68 of 91 in Kinetics & Equilibrium.