Chapter 16 – Summary

16.1 Acids and Bases

A summary of key properties and characteristics of acids and bases commonly encountered in everyday life was provided.

  • Both acids and bases release ions in water solution.
  • Since both are capable of producing ions when dissolved in water, they are electrolytes.
  • Since they are electrolytes, they are both capable of conducting electricity.
  • Both acids and bases undergo neutralization reaction

Strong acids and bases are 100% ionized in aqueous solution. Weak acids and bases are less than 100% ionized in aqueous solution. Salts of weak acids or bases can affect the acidity or basicity of their aqueous solutions. A compound that can donate a proton (a hydrogen ion) to another compound is called a Brønsted-Lowry acid. The compound that accepts the proton is called a Brønsted-Lowry base. The species remaining after a Brønsted-Lowry acid has lost a proton is the conjugate base of the acid. The species formed when a Brønsted-Lowry base gains a proton is the conjugate acid of the base. Thus, an acid-base reaction occurs when a proton is transferred from an acid to a base, with formation of the conjugate base of the reactant acid and formation of the conjugate acid of the reactant base. Amphiprotic species can act as both proton donors and proton acceptors. Water is the most important amphiprotic species. It can form both the hydronium ion, H3O+, and the hydroxide ion, OH− depending on what it is reacting with.

16.2 Ionization of Water

An acid-base reaction occurs when a proton is transferred from an acid to a base, with formation of the conjugate base of the reactant acid and formation of the conjugate acid of the reactant base. Amphiprotic species can act as both proton donors and proton acceptors. Water is the most important amphiprotic species. It can form both the hydronium ion, H3O+, and the hydroxide ion, OH− when it undergoes autoionization:

[latex]2\text{ H}_2\text{O}(l)\;{\leftrightharpoons}\;\text{H}_3\text{O}^{+}(aq)\;+\;\text{OH}^{-}(aq)[/latex]

The ion product of water, Kw is the equilibrium constant for the autoionization reaction:

[latex]K_{\text{w}} = [\text{H}_3\text{O}^{+}][\text{OH}^{-}] = 1.0\;\times\;10^{-14}\;\text{at}\;25\;^{\circ}\text{C}[/latex]

16.3 Reactions in Acids and Bases

Chemical reactions are classified according to similar patterns of behaviour. Acid-base reactions involve the transfer of hydrogen ions between reactants.

General acid-base reactions, also called neutralization reactions can be summarized with the following reaction equation:

ACID(aq) + BASE(aq) → H2O(l) + SALT(aq) or (s)

  • The driving force for a general acid-base reaction is the formation of water.

Gas-forming acid-base reactions can be with carbonates or metals. One example of an acid mixing with a carbonate is summarized in the following reaction equation:

ACID(aq) + NaHCO3 or Na2CO3(aq) → H2O(l) + CO2(g) + SALT(aq) or (s)

  • The driving force for a gas-forming acid-base reaction is the formation of gas.

Reactions of acids with metal oxides results in the formation of salt and water.

16.4 Introduction to pH and pOH

The concentration of hydronium ion in a solution of an acid in water is greater than 1.0 × 10−7M at 25 °C. The concentration of hydroxide ion in a solution of a base in water is greater than 1.0 × 10−7M at 25 °C. The concentration of [latex]\text{H}_3\text{O}^{+}[/latex] in a solution can be expressed as the pH of the solution

[latex]\text{pH} = -\text{log[H}_3\text{O}^{+}][/latex]

The concentration of OH− can be expressed as the pOH of the solution:

[latex]\text{pOH} = -\text{log[OH}^{-}][/latex]

In pure water, pH = 7.00 and pOH = 7.00.

16.5 Neutralization

The characteristic properties of aqueous solutions of Brønsted-Lowry acids are due to the presence of hydronium ions; those of aqueous solutions of Brønsted-Lowry bases are due to the presence of hydroxide ions. The neutralization that occurs when aqueous solutions of acids and bases are combined results from the reaction of the hydronium and hydroxide ions to form water. A strong acid will neutralize a strong base and result in a neutral solution. However, some salts formed in neutralization reactions may make the product solutions slightly acidic or slightly basic.

16.6 Titrations and Neutralization Calculations

Acids and bases neutralize each other, forming a salt and water. A strong acid-strong base neutralization results in a neutral solution with a pH of 7. A titration is an experiment in which a controlled acid-base neutralization reaction is used to determine the unknown concentration of an acid or a base. The equivalence point is reached when the number of hydrogen ions is equal to the number of hydroxide ions. Acid-base indicators are used in a titration in order to detect the end point of the titration. Neutralization calculations can be performed to determine the concentration or volume of a strong acid or base used in a titration.

16.7 Buffers

Buffering systems have the following characteristics:

  • The solution contains a weak acid and its conjugate base OR a weak base and its conjugate acid
  • The buffer resists changes in pH by reacting with added acid or base, so these ions do not accumulate.
  • Any added acid reacts with the conjugate base to resist pH changes
  • Any added base reacts with the conjugate acid to resist pH changes

Buffers cannot be made from a strong acid (or strong base) and its conjugate since these solutions ionize completely in water. Also recognize that water is not a buffer.

Attribution & References

Except where otherwise noted, this page is adapted by Jackie MacDonald from:

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Enhanced Introductory College Chemistry Copyright © 2023 by Gregory Anderson; Caryn Fahey; Jackie MacDonald; Adrienne Richards; Samantha Sullivan Sauer; J.R. van Haarlem; and David Wegman is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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