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Difference Between Strong and Weak Acid

Acids are chemical substances that donate hydrogen ions or protons when mixed in solutions. The number of protons given off by a particular acid actually determines the strength of the acid – whether it is a strong acid or a weak acid. In order to understand the strength of the acids, one need to compare their tendency to donate protons to the similar base (mostly water). The strength is denoted by a number called pKA.


What is a Strong acid?

An acid is said to be strong if it dissociates or ionizes completely in a solution. That means, it is able to give the largest number of H+ ions or protons when mixed in a solution.  These ions are the charged particles. Since a strong acid donates a greater number of ions as it breaks down, or ionizes, that means a strong acid is a conductor of electricity.

When an acid mixes in H2O, a proton (H+ ion) is carried to a H2O molecule to generate a H3O+ (Hydroxonium ion) and a – ion based on which acid is involved to begin with.

In a general scenario,

Such chemical reactions can be revered, but in few cases, the acid is gives away H+ ion quite easily and the reaction looks like being one-way. And the acid is complete dissociated.

For e.g., when hydrogen chloride dissolves in H2O to make HCl, so little of the reverse reaction happens that we can write:

At one time, hundred percent virtual reaction will take place wherein hydrogen chloride will show reaction with H3O+ (Hydroxonium ion) and Cl ions. Here, the strong acid is Hydrogen Chloride.


What is a Weak acid?

An acid is said to be weak if it ionizes partially or incompletely, giving off only some of its hydrogen atoms into the solution. Hence, it is less capable as compared to a strong acid in giving off protons. Weak acids have higher pKa than strong acids.

Ethanoic acid is a good example of a weak acid. It shows reaction with H2O for producing H3O+ (Hydroxonium ions) and CH3COOH (ethanoate ions), but the reverse reaction shows more success than the forward one. The molecules react quite easily to ameliorate the acid and the H2O.

At any one time, only about one percent of the CH3COOH acid molecules show conversion into ions. Whatever is left is the simple acetic acid (systematically called ethanoic acid) molecules.


Difference between Strong acid and Weak acid

  1. Definition

Strong acid

A strong acid is an acid that ionizes completely in an aqueous solution. A strong acid will always loose a proton (A H+) when dissolved in H2O. In other words, a strong acid is always on its toes and quite efficient in giving off protons.

Weak acid

A weak acid is one that ionizes partially in a solution. It gives off only few of its hydrogen atoms in to the solution. Hence it is less capable than a strong acid.

  1. Electrical conductivity

Strong acid

Strong acids will always show strong conductivity. Strong acids usually pass more current as compared to the weak acids for the same voltage and concentration.

Weak acid

Weak acids have a low conductivity. They are poor conductors and show a low value for current passing

  1. Rate of reaction

Strong acid

Rate of reaction is faster in the strong acids

Weak acid

Rate of reaction is slower in weak acids

  1. Examples

Strong acid

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

Weak acid

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

  1. pH

Strong acid

In a strong acid, the pH is lower than, generally 3. Strong acids possess a very high concentration of H+ ions (an acid having a pH of 3 has 0.001 moles per liter of Hydrogen ions).

Weak acid

A weak acid has a pH ranging between 3-7.

  1. Value of pKa

Strong acid

In a strong acid, the value of pKa is quite low.

Weak acid

In a weak acid, the value of pKa is quite high.

  1. Dissociation

Strong acid

HCl(g) + H2O(l)  ≈  H3O+(aq) + Cl(aq)

Weak acid

CH3COOH(l) + H2O(l) ≈  H3O+(aq) + CH3COO(aq)

Summary of Strong acid Vs. Weak acid

The points of difference between Strong and Weak Acids have been summarized below: Comparison chart

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References :

[0]Kelly, R. M., & Akaygun, S. (2016). Insights into how students learn the difference between a weak acid and a strong acid from cartoon tutorials employing visualizations. Journal of Chemical Education, 93(6), 1010-1019.

[1]Ring, T., & Kellum, J. A. (2016). Strong Relationships in Acid-Base Chemistry–Modeling Protons Based on Predictable Concentrations of Strong Ions, Total Weak Acid Concentrations, and pCO2. PloS one, 11(9), e0162872.

[2]Image credit: https://upload.wikimedia.org/wikipedia/commons/1/15/Weak_acid_pfe.png

[3]Image credit: https://upload.wikimedia.org/wikipedia/commons/7/71/Weak_acid_speciation.png

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