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20.2: Arrhenius Acids and Bases

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Lesson Objectives

The student will:

  • define an Arrhenius acid and list some substances that qualify as acids under this definition.
  • define an Arrhenius base and list some substances that qualify as bases under this definition.

Vocabulary

  • Arrhenius acid
  • Arrhenius base

Introduction

Work done by Svante Arrhenius in the late 19th century set the groundwork for our current understanding of acid-base theory. Despite the relatively limited technology available to him, he was able to develop theories that adequately explained the properties and chemical reactivity of many acids and bases. Subsequent research led to the development of other commonly used definitions for acids and bases, but for now, we will focus on those proposed by Arrhenius.

Definitions

Take a look at the following dissociation reactions:

\mathrm{HCl}_{(aq)} \rightarrow \mathrm{H}^+_{(aq)} + \mathrm{Cl}^-_{(aq)}
\mathrm{HNO}_{3(aq)} \rightarrow \mathrm{H}^+_{(aq)} + \mathrm{NO}^-_{3(aq)}
\mathrm{HClO}_{4(aq)} \rightarrow \mathrm{H}^+_{(aq)} + \mathrm{ClO}^-_{4(aq)}
\mathrm{NaOH}_{(s)} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{OH}^-_{(aq)}
\mathrm{Ba(OH)}_{2(s)} \rightarrow \mathrm{Ba}^{2+}_{(aq)} + 2 \ \mathrm{OH}^-_{(aq)}

If you look at the products, you will notice some common features. All of the acids have \mathrm{H}^+ as one of the resulting ions, and all the bases produce \mathrm{OH}^-. It was on the basis of this observation that Arrhenius settled on his definitions for acids and bases. An Arrhenius acid is a substance that produces H+ ions in solution, and an Arrhenius base is a substance that produces OH- ions in solution.

Identifying Arrhenius Acids and Bases

How can we identify an acid or a base simply by looking at the chemical formula? Since we have defined acids and bases by the ions they release in solution, the first requirement is that they contain H or OH, respectively. However, there are plenty of compounds that contain oxygen and hydrogen atoms that are not acids and bases. What other clues can we look for?

Arrhenius bases are easy to spot because basically all of them are salts where the cation is a metal (or ammonium) and the anion is hydroxide. Due to the way we name ionic compounds, these substances will also have “hydroxide” in the name.

Acids can sometimes be a little trickier if all you have is the formula. For many of the acids that you will encounter, the formula will begin with an H: \mathrm{H}_2\mathrm{SO}_4, \mathrm{HCl}, \mathrm{HNO}_3, and \mathrm{HClO}_4 are all acids. When a formula involves hydrogen plus an anion that you are familiar with, it is highly likely that the compound is an acid.

One common type of weak acid is generally written in the form R-COOH or R-CO2H, where R is usually a hydrocarbon. An example is acetic acid, which is usually written as \mathrm{CH}_3\mathrm{CO}_2\mathrm{H}. In solution, acetic acid dissociates to form acetate (\mathrm{CH}_3\mathrm{CO}_2^-) and \mathrm{H}^+. Other acids of this type have similar dissociation reactions. Note that when written in the form R-COOH, these compounds may appear to have an available hydroxide anion. Don’t be fooled! When OH is attached to something covalently, it will not release when dissolved into solution.

Example:

Write the dissociation equation for the following compounds and state whether each is an acid, a base, or neither.

  1. \mathrm{H}_2\mathrm{SO}_4
  2. \mathrm{LiOH}
  3. \mathrm{NaNO}_3
  4. \mathrm{C}_6\mathrm{H}_5\mathrm{COOH}
  5. \mathrm{NaOH}
  6. \mathrm{KOH}

Solution:

  1. acid, \mathrm{H}_2\mathrm{SO}_{4(aq)} \rightarrow 2 \ \mathrm{H}^+_{(aq)} + \mathrm{SO}^{2-}_{4(aq)}
  2. base, \mathrm{LiOH}_{(s)} \rightarrow \mathrm{Li}^+_{(aq)} + \mathrm{OH}^-_{(aq)}
  3. neither, \mathrm{NaNO}_{3(s)} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{NO}^-_{3(aq)}
  4. acid, \mathrm{C}_6\mathrm{H}_5\mathrm{COOH}_{(aq)} \rightarrow \mathrm{H}^+_{(aq)} + \mathrm{C}_6\mathrm{H}_5\mathrm{COO}^-_{(aq)}
  5. base, \mathrm{NaOH}_{(s)} \rightarrow \mathrm{Na}^+_{(aq)} + \mathrm{OH}^-_{(aq)}
  6. base, \mathrm{KOH}_{(s)} \rightarrow \mathrm{K}^+_{(aq)} + \mathrm{OH}^-_{(aq)}

Lesson Summary

  • Arrhenius defined an acid as a substance that donates \mathrm{H}^+ ions when dissociating in solution.
  • An Arrhenius base is a substance that releases \mathrm{OH}^- ions in solution.

Review Questions

  1. Which of the following is the Arrhenius definition of an acid?
    1. An acid is a substance that donates protons.
    2. An acid is a substance that accepts protons.
    3. An acid is a substance that dissolves in water to form \mathrm{OH}^- ions.
    4. An acid is a substance that reacts with water to form \mathrm{H}^+ ions.

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