There are many different types of chemical reactions and processes that occur in real-world settings or are performed in labs. Dissociation is one of them. Dissociation in chemistry generally describes the process in which molecules break up into simpler components.
Dissociation occurs when solid ionic and polar covalent compounds dissolve in water and split into atoms, ions, or radicals.
While dissociation in chemistry is one of the basic concepts students learn, some find it a little bit tricky to understand how it works. However, as long as you consider a few aspects of dissociation instead of memorizing the rules, you can easily comprehend the topic.
To help you get started, below is the definition of dissociation in chemistry and a brief explanation of the dissociation constant. We also highlight some differences between dissociation and ionization while teaching you how to write dissociation equations in chemistry. Last but not least, we provide a few dissociation examples in chemistry, including the dissociation of ionic compounds, acids, bases, and salts.
What Is Dissociation? – Definition
In general terms, dissociation is a reaction in chemistry in which a given compound breaks into two or more constituents.
In a dissociation reaction, ionic or polar covalent compounds in an aqueous solution (water) dissociate. In this process, water molecules break the water-soluble compounds apart, forming ions. Usually, the energy required to break the bonds is present in the form of heat or electricity.
The general formula for a dissociation reaction is as follows:
- AB(aq) ⇌ A(aq) + B(aq)
Since dissociation reactions are typically reversible, the resulting atoms or ions can recombine to form the original compound. Therefore, we use the symbol “⇌” with two half arrowheads to show the forward and backward reactions.
What Is the Dissociation Constant?
While we know as a fact that some acids and bases are strong and others are weak, there’s a constant that describes how strong or weak an acid or base actually is.
A dissociation constant KD is defined as the ratio of the concentrations of dissociated ions to the concentration of the original acid or base. The larger the KD value, the stronger the acid or base.
The general formula for the dissociation constant KD is as follows:
- KD = [A][B][AB]
Where [A] and [B] are the concentrations of the dissociated ions (products) and [AB] is the concentration of the original compound before dissociation.
In case of the dissociation of acids, the dissociation constant is written as Ka.
In case of the dissociation of bases, the dissociation constant is written as Kb.
To learn more about acid/base dissociation constants, check this video by The Science Classroom.
What Is the Difference between Dissociation and Ionization?
Since both dissociation and ionization are based on the same principle of the separation of the constituents, many students often mistake dissociation for ionization and vice versa. Still, dissociation and ionization are two different processes.
The main difference between dissociation and ionization is that dissociation refers to the breakdown of a compound into smaller constituents (atoms, ions, molecules, or radicals), while ionization is the process in which atoms or molecules gain a positive or negative charge.
To make sure you never confuse these two processes with one another, here’s a summary of the key differences between dissociation and ionization.
|Dissociation is the process by which a substance is broken down into smaller constituents.||Ionization is the process by which compounds gain a positive or negative charge.|
|Dissociation occurs only when a substance is dissolved in water and energy is applied.||During ionization, energy is absorbed when an atom loses an electron and energy is released when an atom gains an electron.|
|Dissociation can produce both charged and electrically neutral particles.||Ionization produces charged particles only.|
|Dissociation can form atoms, molecules, ions, or radicals that are smaller than the original compound.||Ionization forms ions only.|
|Dissociation produces the ions that already exist before the compound is dissolved in water.||Ionization produces new ions.|
|Dissociation occurs only in ionic and polar covalent compounds.||Ionization can occur in polar covalent compounds and metals.|
|Dissociation is a reversible process.||Ionization is an irreversible process.|
How to Write Dissociation Equations in Chemistry?
As we’ve already discussed the basics of dissociation, let’s move on to explaining a few rules to write dissociation equations in chemistry.
To write a dissociation equation, you first determine if a given compound is soluble in water. Next, write down the dissociation equation and indicate the aqueous states (aq) of the original compound and the dissociated particles. Do not forget to put the positive and negative charges of the ions. Lastly, balance the dissociation equation and you’re ready to go.
Check out this educational video by Melissa Maribel explaining how to write dissociation equations of strong electrolytes.
To help you better understand the concept of writing balanced dissociation equations in chemistry, the following section will share a few common examples.
Dissociation Examples in Chemistry
To make the article even more informative and explicit, let’s explain how the dissociation of different compounds works while providing a few specific examples to help you visualize the process.
Here’s what you need to know about the dissociation of ionic compounds, acids, bases, and salts.
Dissociation of Ionic Compounds
As we’ve already mentioned above, dissociation typically occurs in ionic compounds only. An ionic compound is a compound made of ions with strong ionic bonds between the particles.
Ionic compounds dissociate when dissolved in water, forming anions and cations. The resulting solution consisting of negative and positive ions is known as an electrolytic solution. Electrolytic solutions can conduct electricity.
The ionic compounds that dissociate in water include water-soluble acids, bases, and salts. To learn the basics of solubility rules, check out this video by FuseSchool mentioning some of the soluble and insoluble acids, bases, and salts.
Dissociation of Covalent Compounds
When it comes to the dissociation of covalent compounds, there are a few factors we should consider. Since water is a polar solvent and most covalent compounds are nonpolar, covalent compounds do not dissociate into individual atoms when dissolved in water. Instead, they break down into molecules. However, polar covalent compounds dissociate in water and form electrically charged particles, ions.
While nonpolar covalent compounds do not dissociate when dissolved in water, polar covalent compounds dissociate in water and form anions and cations.
As an example, H2, O2, Cl2, and Br2 are nonpolar covalent compounds and they don’t dissociate in water.
On the other hand, HCl, HBr, HF, H2SO4, and HNO3 are strong acids that are polar covalent compounds and dissociate in water, forming ions.
To read more about how covalent compounds dissociate in water, explore this thread on Chemistry StackExchange.
Dissociation of Acids
As a matter of fact, acids are polar covalent or molecular compounds that dissociate when dissolved in water. In terms of acid strength, there are strong acids and weak acids.
A strong acid is an acid that dissociates completely in water. On the other hand, a weak acid is an acid that dissociates partially in water. While strong acids produce a large amount of H+ ions, weak acids produce only a small amount of hydrogen ions.
Here’s what you need to know about the dissociation of strong and weak acids.
Writing Dissociation Equations for Acids
When writing the dissociation equations for acids, there are a few aspects you should consider.
To simplify the equations, we often write the dissociation of acids as follows:
- HCl (aq) ⇌ H+ (aq) + Cl– (aq)
In reality, the dissociation of acids occurs in two steps. The first step is dissociation and the second step is ionization. In the first step, the acid is dissolved in water, forming the corresponding ions. In the second step, hydrogen ions react with water molecules, forming the hydronium ions.
So, the complete dissociation/ionization equation of acids should look like this.
- Step 1 – Dissociation
HCl (g) + H2O (l) ⇌ H+ (aq) + Cl– (aq)
- Step 2 – Ionization
H+ (aq) + H2O (l) ⇌ H3O+ (aq)
Besides, we can combine these two steps and write a single equation for the dissociation of HCl.
- HCl (g) + H2O (l) ⇌ H3O+ (aq) + Cl– (aq)
Simple Science on YouTube explains the dissociation and ionization of acids in this video, so make sure to check it out.
The same rule applies to the dissociation of other acids. Let’s take a look at the simplified dissociation equations for HNO3, H2SO4, and HClO4.
- HNO3 (aq) ⇌ H+ (aq) + NO3 (aq)
- H2SO4 (aq) ⇌ 2H+ (aq) + SO42- (aq)
- HClO4 (aq) ⇌ H+ (aq) + ClO4– (aq)
For further information about the ionization of monoprotic, diprotic, and triprotic acids, check out this video tutorial by Teach Me Chemistry.
Dissociation of Bases
Generally speaking, bases are ionic compounds in which a metal cation is connected to a hydroxyl anion by means of an ionic bond.
When dissolved in water, bases dissociate into OH– anions and metal cations. While strong bases dissociate completely in water, weak bases dissociate only partially.
Writing dissociation equations for bases is pretty simple. Let’s consider a few examples to help you better understand the concept.
Writing Dissociation Equations for Bases
Here are the equations for the dissociation of NaOH, Ca(OH)2, and Al(OH)3.
- NaOH (aq) ⇌ Na+ (aq) + OH– (aq)
- Ca(OH)2 (aq) ⇌ Ca2+ (aq) + 2OH– (aq)
In case of Al(OH)3, you may have written the following equation:
- Al(OH)3 (s) + H2O (l) ⇌ Al3+ (aq) + 3OH– (aq)
This equation isn’t quite accurate. Looking at the solubility table, Al(OH)3 doesn’t dissolve in water, meaning that it wouldn’t dissociate into the ions we’ve written above. Wayne Breslyn explains what happens to aluminum hydroxide when dissolved in water.
Dissociation of Salts
Salts are ionic compounds that are produced when acids react with bases. As a result, salt obtains a positively charged ion from a base and a negatively charged ion from an acid. Just like other ionic compounds, most salts dissociate in water.
When dissolved in water, salts dissociate and form positively and negatively charged ions. While water-soluble salts dissociate completely, water-insoluble salts don’t dissociate. Still, water-insoluble salts may still produce a small amount of ions in the aqueous solution.
Here’s what to take into account when it comes to the dissociation of salts.
Writing Dissociation Equations for Salts
Writing the dissociation equations for salts is quite straightforward. As long as you know the salts that are soluble or insoluble in water, you can easily break them into corresponding anions. To get started, you should first take a look at the solubility rules.
If you’ve already checked the solubility rules, let’s consider a few examples and write dissociation equations for NaCl, AgNO3, and CaSO4.
Looking at the solubility rules, we can see that NaCl is a water-soluble salt. So, here’s the dissociation equation for NaCl:
- NaCl (aq) ⇌ Na+ (aq) + Cl– (aq)
Continuing with AgNO3. While most silver salts are insoluble, AgNO3 is an exception to this rule. So, we can write the dissociation equation for AgNO3:
- AgNO3 (aq) ⇌ Ag+ (aq) + NO3– (aq)
For the last example, we have CaSO4. Although most sulfate salts are soluble in water, CaSO4 is an important exception to this rule and this ionic salt is slightly soluble in water. Therefore, CaSO4 doesn’t dissociate in water completely. Still, some of the CaSO4 will split into ions when dissolved in water and much of it will remain solid. Here’s a video explanation by Wayne Breslyn.