Acidic Strength and Factors Affecting It

Acids are substances whose pH value is below 7. These substances turn litmus paper red. They also have a sour taste and react with alkalis to form a salt. 

When dissolved in water, the strength of an acid or a base is determined by its ionization ability. Acidic strength refers to an acid’s ability to lose its H+ ion. The strength of the H-A bond, rather than polarity, is the primary influencing factor when comparing elements in the same periodic table group.

Except for its most concentrated solutions, a strong acid dissociates completely in the solution. When an acid is weak, it can only be partially dissociated. Titration is usually used to determine the acid dissociation constant (Ka) of acid to determine its strength. 

The logarithmic constant of strong acids is smaller than the one of weaker acids. Strong acids also have a larger Ka. An acid loses a proton more easily if it is stronger. Among the most critical factors in influencing deprotonation are polarity in the H-A bond and ‘A’ atom size, as both affect the strength of the H-A bond. A conjugate base’s stability also affects acid strength.

An acidic solute’s PKa value measures its ability to transfer a proton to a standard solvent; how well an acidic solvent transfers a proton to a reference solute is measured by the Hammett acidity function. PKa and H0 values consist of measures of distinct characteristics which may diverge even when they are both associated with a similar tendency to donate protons.

In weak organic acids, substituent effects may influence their strength. 

An acid’s strength also depends on what it is dissolved in. For example, in an aqueous solution, hydrogen chloride is a strong acid, but when dispersed in glacial acetic acid, it is a weak acid.

The Strength of an Acid and a Base

The strength of an acid or a base measures how strong or weak an acid or base is. This is measured based on their ability to conduct electricity. 

If they are good conductors of electricity, they are considered strong acids or bases. If they are ineffective conductors of electricity, they are classified as weak acids or bases.

Strong Acids

A strong acid is an acid that can dissociate completely into its constituent ions in water. As a result, strong acids generate one or more protons per molecule. 

Strong acids are highly corrosive and can cause severe burns when they come into contact with skin. 

Examples of strong acids include hydrochloric acid and sulphuric acid.

Weak Acids

A weak acid is an acid that cannot dissociate fully into its constituent ions in water. Strong acids are corrosive due to their oxidant nature. They have H+  readily available to oxidise a metal surface whereas weak acids at low concentrations are not corrosive. But they can be harmful if taken in greater concentration. They may even be found in food and our bodies. For example, Formic acid is known to occur inside the bodies of some ants.

About 1% of acids dissociate in water with a 0.1 mol/L solution, making them mostly weak acids. Thus, weak acid solutions contain fewer H+ ions than undissociated species.

Examples of weak acids include citric acid (found in lemons) and acetic acid.

Determining Acid Strength

Acidic strength varies among different acids. Acids with a high dissociation degree behave as stronger acids. The two main factors in determining the degree of dissociation of acid are HA bond strength and HA bond polarity.

HA bond strength

It is often the strength of the HA bond that determines the strength of acids. The acid will be weak if the proton in the HA bond is tightly held by the remaining molecule. This is because it requires a lot of energy to remove the proton. 

Strong acids, on the other hand, possess weakly held protons. 

A decrease in HA bond strength correlates with an increase in acidic strength as the size of the A increases.

HA bond polarity

Bond polarity is the ability of molecules or groups of chemicals to separate electric charges along with their bonds, which causes a dipole moment or electric dipole to form. The electronegativity difference between atoms in a bond determines the polarity of that bond. 

A table of electronegativity may be used to determine which atom will attract more electrons from two atoms when they share electrons. The nature of bonds can range from complete non-polarity to complete polarity.

Stronger acids are generally characterized by weaker H-A bonds. Further, the stronger the acid, the stronger the polarity of the H-A bond. Due to these factors, acid molecules are more easily dissociated into H+ and A-, increasing acidity.

High polarity allows the proton to escape the molecular configuration easily, which results in strong acid. On the other hand, low polarity keeps the proton in place and results in a weak acid. 

The higher the polarity of the HA bond, the stronger the acid; the lower the polarity of the HA bond, the weaker the acid.

Correct Order of Acidic Strength

When determining the correct order of acidic strength, it is important to remember that the acidic nature will increase as oxidation increases. For instance, Cl2O7>SO2>P4O10 is the correct order of acidic strength.

Factors Affecting Acid Strength

• H and A bond strength

H and A bond strength determine acid strength. If the bonds are weaker, it takes less energy to break them, and the resultant acid is strong. If the bond is strong, on the other hand, it takes more energy to break it, and the acid is weak.

It is crucial to consider bond strength when comparing acid strengths among elements in the same group of the periodic table.

• H and A polarity

H and A polarity affect their strength. Since the two bonds have a high polarity, the proton can escape the molecule easily, turning it into a strong acid. On the other hand, low polarity makes it difficult for the proton to escape, resulting in a weak acid.

When comparing the acid strengths of elements in the same row, the polarity of the H-A bond must be taken into account.

• Atomic size of A

A’s atomic size also affects acid strength. As the atom gets larger, the bond becomes weaker, so the acid strength increases.

Conclusion 

Dilution and concentration are not the same as weak and strong. There is a low concentration of H+ ions in a dilute acid due to a large amount of water mixed with the acid molecules. When acids are concentrated, water molecules are rarely mixed with acid molecules, meaning there are high concentrations of H+ ions. 

The rate of reaction between strong acids and substances like metals is higher than that between weak acids because strong acids have a lower pH value. 

Additionally, this explains how stronger acids cause temperatures to rise more rapidly than weak acids.