Intermediate Topic 521 of 1102 Acids, Bases & Electrochemistry
Weak Acids
Acids that only partially ionize in water, described by Ka.
Practise Acids, Bases & Electrochemistry All Acids, Bases & Electrochemistry topics
In-depth explanation of Weak Acids
Weak acids ionize only partially in water, so most of the acid remains as undissociated molecules at equilibrium: HA <-> H+ + A-. The extent of ionization is described by the acid dissociation constant Ka, which is small for weak acids. Because the equilibrium is incomplete, the hydrogen ion concentration cannot be taken equal to the initial acid concentration and must be found from the equilibrium expression. For a monoprotic weak acid, solving x^2/(C - x) = Ka gives the equilibrium concentration of hydrogen ions. Typical examples include acetic acid, hydrofluoric acid and carbonic acid, all of which are common in laboratories and food chemistry.
Going deeper: Weak Acids
Key ideas: Weak acids ionize only partly, HA + H2O giving H3O+ + A-, with Ka well below 1. Typical values: acetic 1.8e-5, HF 6.8e-4, HCN 6.2e-10. Ionized fraction is usually under 5 percent at ordinary concentrations. Strength tracks the stability of A-, bond strength and polarity.
Common mistakes and connections: Mistakes: assuming [H+] equals the initial acid concentration, or comparing acids by pH when concentrations differ. Compare Ka or pKa instead. Also, HF is weak despite being a halogen acid, owing to the strong H-F bond and tight hydration of F-. Leads into the ICE table and percent ionization.
Visualization of Weak Acids
In 0.10 M acetic acid only about 1.3 percent of molecules ionize; most stay as HA, so H+ is far below the formal concentration. The interactive lesson shows this as a live animation or chart.
Worked example and use case
A 0.10 M acetic acid solution has pH 2.88, far less acidic than a 0.10 M HCl solution.
Frequently asked questions about Weak Acids
What is Weak Acids?
Acids that only partially ionize in water, described by Ka.
Is Weak Acids a beginner, intermediate or advanced topic?
Weak Acids is a intermediate topic in Acids, Bases & Electrochemistry. It builds on the vocabulary of beginner topics in this area.
What should I learn before Weak Acids?
Study these earlier topics in Acids, Bases & Electrochemistry first: pOH and pKw, Strong Acids, Strong Bases.
What comes after Weak Acids?
The next topics in Acids, Bases & Electrochemistry are Weak Bases and Acid Dissociation Constant.
Can I practise Weak Acids with quizzes?
Yes. The interactive lesson includes a self-check question on Weak Acids, and the Acids, Bases & Electrochemistry quiz draws on every topic in this area.
Where Weak Acids fits in Acids, Bases & Electrochemistry
Weak Acids is topic 11 of 105 in Acids, Bases & Electrochemistry.