5.9: Acids

A spot test for gold has been in use for decades. The sample is first treated with nitric acid. Other metals may react or dissolve in this acid, but gold will not. Then the sample is added to a mixture of nitric acid and hydrochloric acid. Gold will only dissolve in this mixture. The term "acid test" arose from the California gold rush in the late 1840's when this combination was used to test for the presence of real gold. It has since come to mean "tested and approved" in a number of fields.
 

Acids

An acid can be defined in several ways. The most straightforward definition is that an acid is a molecular compound that contains one or more hydrogen atoms and produces hydrogen ions (H⁺) when dissolved in water.
 

                 

Figure \(\PageIndex{1}\): (A) Vinegar comes in a variety of types, but all contain acetic acid. (B) Citrus fruits like grapefruit contain citric and ascorbic acids.

This is a different type of compound than the others we have seen thus far. Acids are molecular, which means that in their pure state they are individual molecules and do not adopt the extended three-dimensional structures of ionic compounds like NaCl. However, when these molecules are dissolved in water, the chemical bond between the hydrogen atom and the rest of the molecule breaks, leaving a positively-charged hydrogen ion and an anion. This can be symbolized in a chemical equation:

\[\ce{HCl} \rightarrow \ce{H^+} + \ce{Cl^-}\]

Since acids produce H⁺ cations upon dissolving in water, the H of an acid is written first in the formula. The remainder of the acid (other than the H) is the anion after the acid dissolves. 
 

Binary Acids

The name of an acid is based on the other elements with which it is combined. A binary acid is an acid that consists of hydrogen combined with one other element. The most common binary acids contain a halogen. The convention for naming binary acids is

hydro- + root name of the other element + -ic acid

In other words, the acid name begins with the prefix hydro-, followed immediately by the root name of the other element, which is then followed by the suffix -ic acid. It should also be noted that binary acids are only assigned their acid name when they are dissolved in water, which we designate with a phase label of (aq), which stands for aqueous (pronounced AY-quee-us).

Therefore, the name of HCl (aq) is hydrochloric acid, since the root name of chlorine is chlor- and we are showing that the HCl is dissolved in water. As a gas, HCl (g) is called hydrogen chloride. While this distinction is important, the phase label is usually left off and the correct name is usually based on the context in which it is used. In the absence of a phase label, one is usually safe to assume that a binary acid is named as an acid, in which the hydro- prefix and -ic acid suffix are used.

When writing the formula of an acid, the formula of a hydrogen ion, H⁺, is combined with the formula and charge of the nonmetal anion in the acid. To write the formula of hydroiodic acid, H⁺ would be combined with I^–, since iodine is located in Group VIIA of the periodic table (see Figure 5.5.1). The charges H⁺ and I^– are already balanced, so the formula of hydroiodic acid is HI (aq). The formula could also be simply written as HI, since we know the context in which it was used (as an acid, according to its name).
 

✅ Example \(\PageIndex{1}\): Binary Acids

If the chemical formula is provided, write the name. If the name is provided, write the formula.

1. HF (aq)
2. hydrosulfuric acid

Solution

1. Since there are just two elements and the first element in the formula is H, this a binary acid. Binary acids have a hydro- prefix and an -ic acid suffix. The root name of fluorine is fluor-. HF (aq) is hydrofluoric acid.
        
2. The name has a hydro- prefix and an -ic acid suffix, making it a binary acid. H⁺ would be combined with S^2–. The charge on sulfide is 2– based on its position (Group VIA) on the periodic table. Charge is balanced with two H⁺ for every one S^2–. Hydrosulfuric acid is H₂S (aq) or simply H₂S.

Oxyacids

An acid always contains the element hydrogen and the H is always written at the front of the chemical formula. An oxyacid is an acid that also contains the element oxygen, along with a third (and occasionally fourth) element. The third element is almost always a nonmetal. Oxyacids may also be thought of as a compound where H⁺ has combined with an oxyanion. As we recall, the most common oxyanions end with an -ate suffix, while other oxyanions end with an -ite suffix.

The convention for naming oxyacids:

• Identify and name the oxyanion present in the acid.
• If the oxyanion name ends with -ate, drop the -ate suffix from the anion name and replace with an -ic acid suffix.
• If the oxyanion name ends with -ite, drop the -ite suffix from the anion name and replace with an -ous acid suffix. 
   

✅ Example \(\PageIndex{2}\): Naming Oxyacids

Write the name for each acid.

1. HNO₃
2. HNO₂
    

Solution

	HNO3	HNO2
Identify the oxyanion present in the acid.	NO3^–	NO2^–
Name the oxyanion.	nitrate	nitrite
Drop -ate or -ite from the anion name.	nitr-	nitr-
Replace -ate with -ic acid. Replace -ite with -ous acid.	nitric acid	nitrous acid

 

 

✅ Example \(\PageIndex{3}\): Writing Formulas for Oxyacids

Write the chemical formula for each acid.

1. chlorous acid
2. chloric acid
    

Solution

The process of arriving at a name now needs to be reversed.

	chlorous acid	chloric acid
Identify suffix as -ic acid or -ous acid.	chlorous acid	chloric acid
Replace -ic acid with -ate. Replace -ous acid with -ite.	chlorite	chlorate
Since it is an acid, combine H^+ with the formula of the oxyanion.	H^+ with ClO2^–	H^+ with ClO3^–
Balance charges.	one H^+ balances one ClO2^–	one H^+ balances one ClO3^–
Write the final formula. Charges are left off.	HClO2	HClO3

 

 

⚡️ Sulfur and Phosphorus

When sulfur or phosphorus are present in acids, their root names are different than when they are present in anions. Everything else about naming acids that contain sulfur or phosphorus remains the same.

Element	Root Name in Anions	Root Name in Acids
sulfur	sulf-	sulfur-
phosphorus	phosph-	phosphor-

 

 

✅ Example \(\PageIndex{4}\): Naming Oxyacids

Write the name for H₂SO₄.

Solution

	H2SO4
Identify the oxyanion present in the acid.	SO4^2^–
Name the oxyanion.	sulfate
Drop -ate or -ite from the anion name.	sulf-
Replace -ate with -ic acid. Replace -ite with -ous acid. Sulfur has a root name of sulfur- in acids.	sulfuric acid

 

 

✏️ Exercise \(\PageIndex{1}\)

If the chemical formula is provided, write the name. If the name is provided, write the formula. 

1. sulfurous acid
2. HBr (aq)
3. acetic acid
4. H₂CO₃

Answer A

H₂SO₃

Answer B

hydrobromic acid

Answer C

HC₂H₃O₂

Answer D

carbonic acid

Summary

• Acids are molecular compounds that release hydrogen ions when dissolved in water.
• A binary acid consists of hydrogen and one other element.
• Oxyacids contain hydrogen, oxygen, and at least one other element.
• The name of the acid is based on the anion attached to the hydrogen.

 

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This page is shared under a CK-12 license and was authored, remixed, and/or curated by Lance S. Lund (Anoka-Ramsey Community College), Melissa Alviar-Agnew, and Henry Agnew. Original source: https://www.ck12.org/c/chemistry/.

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