Acid strength

Introduction :

We have learnt about acids and bases and their chemical properties from an early age. Some simple properties are – Acids have a pH below 7; when performing a litmus test, it turns blue litmus to red, they are sour to taste, and they can react with alkalis and form salts. The most interesting aspect of acids is acidic strength. This article will elaborate upon the concept of acidic strength and the factors affecting acidic strength.

What is Acidic Strength?

Acidic strength measures an acid’s ability to lose its H+ ion. A weak organic acid’s strength depends upon the substituent effects. An inorganic acid’s strength depends upon the atom’s oxidation state to which the proton attaches itself. Acidic strength is also always solvent-dependent. For example, hydrogen chloride is a strong acid in an aqueous solution but a weak acid when dissolved in glacial acetic acid.

 Strong Acids

Strong Acids are the acids that completely ionizes in any solution. Strong acids are corrosive and can cause severe burns when they contact skin. Examples are hydrochloric acid and sulphuric acid.

They dissociate according to the following reaction:

HA + S ⇌SH⁺+A⁻

For example, HCl→H⁺ + Cl⁻

S here represents a solvent molecule, for instance, a molecule of water. This is the leveling effect, and it results from the high buffer capacity of solutions having a pH value of 1 or less than 1.

Weak Acids

A weak acid only partially ionizes in a solution. Weak acids are present in our food and body and are only mildly corrosive. A few examples of weak acids are citric acid, ethanoic acid and acetic acid. Acetic acid is found in vinegar, whilst citric acid is naturally present in fruits.

Weak acids partially dissociate when dissolved in a solvent.

HA ⇌ H⁺ + A⁻

Here, water, which is solvent-like, omits this expression when its concentration becomes finely unchanged by acid dissociation’s process. A weak acid’s strength can be measured in terms of a dissociation constant.

Kₐ = [H⁺][A⁻][HA]

Factors that determine Acidic Strength 

Different acids possess different strengths. Any acid possessing a greater degree of dissociation behaves as a strong acid. Two factors determine Acidic strength:

• The inductive effect: Due to Negative inductive effect shown by substituent groups with higher electronegativity present in the carboxylic acid increases its acidic strength.
• Effect of oxidation state: In a set of oxoacids of an element, the pKa  value decreases with the decrease of the element’s oxidation state.

Both these factors make the dissociation of acid molecules into H⁺ and A⁻ easier, thereby increasing the acidity. 

The acid strength order

The order of acidic strength denotes that when comparing the elements in the same group of the periodic table, the strength of the H-A bond is an essential factor when deciding the acidity more than the polarity of the acid. As A’s size increases on a descending of a group, the H-A bond strength decreases, and the acidic strength increases.

Factors Affecting Acidic Strength

There are many factors affecting acidic strength:

1. Acidic strength is dependent on the H and A bond’s strength. The weaker the bond is, the less energy will be required to break it. Thus, the acid becomes strong.
2. The H and A bond’s polarity affects its strength. If the existing bond between them is highly polar, the proton may leave the molecule easily by making it a strong acid.
3. Bond strength is an essential factor affecting acidic strength. It is important when considering and comparing the acid strength of elements of the same group of the periodic table while using the above two factors.
4. When comparing the acidic strengths of elements in the same rows, the polarity of the H and A bond is given priority.
5. The atomic size of A affects the strength of acid and base. As the atom gets larger, the bond starts becoming weaker. Hence, increasing acidic strength.
6. Inorganic carboxylic acids are an electronegative substituent which can easily pull the electron density out from acidic bonds through the inductive effect.

Acids in non-aqueous solvents

The strength of acids varies from solvent to solvent. Any acid that is strong in water might be weak in a less basic solvent. And any acid that is weak in water, might be stronger in a more basic solvent. 

Conclusion

Acid strength is the tendency of acids to dissociate into protons and anions. Based on its strength, an acid or base is classified as strong or weak. Organic acids may occur naturally in food and our body. Strong acids are highly corrosive. The acid strength is affected by two factors; the inductive effect and the effect of the oxidation state. Various solvents affect the strengths of acids. Acids strong in water might be weak in less basic solvents. Whereas any acid weak in water might be stronger in more basic solvents. This article highlights the concepts of acid strength and the factors affecting acid strength.