There are many acids in chemistry. These can be further subdivided. A distinction can be made between weak acids, such as acetic acid, and strong acids, such as HCl.
Weak and strong acids
In chemistry, a strong acid is an acid that completely dissociates in an aqueous solution.
In chemistry there are bases and acids. These differ, among other things, in the pH value and also in their behavior from one another.
In the case of acids and bases, one more time between weak acidsuch as acetic acid, and strong acid respectively weak base and strong base distinguished. This distinction between strong acid and weak acid is based on the acid strength.
In chemistry, strong acids are acids that are completely dissolved in an aqueous solution dissociate. With a weak acid, such as acetic acid, only part of the acid dissociates. The rest of the weak acid does not dissociate. Thus, a weak acid is only partially ionized. A strong acid, such as HCl, on the other hand, dissociates completely.
Dissociation describes the breaking of chemical bonds between a molecule, ion or atom. As a result, the molecule breaks down into its individual components. Most compounds in water dissociate.
The reactivity of strong acids
What is special about strong acids is that they dissociate completely and are therefore called strong acids electrolytes are. These electrolytes can conduct electricity in an aqueous solution. With weak acids, such as acetic acids, the situation is somewhat different. Weak acids are just weak electrolytes.
A combination of an acid and water leads to a protolysis reaction. A proton is transferred to the water.
Example of a protolysis equation using hydrochloric acid, HCl:
hydrochloric acid + water ⇾ chloride ion + oxonium ion
It’s very important that you don’t confuse the strength of an acid with its reactivity. For example, hydrofluoric acid is a very reactive acid that can break down materials such as glass or plastic. However, according to the pKa scale, it is merely a weak acid that does not completely dissociate.
Strong acids – pKa value
Strong acids are mainly determined via the pKa value. Of the pKa value, the acidity constant, indicates the extent to which an acid is protolyzed in an equilibrium reaction with water. The smaller the value, the higher the acid strength. For example, a high value indicates a weak acid, such as acetic acid.
Here is a table of values used to categorize acid strengths. However, these are only approximate values, since the limits are fluid and no exact values have been defined.
Strong acids – pH
Acids of different strengths with the same concentration lead to different pH values. For example, acetic acid results in a different pH than phosphoric acid, even though they are in the same concentration.
The pH value (pondus hydrogenii) indicates how many oxonium ions are present in the solution to be tested. The more oxonium ions in the solution, the lower the pH.
The pH of a strong acid is approximately calculated using the following formula:
When different acids are dissolved in water, different amounts of oxonium ions (H3O+) released into the solution. This can be seen from the different pH values of the acids. In contrast to weak acids, such as acetic acid, strong acids dissociate completely, with many oxonium ions also being formed. The formula above is derived from the assumption that each molecule of strong acid added reacts to form an oxonium ion. The negative logarithm of the oxonium ion concentration is defined as the pH. Therefore, particularly strong acids usually lead to a particularly low pH value.
The lower the resulting pH value from your acid at the same concentration, the more strongly it is dissociated.
Causes of strong acidity
There are several causes of high acidity.
In acids, where a hydrogen atom is bonded to an element rather than an oxygen atom, the electronegativity and atomic size of that element play a role. When the element that is bonded to the hydrogen atom has high electronegativity, the bonding electrons are strongly on the side of the element. The hydrogen atom can easily be split off as a proton.
With a large atomic size of the element, there is a weaker bond between the element and the hydrogen atom and the corresponding base is more stable because the negative charge is distributed over a larger space. Because of this, the hydrogen atom can also be split off more easily here.
Electronegativity also plays a role in oxoacids. In oxo acids, there is a bond between an element, an oxygen atom and a hydrogen atom. In such compounds, the acid strength also increases with increasing electronegativity, since the hydrogen atom can more easily be split off as a proton. This is especially the case when the element is a nonmetal, such as hypochlorous acid.
Many oxo acids can have additional oxygen atoms attached to the element. These oxygen atoms exert an additional electron pull due to electronegativity. At the same time, they lead to a mesomeric stabilization of the corresponding base. For this reason, it can be said that as the number of oxygen atoms increases, the acid strength increases.
Another factor in acid strength is the corresponding base. If the corresponding base is very weak or stable, it is a strong acid. Weak bases are often stabilized by mesomeric or electron-withdrawing effects. Because of this, these bases do not accept a proton.
Mesomerism describes the phenomenon that the bonding relationships between atoms or groups of atoms cannot be described exactly. There are several boundary structures, with reality lying somewhere in between. The bonding electrons and electron pairs are also delocalized due to the mesomerism.
At a acid-base reaction an acid becomes a base. The acid donates a proton. Thus, after the reaction, the original acid has the ability to accept a proton again. Accordingly, the acid became a base. This base will corresponding base called. In an acid-base reaction, there are always corresponding acid-base pairs.
Strong acids – list
The list below shows some strong acids used in chemistry. The further the pKa value is in the negative range, the greater the acid strength.
Sulfuric acid is one of the strongest acids. Special care must be taken when handling sulfuric acid as it is highly corrosive. Sulfuric acid is an important basic chemical used in numerous reactions in the laboratory.
Hydrochloric acid is an inorganic acid. Hydrochloric acid is also an important chemical used both in the laboratory and in industry.
Nitric acid is a strong acid that can be made using the Ostwald procedure is produced. Nitric acid is used both in the laboratory and in industry. The production of fertilizers, explosives and dyes deserves special mention here.
Strong Acids – The Most Important
- In chemistry, a distinction is made between strong acids, which dissociate completely, and weak acids, which dissociate incompletely.
- Strong acids are mainly classified according to the acidity constant, the pKa value.
- The following applies to the pKa value: the smaller the value, the greater the acid strength.
- Strong acids are completely dissociated, so there are many oxonium ions in solution.
- The more oxonium ions in a solution, the lower the pH.
- There are several causes of high acidity, with electronegativity and mesomerism playing an important role.
- Examples of strong acids in chemistry are hydrochloric acid (HCl), nitric acid and sulfuric acid.