Weak acids as the name suggests are the acid with less acidic characteristics i.e., less concentration of Hydrogen ions. Weak acids are much more useful than strong acids in our daily life, as strong acids are corrosive to touch. All the daily activities involved using of weak acids, from food to cosmetics, and pharmaceuticals to farming, use of weak acids can be seen extensively. In this article, we will learn about weak acids, various examples, characteristics, and their use in various daily activities. Other than that, we will also learn about the acid dissociative constant and pH of weak acids. So, let’s start our journey into the world of acetic acid.

Weak Acids

By designating a proton to another molecule, acids react with water to form H⁺ ions. They contain hydrogen, which when dissolved in water forms both an anion and a hydrogen ion. The nature of these hydrogen ions is one of extreme reactivity. A blue litmus paper will change color when dipped into an acidic solution to become a red one. Additionally, acids have a pH (power of hydrogen) value lower than 7, and their flavor is sour.

According to the definition, weak acids are those that do not completely release all of their hydrogen ions into the solution and have low values for K_a (a measure of an acid’s strength) in comparison to strong acids.

The process, known as a neutralization reaction, occurs when acids and bases react to produce salt and water. Weak acids are those whose solutions do not entirely ionize them. They frequently do not entirely separate into their component ions in the solution. Strong acids are those that totally break down into their ions in the solution and dissolve entirely.

Examples of Weak Acids

Some examples of weak acids are:

• Ethanoic or Acetic Acid (CH₃COOH)
• Formic Acid (HCOOH)
• Benzoic Acid (C₆H₅COOH)
• Oxalic Acid (C₂H₂O₄)
• Sulfurous Acid (H₂SO₃)

Characteristics of Weak Acid

Some of the key characteristics of weak acids are:

• Ionization: Weak acids ionize only partially when dissolved in water. They do not completely dissociate into ions. Instead, they establish an equilibrium between the undissociated acid molecules and the dissociated ions.
• Weak electrolyte: Since weak acids only partially dissociate, they are considered weak electrolytes. This means that they conduct electricity to a lesser extent compared to strong acids.
• Dilute solutions: Weak acids are typically used in dilute solutions because their ionization is more significant at lower concentrations. As the concentration increases, the extent of ionization decreases.
• pH: The pH of a solution containing a weak acid is usually higher than the pH of a solution containing a strong acid of the same concentration. This is because weak acids produce fewer hydrogen ions (H+) when they ionize.
• Acid dissociation constant (Ka): Weak acids have a small acid dissociation constant (Ka) compared to strong acids. Ka represents the degree of ionization of an acid in water. A smaller Ka value indicates less ionization.
• Buffering capacity: Weak acids play a significant role in buffering solutions. They can resist large changes in pH when small amounts of acid or base are added. This is due to the reversible nature of their ionization.

Acetic Acid

Acetic acid, often known as Ethanoic acid, has the chemical formula CH₃COOH (CH₃CO₂H, C₂H₄O₂, or HC₂H₃O₂). This is a type of carboxylic acid as well, the second most basic type in which methane is joined to the COOH group. Acetic acid’s chemical structure is as follows:

 

After water, acetic acid makes up the majority of vinegar and makes up 4 to 7% of the total volume of the solution in water. Acetic acid is the main component of vinegar, which is diluted in water and is most likely created through fermentation and further oxidation with ethanol. Since acetic acid in its concentrated form can damage human skin, it should be handled carefully and away from direct contact.

In order to create cellulose acetate, an essential chemical reagent called acetic acid is also a common industrial ingredient used in photographic film. This acid also aids in the manufacturing of synthetic fibers, polyvinyl acetate for wood glue, and other fibers. Acetic acid is a weak acid because, when dissolved in water, it partially dissociates into its component parts. Under normal conditions of pressure and temperature, acetic acid has a smell similar to vinegar and a molecular mass of 60.052 grams per mole. Acetic acid has a density of 1.27 grams per cubic cm in its solid form compared to 1.049 grams per cubic cm in its liquid form. It is well known that hydrogen bonding exists in the acetic acid solid state.

Acetic acid has a melting point of 16 and 17 degrees Celsius and a boiling point of 118 degrees Celsius. It produces miscible mixes when it is a byproduct of water-based combinations. The acid’s pKa value is 4.756. Methanol is used in one of the procedures used to make acetic acid.

Dissociation of Acetic Acid

Acetic Acid is dissociated into acetate ion (CH₃CO₂⁻) and hydrogen ion(H⁺) as follows:

CH₃COOH ⇔ CH₃CO₂⁻ + H⁺

Formic Acid

The most basic type of carboxylic acid is formic acid, also referred to as methanoic acid. Formic acid has the chemical formula HCOOH (CH₂O₂). The following diagram depicts formic acid’s structure:

 

Ants produce formic acid. Keep in mind how it feels to get bitten by an ant. Right, it hurts like it’s burning there. The acid from the ant’s body penetrates our body, causing pain. One of the most significant weak acids is regarded to be formic acid. Under normal temperature and pressure, it appears to be a fuming, white liquid. Additionally, it has an unpleasant smell that is both potent and penetratingly pungent. Formic acid is frequently used to treat leather and textiles, which is one of its common applications or uses. Formic acid can be produced in the form of its esters in addition to being a naturally occurring substance in ant bodies. Methyl alcohol and carbon monoxide combine when a catalyst is present.

Here we cover some of the fundamental characteristics of formic acids. This acid has a molar mass of 46.03 grams and a density of 1.22 grams per milliliter. Formic acid freezes at 8.4 degrees Celsius while boiling at 100.3 degrees Celsius. Given that it has a pKa value of 3.745, it can be easily dissolved in water.

Additionally, formic acid and other organic solvents like acetone are miscible. The mixture of glycerol, ethanol, and methanol may also be only partially soluble in other aromatic substances, such as benzene and toluene. In hydrocarbons that form hydrogen-bonded dimers rather than as individual molecules, this acid is slightly miscible. The ideal gas law is broken by formic acid.

Benzoic Acid

With the molecular formula C₆H₅COOH, benzoic acid is the most basic aromatic carboxylic acid. The acid is also referred to as Benzene Carboxylic Acid and Carboxy Benzene, as you must be aware. This is a weak acid that is easily found in nature in plants and gum benzoin. Its salts are widely employed in the food industry as preservatives. Under normal temperature and pressure, benzoic acid exists as a crystalline solid that is either colorless or white and is not highly soluble in water. Because benzoic acid has an aromatic structure, it has a somewhat pleasant scent. The structure of benzoic acid is:

 

Under normal temperature and pressure conditions, benzoic acid has a molar mass of 122.123 grams and a density of 1.26 grams per cubic cm. This acid’s boiling point is 250 degrees Celsius, whereas its melting point is 122 degrees Celsius.

As the temperature rises, benzoic acid becomes more soluble in water. The solubility of benzoic acid in water is 3.44 grams at a temperature of 25 degrees Celsius; however, if the temperature is raised to 100 degrees Celsius, the solubility of benzoic acid in water increases dramatically to 56.31 grams per liter. Regarding solubility, we must point out that this acid is soluble in a few organic solvents, including benzene, acetone, carbon tetrachloride, and hexane.

Oxalic Acid

With the chemical formula C₂H₂O₄, oxalic acid is the most basic type of dicarboxylic acid and is regarded as a weak acid because it does not separate into its component parts when dissolved in water. Oxalic acid has substantially higher acidity as compared to acetic acid. The oxalic acid’s chemical structure:

 

The molar mass of oxalic acid is 90.03 grams per mole; however, the molar mass of the dihydrated form of the same acid, at standard pressure and temperature, is 126.06 grams per mole. The mass density of oxalic acid in its anhydrous state, under identical circumstances, is 1.9 grams per cm cube. This acid has a melting point of 190 degrees Celsius and a boiling point of 149 to 160 degrees Celsius.

It is completely soluble in water with regard to solubility. Its structure contains hydrogen bonds and it functions as a reducing agent.

Acid Dissociation Constant (Ka) and pKa

The acid dissociation constant, commonly denoted as K_a, is a quantitative measure of the strength of an acid in solution. It expresses the degree to which an acid dissociates or ionizes into its conjugate base and hydrogen ions in water.

The general equation of the dissociation reaction of the weak acid HA can be represented as follows:

HA ⇌ H+ + A-

The acid dissociation constant, K_a, is defined as the ratio of the concentrations of the products (H⁺ and A_–) to the concentration of the undissociated acid (HA) at equilibrium. Mathematically, it is expressed as:

Ka = [H⁺][A^–]/[HA]

Where [H+], [A-], and [HA] represent the concentrations of hydrogen ions, conjugate base, and a weak acid, respectively.

The acid dissociation constant is related to the pK_a value, which is the negative logarithm (base 10) of the K_a. The pK_a is often used to express the acidity of a compound on a logarithmic scale. The relationship between Ka and pKa can be expressed as:

pKa = -log10(Ka)

pH of Weak Acids

Let’s consider the concentration of weak acid (HA) before dissociation to be C₀, and α to be the degree of dissociation i.e., the fraction of weak acid molecules that have dissociated. Thus, after the dissociation of a weak acid, we can write:

[HA] = (1 – α)C₀, [H+] = αC₀, and [A-] = αC₀

Substituting these values into the equilibrium constant expression, we have:

K_a = (αC₀)(αC₀)/[(1 – α)C₀]

⇒ K_a = α²C₀/(1 – α)

⇒ α² + (K_a/C₀)α – (K_a/C₀) = 0

From this equation, we can calculate the value of α, and using α we can calculate the concentration of hydrogen ions [H+] using [H+] = αC₀. Thus, the pH of a weak acid can be calculated as:

pH = -log[H+]

Let’s consider an example of dissociation of acetic acid i.e.,

CH₃COOH ⇔ CH₃COO⁻ + H⁺

	CH3COOH	CH3COO–	H+
Moles	1-α	α	α
Concentration	[(1-α)/v]	[α/v]	[α/v]

K_a = [H⁺]×[CH₃COO^–] / [CH₃COOH]

∴ K_a = (α/v × α/v) / ((1-α)/v)

∴ K_a = α² / v(1-α)

∴ K_a = α²c / (1-α)  …(c = 1/v)

Very weak acid, α <<<<< 1

(1-α) ≈ 1

So, K_a = α²c

∴ α = √K_a/c

Degree of Dissociation.

Concentration of [H⁺] = αc = √K_a/c × c = √K_ac 

pH = 1/2[pK_a – log₁₀c]

Where, 

K_a is dissociation constant of weak acid
c is concentration of solution

Uses of Weak Acids

Uses of some weak acids are as follows:

• pH control: Weak acids are frequently used in industries and laboratories to control the pH of solutions. Their ability to partially ionize makes them suitable for adjusting and maintaining specific pH levels in various processes.
• Food and beverage industry: Weak acids such as acetic acid (vinegar) and citric acid are widely used in the food and beverage industry as flavor enhancers, preservatives, and acidifiers. They provide a tart taste and help inhibit the growth of bacteria and other microorganisms.
• Household cleaning products: Weak acids like acetic acid (found in vinegar) are commonly used in household cleaning products due to their mild acidity. They are effective in removing mineral deposits, soap scum, and stains from various surfaces.
• Analytical chemistry: Weak acids are essential in analytical chemistry for various purposes. They are used as titrants in acid-base titrations to determine the concentration of bases. Weak acid solutions can also be used as reference standards to calibrate pH meters.

Weak Acid vs Strong Acid

The differences between weak and strong acids are discussed in the table below, 

Keep in Mind

• If the K_a and concentration of the solution are known, it is possible to compute the pH of any solution.
• The calculation methods used to determine pH might vary and are predicated on assumptions.
• Making buffer solutions requires the use of weak acids.
• Weak acids have a sour taste, are sticky in consistency, and can cause nose burns when smelled.
• Strong concentrations of weak acids can be destructive and harmful.

Read More,

• Acids, Bases, and Salts
• Importance of pH in Everyday Life
• Ionization of Acids and Bases

Solved Examples of Weak Acids

Example 1: KOH_(aq) + HCN_(aq) ⇔ H₂O_(l) + KCN_(aq) According to the given chemical equation, KOH, and HCN react in an aqueous solution. What was the concentration of the initial HCN solution if 36 mL of a KCN solution required 32.9 mL of a 0.21 M KOH solution to titrate?

Solution:

KOH_(aq) + HCN_(aq) ⇔ H₂O_(l) + KCN_(aq)

K_a = [KOH(aq)]×[KCN_(aq)] / [HCN_(aq)]

∴ K_a = 32.9 × 0.21 / 36

∴ K_a = 6.909 / 36

∴ K_a = 0.19 M

Example 2: HF_(aq) + H₂O(l) ⇔ H₃O⁺_(aq) + F^–_(aq) According to the given chemical equation, the concentration of H₃O⁺ solution is 21.8 mL, F^– the solution is 1.23 M and HF solution concentration is 38 mL. What is the value of K_a?

Answer:

HF_(aq) + H₂O_(l) ⇔ H₃O⁺_(aq) + F^–_(aq)

K_a = [H₃O⁺_(aq)]×[F^–_(aq)] / [HF_(aq)]

∴ K_a = 21.8 × 1.28 / 38

∴ K_a = 27.904 / 38

∴ K_a = 0.73 M

Example 3: CH₃COOH_(aq) + H₂O_(l) ⇔ H₃O⁺_(aq) + CH₃COO⁻_(aq) according to the given chemical equation, concentration of H₃O⁺ solution is 31.1 mL, CH₃COO⁻ solution is 2 M and CH₃COOH solution concentration is 29.2 mL . Then Find the K_a.

Answer:

CH₃COOH_(aq) + H₂O_(l) ⇔ H₃O⁺_(aq) + CH₃COO⁻_(aq)

K_a = [H₃O⁺]×[CH₃COO⁻] / [CH₃COOH]

∴ K_a = 31.1 × 2 / 29.2

∴ K_a = 62.2 / 29.2

∴ K_a = 2.13 M

Example 4: B + H₂O ⇔ BH⁺ + OH^–, BH⁺ solution concentration is 20.1 mL, OH^– solution concentration is 3.7 M, and B solution concentration is 29.2 mL, all in accordance with the chemical equation provided. Calculate K_a.

Answer:

B + H₂O ⇔ BH⁺ + OH^–

K_a = [BH⁺]×[OH⁻] / [B]

∴ K_a = 20.1 × 3.7 / 29.2

∴ K_a = 74.37 / 29.2

∴ K_a = 2.54 M

FAQs on Weak Acid

Q1: Define Weak Acid.

Answer:

 A weak acid is an acid that partially dissociates in water, meaning it releases only a small fraction of its hydrogen ions (H+) when dissolved in water.

Q2: How do Weak Acids differ from Strong Acids?

Answer:

The key difference between both weak and strong acids is degree of dissociation. Strong acids dissociate in water completely which results in a higher concentration of hydrogen ion in water, whereas weak acids only partially dissociate in water.

Q4: What are Some Examples of Weak Acids?

Answer:

Some examples of weak acids are acetic acid (CH₃COOH), citric acid (C₆H₈O₇), formic acid (HCOOH), and carbonic acid (H₂CO₃).

Q5: Can Weak Acids Fully Ionize in Water?

Answer:

No, weak acids do not fully ionize in water. They only partially dissociate, meaning only a small fraction of the acid molecules release hydrogen ions.

Q6: Can Weak Acids be Dangerous?

Answer:

While weak acids generally have a lower degree of acidity compared to strong acids, they can still be corrosive or harmful in concentrated forms or at high concentrations.

Q7: Which Acid is thought to be the Least Powerful Acid?

Answer:

In addition to the fact that there are numerous other weak acids, hydrocyanic acid is regarded as the weakest acid. Its p_Ka value is4.9 × 10^-10.