Imagine you are boiling some water for pasta. When you pour in salt and stir, the salt begins to disappear. Well, it isn't actually disappearing, instead, it is dissociating (meaning it is breaking down into its ions). However, if you were to accidentally completely boil off the water (for example, you were too busy studying and forgot about it), the salt would reappear.
In this article, we will be learning about the different types of dissociation constant: what they are, what they mean, and how to calculate them
This article covers the dissociation constant.
First, we will define what the dissociation constant is and what it measures.
Then, we will look at the dissociation constant (Kd).
Next, we will cover the Acid Dissociation Constant (Ka) and the base dissociation constant (Kb). and see how they measure the strength of their respective species.
Lastly, we will learn about the water dissociation constant (Kw).
Test your knowledge with multiple choice flashcards
Dissociation constant are Equilibrium Constants, so they tell us which "side" of the equilibrium is favored. If the dissociation constant is large (>1), it means that products are favored (i.e. the dissociation is favored). However, if the dissociation constant is small (<1), it means that the reactant is favored (i.e. the species tends not to dissociate)There are several types of dissociation constants that we will be discussing today. These are: 1) The general dissociation constant: Kd. 2) The Acid Dissociation Constant: Ka.3) The base dissociation constant: Kb.4) The water dissociation constant: Kw.
Dissociation Constant Kd
The dissociation constant (Kd) measures the tendency of a species to break up into its components.
For a general dissociation:
The formula for the dissociation constant is:
Where [A] is the Concentration of species A, [B] is the concentration of species B, [AaBb] is the Concentration of species AaBa, and Kd is the dissociation constant
The dissociation constant can be used for things like the dissociation of a coordination complex (compound with a metal center bonded to several other species called ligands) or the dissociation of a salt.
For example, here is the dissociation of [Ag(NH3)2]+ (a coordination complex):
And here is the dissociation of NaCl (a salt):
Acid Dissociation Constant
The acid dissociation constant (Ka) measures the strength of an acid.
The conjugate base is the species that results from the losing its proton (and can now act as a base).
The acid dissociation can be written one of two ways. 1) Water is includedFor a general dissociation:Where HA is our acid and A- is our conjugate baseThe equation for Ka is:
Where [H_3O^+] is the concentration of the hydronium ion, [A-] is the concentration of the conjugate base and [HA] is the concentration of the acid
Liquids (and solids) are not included in Equilibrium Constants, so water is left out
2) Water is excludedFor a general dissociation:$$HA_{(aq)} \rightleftharpoons H^+_{(aq)} + A^-_{(aq)} The equation for Ka is:Ka measures the strength of an acid. The larger the Ka, the stronger the acid, since there is a higher concentration of H+/H3O+ ions.
Do you want to see this and many more great infographics?
The base dissociation constant (Kb) measures the strength of a base
The conjugate acid is the species that results from the base gaining a proton (and can now act as an acid)
Like with the acid dissociation constant, there are two ways to write it:
1) Water is included
For a general dissociation:
Where B is our base and BH+ is our conjugate acid
The equation for Kb is:
Where [BH^+] is the concentration of the conjugate acid, [OH-] is the concentration of the hydroxide ion, and [B] is the concentration of the base
2) Water is excluded
For a general dissociation:
Where BOH is our base and B^+ is the conjugate acid
The equation for Kb is:
Like with Ka, the magnitude of Kb determines a base's strength. However, instead of the strength coming from the concentration of H+/H3O+, it instead comes from the concentration of OH-.
Here is a table with some common bases and their Kb values:
Name of Base
Kb value
Lithium hydroxide (LiOH)
2.29x100
Potassium hydroxide (KOH)
3.16x10-1
Sodium hydroxide (NaOH)
6.31x10-1
Ammonia (NH3)
1.77x10-5
Ammonium hydroxide (NH4OH)
1.79x10-5
Pyridine (C5H5N)
1.78x10-9
Water Dissociation Constant
The water dissociation constant (Kw) describes how water dissociates into its ions
The dissociation reaction is:So the formula for Kw is:Where [OH-] is the concentration of the hydroxide ion and [H+] is the concentration of the hydrogen ionThe value of Kw is dependent on temperature. The standard value (at room temperature, which is ~25°C) is 1.00·10-14.Below is a table of Kw values based on temperature:
Temperature (°C)
Kw
10
0.29x10-14
15
0.45x10-14
20
0.69x10-14
25
1.01x10-14
30
1.47x10-14
Based on this, we can see that an increase in temperature causes an increase in dissociation
Kw and acid/base strength
For any acid/base pair:
Because of this, this can tell us two things:
We can calculate Ka when given Kb and vice versa
The strength of the acid and conjugate base are inversely related
If an acid is very strong, this means that its conjugate base will be weak and vice versa. For example, take hydroiodic acid (Ka=2x109):
Therefore, the conjugate base, iodide (I-) is a very weak base
Dissociation Constant - Key takeaways
A dissociation constant is a type of equilibrium constant that measure the tendency of a species to dissociate (separate) into smaller components
The dissociation constant (Kd) measures the tendency of a species to break up into its components.
For a general dissociation:
The formula for the dissociation constant is:
The acid dissociation constant (Ka) measures the strength of an acid
For a general dissociation:$$HA_{(aq)} \rightleftharpoons H^+_{(aq)} + A^-_{(aq)}The equation for Ka is:
The base dissociation constant (Kb) measures the strength of a base
For a general dissociation:
The equation for Kb is:
The water dissociation constant (Kw) describes how water dissociates into its ions
The dissociation reaction is:So the formula for Kw is:
How we ensure our content is accurate and trustworthy?
At StudySmarter, we have created a learning platform that serves millions of students. Meet
the people who work hard to deliver fact based content as well as making sure it is verified.
Content Creation Process:
Lily Hulatt
Digital Content Specialist
Lily Hulatt is a Digital Content Specialist with over three years of experience in content strategy and curriculum design. She gained her PhD in English Literature from Durham University in 2022, taught in Durham University’s English Studies Department, and has contributed to a number of publications. Lily specialises in English Literature, English Language, History, and Philosophy.
Gabriel Freitas is an AI Engineer with a solid experience in software development, machine learning algorithms, and generative AI, including large language models’ (LLMs) applications. Graduated in Electrical Engineering at the University of São Paulo, he is currently pursuing an MSc in Computer Engineering at the University of Campinas, specializing in machine learning topics. Gabriel has a strong background in software engineering and has worked on projects involving computer vision, embedded AI, and LLM applications.
StudySmarter is a globally recognized educational technology company, offering a holistic learning platform designed for students of all ages and educational levels. Our platform provides learning support for a wide range of subjects, including STEM, Social Sciences, and Languages and also helps students to successfully master various tests and exams worldwide, such as GCSE, A Level, SAT, ACT, Abitur, and more. We offer an extensive library of learning materials, including interactive flashcards, comprehensive textbook solutions, and detailed explanations. The cutting-edge technology and tools we provide help students create their own learning materials. StudySmarter’s content is not only expert-verified but also regularly updated to ensure accuracy and relevance.