Is H2CO3 a Potassium Carbonate? – the H2CO3 pH

Carbonic acid (H2CO3) is a diprotic acid with the formula H2CO3. It’s a carbon dioxide hydride that decomposes at temperatures above -80°C.

It produces two types of salts, hydrogen carbonates (HCO3-) and carbonates, due to its diprotic nature (CO3-2).

Many students are curious about the acidity of H2CO3 (carbonic acid), specifically whether it is a strong or weak acid.

So, in this article, we’ll look at the acidity of H2CO3 and go over all of the basics.

Is carbonic acid a powerful acid? No, H2CO3 is not a strong acid since it does not entirely dissociate in water. Carbonic acid is a diprotic acid, meaning it has two hydrogen atoms to lose, and thus has two acid dissociation constants, Ka. The strength of an acid is reflected in the value of the acid dissociation constant. The higher the value of Ka, the more powerful the acid is.

The reaction equations are listed below, along with their Ka values:

H2CO3(aq)    <=====>    HCO3-    +    H+         Ka1 = 4.3 X 10−7 mol/L; pKa1 = 6.36 at 25°C

HCO3-   <=====>     CO3-2   +    H+              Ka2 = 4.8 X 10−11 mol/L; pKa2 = 10.25 at 25°C

The value of the acid dissociation constant is very low, while the logarithmic constant is higher for carbonic acid than for strong acids, as shown by the above equations.

The Ka value for strong acids is expected to reach infinity, while the pKa value should be less than 2.

As a result, carbonic acid is a weak acid. Carbonic acid with a concentration of 0.1M has a pH of 3.68.

What Makes Carbonic Acid So Weak?

When carbonic acid is dissolved in an aqueous solution, it first ionises into bicarbonate and proton, as discussed above.

This means that the bicarbonate ion acts as a conjugate base for carbonic acid in this process.

Furthermore, because the bicarbonate ion is a strong conjugate base, it has a tendency to stay connected with its proton, causing acid molecules to only ionise partially in aqueous solution.

H2CO3 or CH3COOH, which is more acidic?

Both carbonic acid and acetic acid are classified as weak acids because they do not completely ionise in an aqueous solution.

Carbonic acid, on the other hand, has a Ka value of 4.3 X 10-7 mol/L, whereas acetic acid has a Ka value of 1.8 X 10-5 mol/L.

Carbonic acid has a pKa of 6.36 in the first dissociation equation, whereas acetic acid has a pKa of 4.76.

These figures show that acetic acid is more powerful than carbonic acid.

Let us now try to figure out why acetic acid is more powerful than carbonic acid. Begin by formulating the dissociation equations for both acids:

For carbonic acid, H2CO3(aq)   <====>    HCO3-   +   H+

For acetic acid, CH3COOH(aq)   <====>   CH3COO-   +   H+

According to the preceding equations, the conjugate base for carbonic acid is bicarbonate ion, whereas the conjugate base for acetic acid is acetate ion.

Check read my in-depth post on whether acetic acid is a strong acid.

For the time being, we can disregard the second dissociation equation of carbonic acid because the strength of the acid is determined solely by the first proton released.

We must now assess the strength of these conjugate bases, as a stronger conjugate base indicates a weaker acid, and vice versa.

Because of the resonance effect of the carboxyl ion, the acetate ion is a stronger conjugate base than the bicarbonate ion.

Another factor is the instability of carbonic acid, which decomposes primarily to water and carbon dioxide molecules, leaving only a small amount of carbonic acid in the solution, causing the acidity to naturally decrease.

If you’re interested in learning more about the chemical structure of carbonic acid, click here. Read the article on carbonic acid’s Lewis structure.

Carbonic Acid’s pH is a measurement of how acidic it is.

In an aqueous solution, the pH scale is used to express the acidity or basicity of a substance.

The pH scale has values ranging from 1 to 14. The pH of all acids is less than 7, while the pH of all bases is greater than 7.

The pH of neutral solutions, such as pure water, is 7.

Now, when calculating the pH value of carbonic acid, it’s important to remember that it’s a polyprotic acid, which means it dissociates multiple times in a solution, releasing different ions along with protons.

As a result, calculating the pH of 0.1 M carbonic acid differs from calculating the pH of other simple acids.

To begin, consider the first dissociation equation, which results in the release of bicarbonate ions as well as protons.

The following is the chemical equation:

H2CO3(aq)   <====>   HCO3-   +    H+

Now, using the Ka1 value of 4.3 X 107 mol/L, calculate the hydrogen ion concentration in this equation.

The following is the equation:

Ka = ([HCO3-] [H+]) / H2CO3

Because we’re calculating pH for a 0.1 M solution, we’ll use 0.1 as the starting concentration of H2CO3. Assuming the formation of x protons and x bicarbonate ions, the equation above can be written as:

x2/ 1-x = 4.3 X 107%

x = 2.1 X 10-4 x = 2.1 X 10-4 x = 2.1 X 10-4

Now, the value of Ka2 in the second dissociation equation is 4.8 X 1011 mol/L. The equation is as follows:

HCO3-    <====>    CO3-2   +   H+

Again, Ka2 = ([CO3-2] [H+]) / [HCO3-]

The concentration of HCO3- is 2.1 X 10-4, as calculated by the above equation. Now, assuming that y protons and y CO3-2 ions are formed, plug the values into the above equation.

HCO3-    <====>    CO3-2   +   H+

Again, Ka2 = ([CO3-2] [H+]) / [HCO3-]

The effect of the second proton’s release is completely negligible in carbonic acid, and the value of pH is derived from the first equation, i.e. the value of Ka1.

As a result, pH = -log [H+]

= -log[2.1 X 10-4] = -log[2.1 X 10-4] = -log[2.1 X 10

equals 3.68

What exactly are acids?

An acid is a chemical that readily releases protons when dissolved in water. These are substances with a sour taste, a pH below 7, the ability to turn blue litmus red, and the ability to form a salt with alkalis.

Acids are classed as strong or weak based on the extent of their dissociation in an aqueous solution.

Strong acids are those that totally ionise and weak acids are those that do not entirely dissociate in aqueous solutions.

The properties of acids are believed to have been correctly defined by three theories.

Acids, according to the Bronsted-Lowry Theory, are substances that give away protons in an aqueous solution or are proton donors.

According to the Arrhenius Theory, acids produce hydrogen ions in water.

Acids are electron acceptors, according to Lewis Theory.

An acid is defined as a substance that has any or all of the qualities listed above.

Pairs of Conjugate Acid-Base

A substance that gives away its proton is an acid, while a substance that accepts the protons is a base, according to the Bronsted-Lowry theory of acids and bases.

The dissociation equation for carbonic acid, for example, is stated as follows:

Is H2CO3 a Strong Acid? – pH of H2CO3

Is H2CO3 a strong acid

Carbonic acid is a diprotic acid which is denoted by the formula H2CO3. It is a hydride of carbon dioxide and decomposes at temperatures above -80°C.

Being diprotic it is responsible for the formation of two types of salts viz. hydrogen carbonates (HCO3-) and carbonates (CO3-2).

Many of the students have questions about the acidity of H2CO3 (carbonic acid), whether it is a strong or a weak acid. 

So, In this article, we will study the acidity of H2CO3 and will let you know all the fundamentals.

So, Is carbonic acid a strong acid? No, H2CO3 is not a strong acid as it does not dissociate completely in an aqueous solution. The carbonic acid molecule has two hydrogen atoms to lose i.e. it is a diprotic acid and, therefore, has two acid dissociation constants, Ka. The value of the acid dissociation constant is the reflection of the strength of an acid. The higher value of Ka indicates the higher strength of the acid.

The reaction equations along with their Ka values are given below:

H2CO3(aq)    <=====>    HCO3-    +    H+         Ka1 = 4.3 X 10−7 mol/L; pKa1 = 6.36 at 25°C

HCO3-   <=====>     CO3-2   +    H+              Ka2 = 4.8 X 10−11 mol/L; pKa2 = 10.25 at 25°C

It is clear from the above equations that the value of the acid dissociation constant is very low while that of the logarithmic constant is higher for carbonic acid in comparison to strong acids.

In the case of the strong acids, the Ka value is expected to reach infinity while the value of pKa should lie below 2.

Hence, carbonic acid is not a strong acid. The pH of 0.1M carbonic acid is 3.68.

Contents  show 

Why is Carbonic Acid a Weak Acid?

As discussed in the above section carbonic acid at first ionizes into bicarbonate and proton when it is dissolved in an aqueous solution.

This means that the bicarbonate ion in this reaction works as the conjugate base for carbonic acid.

Also, it is well known that bicarbonate ion is a strong conjugate base and therefore, has the tendency to remain associated with its proton due to which the acid molecules only ionize partially in the aqueous solution.

Which one is more acidic H2CO3 or CH3COOH?

Both the carbonic acid as well as acetic acid are placed under the category of weak acids and they do not ionize completely in an aqueous solution.

However, the Ka value for carbonic acid is 4.3 X 10−7mol/L while that of acetic acid is 1.8 X 10-5 mol/L.

Also, the pKa value of carbonic acid is 6.36 for the first dissociation equation while it is 4.76 for acetic acid.

Is CH3COOH a strong acid? - Quora

These values indicate that acetic acid is stronger than carbonic acid.

Now let us try to understand the reason for the greater strength of acetic acid in comparison to carbonic acid. Start with writing the dissociation equation for both the acids:

For carbonic acid, H2CO3(aq)   <====>    HCO3-   +   H+

For acetic acid, CH3COOH(aq)   <====>   CH3COO-   +   H+

As per the above equations, the conjugate base for carbonic acid is bicarbonate ion while for acetic acid the acetate ion serves as the conjugate base.

Check out the detailed article I wrote on is Acetic acid a strong acid.

For now, we can leave the second dissociation equation of carbonic acid as the strength is measured only through the first proton released.

Now, we have to analyze the strength of these conjugate bases as a stronger conjugate base means a weaker acid and vice versa.

The acetate ion is a stronger conjugate base than the bicarbonate ion due to the resonance effect of the carboxyl ion.

Another factor is the instability of carbonic acid which mostly decomposes to water and carbon dioxide molecules and a very little amount of carbonic acid is left in the solution due to which the acidity declines automatically.

If you also want to know the chemical structure of Carbonic acid. read out the article on lewis structure of Carbonic acid.

The pH of Carbonic Acid

pH scale

The pH scale is the one used for expressing the acidity or basicity of a substance in an aqueous solution.

The values on the pH scale range between 1 and 14. All the acids have a pH value below 7 while all the bases have a pH value above 7.

The neutral solutions such as pure water have a pH value of 7.

Now, while calculating the pH value of carbonic acid we must keep in mind that it is a polyprotic acid i.e. it dissociates more than one time in a solution and thus releases different ions along with the release of protons.

Hence, the steps for calculating the pH of 0.1 M carbonic acid will differ from other simple acids.

First, let us start with the first dissociation equation resulting in the release of bicarbonate ions along with protons.

The chemical equation is given below:

H2CO3(aq)   <====>   HCO3-   +    H+

Now, calculate the hydrogen ion concentration of this equation knowing that the Ka1 value is 4.3 X 10−7 mol/L.

The equation is written below:

Ka = ([HCO3-] [H+]) / H2CO3

We are calculating pH for 0.1 M solution, therefore, the initial concentration of H2CO3 is taken as 0.1. Assuming that finally x protons and x bicarbonate ions are formed the above equation can be written as:

4.3 X 10^−7 = x^2/ 1-x

x = 2.1 X 10^-4

Now, looking at the second dissociation equation the value of Ka2 is 4.8 X 10−11 mol/L. The equation is written as:

HCO3-    <====>    CO3-2   +   H+

Again, Ka2 = ([CO3-2] [H+]) / [HCO3-]

As calculated in the above equation the concentration of HCO3- is 2.1 X 10-4. Now putting the values in the above equation assuming that y protons and y CO3-2 ions are formed,

4.8 X 10^−11 = (2.1 X 10^-4 + y)y / 2.1 X 10^-4 – y

y = 4.8 X 10^−11

In the case of carbonic acid, the effect of the release of the second proton is completely negligible and the value of pH is derived from the first equation i.e. the value of Ka1.

Therefore, pH = -log [H+]

= -log[2.1 X 10^-4]

= 3.68

What are Acids?

A substance that easily gives away protons when dissolved in water is known as an acid. These are the substances that are sour in taste, have pH below 7, turn blue litmus to red, and form a salt with alkalis.

Based upon the extent of their dissociation in an aqueous solution acids are further classified as strong and weak acids.

The acids that ionize completely are known as strong acids and those that do not dissociate completely in aqueous solutions are known as weak acids.

Three theories are trusted to have correctly defined the properties of acids.

Bronsted-Lowry Theory defines acids as the substance that gives away protons in an aqueous solution or is a proton donor.

Arrhenius Theory states that acids produce hydrogen ions in water.

Lewis Theory defines acids as electron acceptors.

If a substance is said to have any or all of the above-listed properties it can be called an acid.

Conjugate Acid-Base Pairs

Conjugate acids and bases and alkali's - Acids and Bases

As per the Bronsted-Lowry theory of acids and bases, a substance that gives away its proton is an acid while the substance that accepts the protons is a base.

For example, in the case of carbonic acid the dissociation equation is written as follows:

H2CO3   +   H2O   <====>   HCO3-  (conjugate base)   +   H3O+ (conjugate acid)

In this reaction, H2CO3 gives up a proton and thus acts as an acid, whereas water accepts a proton and thus acts as a base.

However, according to the theory, a conjugate base and conjugate acid corresponding to the acid and base involved as reactants are formed as a result of this proton loss and gain.

In the example above, HCO3- is the carbonic acid’s conjugate base, whereas H3O+ is the water molecule’s conjugate acid.

A weak acid is indicated by a strong conjugate base, and vice versa. A strong conjugate acid, on the other hand, indicates a weak base, and vice versa. As a result, these can be used to estimate the strength of acids and bases.

What Causes Carbonic Acid to Form?

Carbonic acid is created when carbon dioxide is dissolved in water.

Carbon dioxide molecules act as an anhydride of carbonic acid, accepting a water molecule readily to produce carbonic acid.

The following is the reaction equation:

CO2   +   H2O    <====>    H2CO3

Because it is unstable and weak, it dissociates easily into bicarbonate ions and hydrogen.

Is carbonic acid a natural or synthetic acid?

Organic compounds are defined as those that contain carbon and hydrogen atoms. The characteristics and reactivity of these compounds are studied in a specific discipline of chemistry.

Despite the fact that carbonic acid contains both carbon and hydrogen, it is not an organic compound.

Carbonic acid, like carbon dioxide, is an inorganic or mineral acid.

The structure of the carbonic acid molecule, which consists of one carbon-oxygen double bond and two carbon-oxygen single bonds, explains why this behaviour occurs.

Carbonic Acid’s Characteristics

Carbonic acid is a dibasic or diprotic acid that is unstable and weak. It can also be found in the human body naturally.

The table below lists a few of carbonic acid’s most important properties:

Chemical NameCarbonic Acid
AppearanceGreyish-white
Chemical FormulaH2CO3
Molecular mass62.024 gm/mol
Melting Point210 °C
Boiling Point-78 °C
Density1.668 gm/cm3
OdorOdorless
Ka14.3 X 10−7 mol/L
pKa16.35
ToxicityNon-toxic

Carbonic Acid’s Applications

The following are some of the most important applications of carbonic acid:

• It’s a component of salt production.

• The conjugate base of carbonic acid, the bicarbonate ion, is used in human bodies to carry CO2 outside via respiratory exchange.

• Helps in the protonation of nitrogen bases in blood serum.

• It also works as a buffer in our bodies and is broken down into carbon dioxide by the enzyme carbonic anhydrase.

• Aerated cocktails, sparkling wine, and carbonated water are all made with it.

• It can also be found in mouthwash and vaginal washes.

• For contact lenses, carbonic acid is utilised as a cleaning.

• It’s utilised in the cosmetics and food processing sectors, among other things.

• It aids in the digestion of starch.

Conclusion

Carbonic acid is a weak acid because it does not entirely dissociate in water.

Carbonic acid has a high acid dissociation constant and a low logarithmic constant, indicating that it is a weak acid.

Because the conjugate base of H2CO3 is more stable than that of carbonic acid and acetic acid, it is weaker. It also has a proclivity towards reforming its constituent compounds, such as water and carbon dioxide.

Carbonic acid’s conjugate base is HCO3-.

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Good luck with your studies!!

Read more: Diagram, Steps To Draw The Chlorine Bohr Model

Misha Khatri
Misha Khatri is an emeritus professor in the University of Notre Dame's Department of Chemistry and Biochemistry. He graduated from Northern Illinois University with a BSc in Chemistry and Mathematics and a PhD in Physical Analytical Chemistry from the University of Utah.

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