Question

Write a formula for each acid. (a) hydrofluoric acid (b) hydrocyanic acid (c) chlorous acid

Short answer

The formulas for the acids are: (a) hydrofluoric acid: HF, (b) hydrocyanic acid: HCN, (c) chlorous acid: HClO2.

Step-by-step solution

Writing the Formula for Hydrofluoric Acid

Hydrofluoric acid is an acid derived from hydrogen and fluorine. Since fluorine is a halogen, the acid will have the prefix 'hydro-' and the ending '-ic.' The chemical symbol for fluorine is F. Therefore, the formula for hydrofluoric acid is HF.

Writing the Formula for Hydrocyanic Acid

Hydrocyanic acid is an acid derived from hydrogen and the cyanide ion. The cyanide ion is CN-. Since it is a compound of hydrogen with a non-oxygen anion, the name is given the prefix 'hydro-' and the ending '-ic.' The formula for hydrocyanic acid is HCN.

Writing the Formula for Chlorous Acid

Chlorous acid is an acid that has chlorine with an oxidation state of +3. The base name for the acid comes from the root name of the chlorine in the form of its anion, which is 'chlorite' with chlorine and oxygen. The ion's formula is ClO2-. Therefore, the acid name does not use 'hydro-' but instead changes the '-ite' ending to '-ous.' Hence, the formula for chlorous acid is HClO2.

Key concepts

Hydrofluoric Acid Formula

Understanding the formula for hydrofluoric acid starts with recognizing its components: hydrogen and fluorine. Hydrofluoric acid is formed by the combination of these two elements. Fluorine, being a halogen, brings us to use the prefix 'hydro-' and the suffix '-ic' for the naming of the acid. Therefore, after combining hydrogen, denoted by the symbol H, with fluorine, denoted by the symbol F, we obtain the simple but crucial formula HF for hydrofluoric acid. This reflects a one-to-one ratio of hydrogen to fluorine atoms in the compound.
Remember, hydrofluoric acid is unique in its ability to etch glass due to the highly reactive nature of fluorine.

Hydrocyanic Acid Formula

Hydrocyanic acid is known for its simplicity and potentially lethal properties. It consists of a hydrogen atom bonded to a cyanide group, which is a carbon atom triple-bonded to a nitrogen atom. The prefix 'hydro-' and the suffix '-ic' are used because cyanide is a non-oxygen anion. As a result, when we combine hydrogen, H, with the cyanide ion, CN-, the formula becomes HCN. It's critical to note that despite its gaseous form at room temperature, when in solution with water, it is called hydrocyanic acid.

Chlorous Acid Formula

Chlorous acid involves a more complex understanding of acid nomenclature due to the presence of oxygen in its chemical structure. Chlorine in chlorous acid is in an oxidation state of +3, and this acid is derived from the 'chlorite' ion. Unlike the previous acids we've discussed, chlorous acid follows a different naming standard. Instead of using 'hydro-', we modify the anionic name 'chlorite' by replacing the '-ite' with '-ous' to communicate the specific oxidation state of chlorine. The resulting formula, after combining hydrogen, H, with the chlorite ion, ClO2-, is HClO2. The presence of oxygen in the acid's formula requires this special naming consideration.

Acid Nomenclature

Understanding the Naming Patterns

Acid nomenclature is a set of rules used for naming inorganic acids. It often relies on the composition of the acid—specifically, whether the acid contains oxygen or not. For binary acids, those without oxygen, such as HF and HCN, we use the 'hydro-' prefix and '-ic' suffix with the root name of the non-metal element. For oxyacids, which contain oxygen, the name is based on the polyatomic ion they are derived from. The suffixes '-ous' and '-ic' are used to denote the oxidation states of the central atom. Polyatomic ions ending in '-ite' lead to acids with '-ous' endings, while '-ate' leads to '-ic'.
Remember, consistent application of these rules is crucial for accurate communication in chemistry.

Oxidation States

Importance in Chemical Formulas

Oxidation states are fundamental to understanding the composition of compounds and the reaction processes they undergo. In essence, the oxidation state is the hypothetical charge an atom would carry if all its bonds were ionic. They are assigned based on a set of rules involving the element's position in the periodic table and its electronegativity. For the acid formulas we have explored—HF, HCN, and HClO2—oxidation states are key to determining their structures and properties. In chlorous acid, for example, knowing that chlorine has an oxidation state of +3 is critical to deducing that the corresponding ion is chlorite (ClO2-) and how it combines with hydrogen to form HClO2. Understanding oxidation states aids in predicting reactivity and stability of chemical species within various chemical reactions.