Acids, Bases and Salts

Acid Strength

Acid strength is the measure of the ability of the acid to lose its \(H^+\) ion. The dissociation of a strong acid in solution is finely complete, omitting in its most concentrated solutions. Examples of strong acids are hydrochloric acid, perchloric acid, nitric acid,  and sulfuric acid.

A weak acid partially dissociates with both the undissociated acid and its dissociation products in the solution, in equilibrium to each other. For example, acetic acid. The strength of a weak organic acid depends on substituent effects. Also, the strength of an inorganic acid depends on the oxidation state of the atom to which the proton might attach. Remember that acid strength is solvent dependent. For instance, in aqueous solution hydrogen chloride i.e., HCl is a strong acid, but when it is dissolved in glacial acetic acid is a weak acid.

Acid strength

Factors Affecting Acid Strength

  • Acid strength depends on the strength of the H and A bond. The weaker the bond, the lesser the energy that will be required to break it. Thus, the acid is strong.
  • The polarity of the H and A bond affects its strength. If the bond between them is highly polar, the proton tends to leave the molecule easily by making it a strong acid.
  • The bond strength is important if we consider and compare acid strengths of elements in the same group of the periodic table by using the above two factors.
  • On comparing the acid strengths of elements in the same row, priority is the polarity of the H and A bond.
  • The atomic size of A affects the acid strength. As the atom gets larger, the bond becomes weaker. Hence, acid strength increases.
  • Inorganic carboxylic acids, an electronegative substituent can easily pull electron density out from an acidic bond through the inductive effect. It results in a smaller \(pK_a\) value.

Strong Acid

A strong acid is an acid that dissociates according to the reaction is below:

\(HA + S \rightleftharpoons  SH^{+} + A^{-}\)

For example: \(HCl\rightarrow H^+ + Cl^-\)

here S represents a solvent molecule like a molecule of water. This results from the high buffer capacity of the solutions with a pH value of 1 or less than 1 and is the levelling effect.

Weak Acid

A weak acid is a substance that partially dissociates when dissolved in a solvent.

\(HA\rightleftharpoons H^+ + A^-\)

The solvent like water omits from this expression when its concentration is finely unchanged by the process of acid dissociation. The strength of a weak acid can be measured in terms of a dissociation constant.

\(K_{a}=\frac{\left[H^{+}\right]\left[A^{-}\right]}{[H A]}\)

FAQs on Acid Strength

Question 1: What makes an acid strong or weak?

Answer: Strong acids have ions in solution, so bonds holding H and A together must be weak. Strong acids split easily into ions. Weak acids exist as molecules with ions in solution. Thus, the bonds that bind H and A together must be solid.

Question 2: What are the seven-strong acids?

Answer: Seven strong acids that exist are chloric acid, hydrobromic acid, hydrochloric acid, hydroiodic acid, nitric acid, perchloric acid, and sulphuric acid. Though being a part of the list of strong acids does not provide any information on how dangerous or harmful acid is.

Question 3: What is the most powerful acid?

Answer: Carborane superacids is the strongest solar acid because fluoroantimonic acid is a mixture of hydrofluoric acid and pentafluoride antimony and carborane comes with a pH value of -18.

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