10.1 Arrhenius Acids and Bases

Learning Objectives

  1. Identify an Arrhenius acid and an Arrhenius base.
  2. Write the chemical reaction between an Arrhenius acid and an Arrhenius base.

Historically, the first chemical definition of an acid and a base was put forward by Svante Arrhenius, a Swedish chemist, in 1884. An Arrhenius acid is a compound that increases the H+ ion concentration in aqueous solution. The H+ ion is just a bare proton, and it is rather clear that bare protons are not floating around in an aqueous solution. Instead, chemistry has defined the hydronium ion (H3O+) as the actual chemical species that represents an H+ ion. H+ ions and H3O+ ions are often considered interchangeable when writing chemical equations (although a properly balanced chemical equation should also include the additional H2O). Classic Arrhenius acids can be considered ionic compounds in which H+ is the cation. Table 10.1 “Some Arrhenius Acids” lists some Arrhenius acids and their names.

Table 10.1 Some Arrhenius Acids
Formula Name
HC2H3O2 (also written CH3COOH) acetic acid
HClO3 chloric acid
HCl hydrochloric acid
HBr hydrobromic acid
HI hydriodic acid
HF hydrofluoric acid
HNO3 nitric acid
H2C2O4 oxalic acid
HClO4 perchloric acid
H3PO4 phosphoric acid
H2SO4 sulfuric acid
H2SO3 sulfurous acid

An Arrhenius base is a compound that increases the OH− ion concentration in aqueous solution. Ionic compounds of the OH− ion are classic Arrhenius bases.

Example 10.1

Identify each compound as an Arrhenius acid, an Arrhenius base, or neither.

  1. HNO3
  2. CH3OH
  3. Mg(OH)2

Solution

  1. This compound is an ionic compound between H+ ions and NO3− ions, so it is an Arrhenius acid.
  2. Although this formula has an OH in it, we do not recognize the remaining part of the molecule as a cation. It is neither an acid nor a base. (In fact, it is the formula for methanol, an organic compound.)
  3. This formula also has an OH in it, but this time we recognize that the magnesium is present as Mg2+ cations. As such, this is an ionic compound of the OH− ion and is an Arrhenius base.

Test Yourself
Identify each compound as an Arrhenius acid, an Arrhenius base, or neither.

  1. KOH
  2. H2SO4
  3. C2H6

Answer

  1. Arrhenius base
  2. Arrhenius acid
  3. neither

Acids have some properties in common. They turn litmus, a plant extract, red. They react with some metals to give off H2 gas. They react with carbonate and hydrogen carbonate salts to give off CO2 gas. Acids that are ingested typically have a sour, sharp taste. (The name acid comes from the Latin word acidus, meaning “sour.”) Bases also have some properties in common. They are slippery to the touch, turn litmus blue, and have a bitter flavor if ingested.

Acids and bases have another property: they react with each other to make water and an ionic compound called a salt. A salt, in chemistry, is any ionic compound made by combining an acid with a base. A reaction between an acid and a base is called a neutralization reaction and can be represented as follows:

acid + base → H2O + salt

The stoichiometry of the balanced chemical equation depends on the number of H+ ions in the acid and the number of OH− ions in the base.

Example 10.2

Write the balanced chemical equation for the neutralization reaction between H2SO4 and KOH. What is the name of the salt that is formed?

Solution
The general reaction is as follows:

H2SO4 + KOH → H2O + salt

Because the acid has two H+ ions in its formula, we need two OH− ions to react with it, making two H2O molecules as product. The remaining ions, K+ and SO42−, make the salt potassium sulfate (K2SO4). The balanced chemical reaction is as follows:

H2SO4 + 2KOH → 2H2O + K2SO4

Test Yourself
Write the balanced chemical equation for the neutralization reaction between HCl and Mg(OH)2. What is the name of the salt that is formed?

Answer
2HCl + Mg(OH)2 → 2 H2O + MgCl2; magnesium chloride

Key Takeaways

  • An Arrhenius acid is a compound that increases the H+ ion concentration in aqueous solution.
  • An Arrhenius base is a compound that increases the OH− ion concentration in aqueous solution.
  • The reaction between an Arrhenius acid and an Arrhenius base is called neutralization and results in the formation of water and a salt.

Exercises

Questions

  1. Define Arrhenius acid.
  2. Define Arrhenius base.
  3. What are some general properties of Arrhenius acids?
  4. What are some general properties of Arrhenius bases?
  5. Identify each substance as an Arrhenius acid, an Arrhenius base, or neither.
    1. NaOH
    2. C2H5OH
    3. H3PO4
  6. Identify each substance as an Arrhenius acid, an Arrhenius base, or neither.
    1. C6H12O6
    2. HNO2
    3. Ba(OH)2
  7. Write the balanced chemical equation for the neutralization reaction between KOH and H2C2O4. What is the salt?
  8. Write the balanced chemical equation for the neutralization reaction between Sr(OH)2 and H3PO4. What is the salt?
  9. Write the balanced chemical equation for the neutralization reaction between HCl and Fe(OH)3. What is the salt?
  10. Write the balanced chemical equation for the neutralization reaction between H2SO4 and Cr(OH)3. What is the salt?
  11. CaCl2 would be the product of the reaction of what acid and what base?
  12. Zn(NO3)2 would be product of the reaction of what acid and what base?
  13. BaSO4 would be product of the reaction of what acid and what base?
  14. Na3PO4 would be product of the reaction of what acid and what base?

Answers

  1. A compound that increases the H+ concentration in water.
  1. Sour taste, react with metals, and turn litmus red
    1. Arrhenius base
    2. neither
    3. Arrhenius acid
  1. 2KOH + H2C2O4 → 2H2O + K2C2O4; K2C2O4
  1. 3HCl + Fe(OH)3 → 3H2O + FeCl3; FeCl3
  1. HCl and Ca(OH)2
  1. H2SO4 and Ba(OH)2

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