Acid dissociation constant

Key takeaways

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

Reference excerpt

In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ⁠ K a {\displaystyle K_{a}} ⁠) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction

HA ↽ − − ⇀ A − + H + {\displaystyle {\ce {HA <=> A^- + H^+}}}

known as dissociation in the context of acid–base reactions. The chemical species HA is an acid that dissociates into A−, called the conjugate base of the acid, and a hydrogen ion, H+. The system is said to be in equilibrium when the concentrations of its components do not change over time, because both forward and backward reactions are occurring at the same rate. The dissociation constant is defined by

K a = [ A − ] [ H + ] [ H A ] , {\displaystyle K_{\text{a}}=\mathrm {\frac {[A^{-}][H^{+}]}{[HA]}} ,} or by its logarithmic form

p K a = − log 10 ⁡ K a = − log 10 ⁡ [ A − ] [ H + ] [ HA ] = log 10 ⁡ [ HA ] [ A − ] [ H + ] {\displaystyle \mathrm {p} K_{{\ce {a}}}=-\log _{10}K_{\text{a}}=-\log _{10}{\frac {[{\ce {A^-}}][{\ce {H+}}]}{{\ce {[HA]}}}}=\log _{10}{\frac {{\ce {[HA]}}}{[{\ce {A^-}}][{\ce {H+}}]}}}

where quantities in square brackets represent the molar concentrations of the species at equilibrium. For example, a hypothetical weak acid having Ka = 10−5, the value of log Ka is the exponent (−5), giving pKa = 5. For acetic acid, Ka = 1.8 x 10−5, so pKa is 4.7. A lower Ka corresponds to a weaker acid (an acid that is less dissociated at equilibrium). The form pKa is often used because it provides a convenient logarithmic scale, and a lower pKa corresponds to a stronger acid.

… excerpt ends here. Continue reading the full article.

Illustrations

Acid dissociation constant: Variation of pKa of acetic acid with ionic strength. — Variation of pKa of acetic acid with ionic strength.

Acid dissociation constant: % species formation calculated with the program HySS for a 10 millimolar solution of citric acid. pKa1 = 3.13, pKa2 = 4.76, pKa3 = 6.40. — % species formation calculated with the program HySS for a 10 millimolar solution of citric acid. pKa1 = 3.13, pKa2 = 4.76, pKa3 = 6.40.

Worked examples

Example 1 — a first encounter with Acid dissociation constant

Start with the simplest possible case. Write down what Acid dissociation constant 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 Acid dissociation constant 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 Acid dissociation constant 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 Acid dissociation constant

In research: Acid dissociation constant 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 Acid dissociation constant 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: Acid dissociation constant is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acids, Analytical chemistry, Bases (chemistry), so understanding it makes those chapters shorter.
In everyday life: Look for Acid dissociation constant 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 Acid dissociation constant in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Acid dissociation constant 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 Acid dissociation constant out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Acid dissociation constant in simple terms?

In chemistry, an acid dissociation constant (also known as acidity constant, or acid-ionization constant; denoted ⁠ K a {\displaystyle K_{a}} ⁠) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction HA ↽ − − ⇀ A − + H + {\displaysty…

Why does Acid dissociation constant 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 Acid dissociation constant?

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 Acid dissociation constant.