Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.
What Is an Arrhenius Acid?
What can make a solution be classified as an acid? There are many definitions from different scientists as to what an acid is. In 1884, the Swedish scientist named Svante Arrhenius came up with the definition of an acid, which is known as the Arrhenius acid.
Let us imagine an aqueous solution, which is a solution containing water. According to Svante Arrhenius, an acid is a compound that, when in an aqueous solution, increases the number of hydrogen ions (H+), which is a proton. This definition became known as the definition for an Arrhenius acid.
In the chemical equation below, it shows that when the acid, hydrochloric acid (HCl) is dissolved in water, the hydrogen (H) atom and the chlorine (Cl) atom dissociate from each other, and so, the concentration of H+ in an aqueous solution increases due to the release of H+ ions from HCl. The Arrhenius acid is HCl.
Equation 1 below shows the chemical compounds involved in the chemical reaction, and equation 2 shows the structure of these chemical compounds so you can visualize how the H and Cl atoms dissociate from each other in HCl.
When an Arrhenius acid is in water, the acid releases hydrogen ions (H+), which are protons, in water, so it protonates water. When water is protonated, it results in the production of hydronium (H3O+) ions. In the chemical equation shown, it shows that the Arrhenius acid, HCl, protonates water (H2O). As a result, the hydrogen atom from HCl transfers to H2O, and this results in the hydronium ion (H3O+) and the chlorine ion (Cl-). Equation 1 below shows the chemical compounds involved in the chemical reaction, and equation 2 shows the structure of these chemical compounds so you can visualize how the acid protonates water.
Hydrogen (H+) and hydronium (H3O+) ions, in the case of Arrhenius acids, are interchangeable terms. So, we can say that an Arrhenius acid is a compound that increases the hydrogen (H+) or hydronium (H3O+) concentration in an aqueous solution, making the solution more acidic.
Examples of Arrhenius Acids
How do we know if a chemical compound is an Arrhenius acid or not? Arrhenius acids can be binary acids, which are made up of a hydrogen (H) atom and another nonmetallic element, and oxoacids, which are acids made of hydrogen (H), oxygen (O) and at least one other atom.
If we look at the periodic table, binary acids can be formed between hydrogen and elements from group 16 and 17. In the illustration below, the elements that are boxed in red are the ones that typically bond with hydrogen to form binary acids. For instance, hydrogen (H) bonds with fluorine (F) to form the binary acid hydrofluoric acid (HF).
The following table shows some examples of Arrhenius acids that are binary acids.
Oxoacids are typically formed between hydrogen and polyatomic anions. Polyatomic anions are negatively charged ions made of more than one element. Below are a few examples of polyatomic anions and the oxoacids they form when they are combined with hydrogen.
The following table shows some examples of Arrhenius acids that are oxoacids:
Let's take a look at a few examples of chemical reactions so that we can recognize what an Arrhenius acid is in a chemical equation.
This shows the reaction between acetic acid (HC2H3O2) and water (H2O).
In this reaction, water is protonated by acetic acid. Since acetic acid is an Arrhenius acid, this produces hydronium ion (H3O+), so the concentration of hydronium ion in an aqueous solution increases and an acetate ion (C2H3O2-) is produced.
This shows the reaction between hydrofluoric acid (HF) and water (H2O).
In this reaction, water is protonated by hydrofluoric acid. Since hydrofluoric acid is an Arrhenius acid, this produces hydronium ion (H3O+), so the concentration of hydronium ion in an aqueous solution increases and a fluoride ion (F-) is produced.
An Arrhenius acid is a compound that increases the concentration of hydrogen ions (H+) or hydronium ions (H3O+) in an aqueous solution. Hydronium ions are produced when the acid protonates water. An Arrhenius acid can be a binary acid, which are acids composed of a hydrogen atom combined with one other element. These elements are typically found in group 16 and 17 in the periodic table.
An Arrhenius acid can also be an oxoacid, which are acids with hydrogen, oxygen and at least one other atom. An oxoacid is formed typically when hydrogen combines with polyatomic anions, which are negatively charged ions consisting of more than one atom.
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