Strength of Acids

Acids are a molecule or other species which can donate a proton or accept an electron pair in reactions. When acids react with H₂O, they create hydrogen ions; the strength of an acid is determined by the concentration of hydrogen ions in a solution. A higher number of hydrogen ions indicates that the acid is stronger, whereas a smaller number of hydrogen ions indicates that the acid is weak. They are categorized as Strong and Weak Acids.

In chemistry, strong and weak acids are important concepts to understand. In water, strong acids totally break down into their ions, whereas weak acids only partially dissociate. There are just a few strong acids, but there are a lot of weak acids.

The ease with which acids dissolve to create ions determines whether they are strong or weak. Acids dissolve in water to produce hydrogen ions, while bases dissolve to form hydroxide ions. Strong acid and base ions quickly dissociate to entirely dissolve in water, creating H hydrogen ions with a positive charge or OH^– hydroxide ions with a negative charge. Weak acids and bases dissociate only partly, leaving fewer ions in solution. The hydrogen ions in acids and the hydroxide ions in bases are responsible for the properties and strength of acids and bases.

What is a Strong Acid?

Strong acids totally breakdown into their ions in water, producing one or more protons or hydrogen cations per molecule. Inorganic or mineral acids are often strong acids.

When exposed to water, strong acids fully break down into their ions. In other words, acids ionise completely and release H⁺ ions into the solution. The acid dissociation constant values define an acid’s strength (K_a). Strong acids often have a very high K_a value.

The more powerful the acid, the more readily it loses protons. Because of the strong polarity of the connection between the H atom and the remainder of the molecule, these protons are easily released. The electronegativity of the two atoms engaged in this connection determines its polarity. The polarity and size of the anion to which the proton is connected determine the deprotonation (removal of a proton) of a strong acid. 

For example, if the acid H-A is considered, the dissociation of HA acid may be written as,

HA (aq) + H₂O (l)  ⇢  A (aq) + H₃O⁺ (aq)

However, if the acid molecule has more than one proton that may be released, it can be represented as illustrated below. The dissociation of a diprotic acid is demonstrated in the example below. This indicates it has the ability to emit two protons.

H₂B (aq) + H₂O (l)  ⇢  B₂ (aq) + H₃O⁺ (aq)

Strong acids, on the other hand, have a large impact on the pH of the solution because they release H⁺ ions into it. The pH is determined by the concentration of H⁺. The connection between H⁺ concentration and pH is shown below.

pH = -log [H⁺ (aq)]

If the acid is really powerful, the pH value is quite low. For example, if a strong monoprotic acid is dissolved in water at a concentration of 0.1 molL-1, the pH of the solution would be,

pH = -log [H⁺ (aq)]

      = -log [0.1 mol/L]

      = 1

Examples of Strong Acids:

• Chloric acid (HClO₃)
• Hydrochloric acid (HCl)
• Sulfuric acid (H₂SO₄)
• Hydroiodic acid (HI)
• Hydrobromic acid (HBr)
• Perchloric acid (HClO₄)
• Nitric acid (HNO₃)

What is a Weak Acid?

There are numerous weak acids, but only a few strong acids. 

Weak acids partially dissociate in water, resulting in an equilibrium state containing the weak acid and its ions. Hydrofluoric acid (HF), for example, is classified as a weak acid because some HF, in addition to H⁺ and F^– ions, persists in an aqueous solution.

In aqueous solutions, weak acids are compounds that partly dissolve into ions. Weak acids do not completely release all of the H+ ions into the solution. The acid dissociation constant (Ka) is lower than in strong acids. The pH of the solution is around 3-5. This is due to the fact that a weak acid does not boost the H⁺ content of a solution in the same way that a strong acid does. There are H⁺ ions, the anion of the molecule, and the weak acid molecule present in a weak acid in a water system.

When describing the dissociation of weak acids, twin arrows should be used instead of a single arrow. This is done to demonstrate that the reaction is in equilibrium.

For example, the dissociation of ethanoic acid, which forms the hydronium cation and ethanoate anion:

CH₃COOH + H₂O  ⇆  H₃O⁺ + CH₃COO^–

Examples of Weak Acids:

• Nitrous acid (HNO₂)
• Sulfurous acid (H₂SO₃)
• Hydrofluoric acid (HF)
• Formic acid (HCOOH)
• Phosphoric acid (H₃PO₄)
• Benzoic acid (C₆H₅COOH)
• Acetic acid (CH₃COOH)

Sample Questions

Question 1: Name the only mineral acid that is a weak acid.

Answer:

Carbonic Acid (H₂CO₃) is the only mineral acid that is a weak acid.

Question 2: Define the degree of ionization.

The degree of ionization is the ratio or percentage that gives tells the number of neutral molecules of a compound breaks into ions when dissolved into water.

Question 3: Explain how does sulfuric acid forms H₃O⁺.

Answer:

When sulfuric acid is dissolved into water. It gets ionized into H+ and SO_42- ions as shown below:

H₂SO₄ ⇄ H⁺ + SO_42-

Now the H+ ions cannot exist independently and then combine with water molecules forming H₃O⁺ ions.  

 H⁺  + H₂O → H₃O⁺

This is how sulfuric acid forms H3O+ ions when dissolved into water.

Question 4: How to distinguish an acidic compound from a basic and neutral compound using the pH scale?

Answer:

We can distinguish between the acidic, basic and neutral compounds using the value of the pH scale as:

• If the value of pH is below 7, then the compound is acidic.
• If the value of pH is equal to 7, then the compound is neutral.
• If the value of pH is greater than 7, then the compound is basic.

Question 5: What is a pH scale?

Answer:

The pH scale is the scale used to measure the acidity or basicity of a solution. It has a value from the range of 0 to 14. If the value of the pH scale is 7, then the solution is neutral.