7.9: Acid-Base and Gas Evolution Reactions

Last updated
Save as PDF

Page ID: 289400

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

⚙️ Learning Objectives

Identify when a reaction will evolve a gas.

Neutralization Reactions

Acids and bases react chemically with each other to form salts. A salt is a general chemical term for any ionic compound formed from an acid and a base. In reactions where the acid is a hydrogen-ion-containing compound and the base is a hydroxide-ion-containing compound, water is also a product. The general reaction is as follows:

acid + base → salt + water

The reaction of acid and base to make water and a salt is called neutralization. Like any chemical equation, a neutralization chemical equation must be properly balanced. For example, the neutralization reaction between sodium hydroxide and hydrochloric acid is as follows:

NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)

with coefficients all understood to be one. The neutralization reaction between sodium hydroxide and sulfuric acid is as follows:

2 NaOH (aq) + H₂SO₄ (aq) → Na₂SO₄ (aq) + 2 H₂O (l)

✅ Example \(\PageIndex{1}\): Neutralizing Nitric Acid

Nitric acid, HNO₃ (aq), can be neutralized by calcium hydroxide, Ca(OH)₂ (aq). Write a balanced chemical equation for the reaction between these two compounds and identify the salt that it produces.

Solution

Write the unbalanced equation. This is a double replacement reaction, so the cations and anions swap to create new products. Referring to the solubility rules, Ca(NO₃)₂ is soluble in water resulting in a phase label of (aq). Water, H₂O, is a liquid at room temperature resulting in a phase label of (l).

Ca(OH)₂ (aq) + HNO₃ (aq) → Ca(NO₃)₂ (aq) + H₂O (l)

At this point, the equation may be balanced by placing a coefficient of "2" in front of HNO₃ (aq) and H₂O (l).

Ca(OH)₂ (aq) + 2 HNO₃ (aq) → Ca(NO₃)₂ (aq) + 2 H₂O (l)

The salt formed is calcium nitrate, Ca(NO₃)₂ (aq).

✏️ Exercise \(\PageIndex{1}\)

Hydrobromic acid, HBr (aq), may be neutralized with potassium hydroxide, KOH (aq). Write a balanced chemical equation for the reaction between these two compounds and identify the salt that it produces.

Answer: KOH (aq) + HBr (aq) → KBr (aq) + H₂O (l)

The salt is potassium bromide, KBr (aq).

Gas Evolving Reactions

Figure \(\PageIndex{1}\) shows an apparatus that may be used for collecting gases in the laboratory.

KClO3 (solid) is placed over a flame in a test tube connected to a beaker with water. Oxygen bubbles are formed. — Figure \(\PageIndex{1}\): An apparatus for collecting gases by the displacement of water.

A gas evolution reaction is a chemical process that produces a gas, such as hydrogen or carbon dioxide. In the following examples, an acid reacts with a carbonate, producing salt, carbon dioxide, and water, respectively. For example, nitric acid reacts with sodium carbonate to form sodium nitrate, carbon dioxide, and water (Table \(\PageIndex{1}\)):

2 HNO₃ (aq) + Na₂CO₃ (aq) → 2 NaNO₃ (aq) + CO₂ (g) + H₂O (l)

Hydrochloric acid reacts with calcium carbonate to form calcium chloride, carbon dioxide, and water:

2 HCl (aq) + CaCO₃ (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)

Table \(\PageIndex{1}\): Types of Compounds That Undergo Gas-Evolution Reactions
Reactant Type	Intermediate Product	Gas Evolved	Example
sulfide	none	H₂S	2 HCl (aq) + K₂S (aq) → H₂S (g) + 2 KCl (aq)
carbonates and bicarbonates	H₂CO₃	CO₂	2 HCl (aq) + K₂CO₃ (aq) → H₂CO₃ (aq) + 2 KCl (aq) Final Equation: 2 HCl (aq) + K₂CO₃ (aq) → H₂O (l) + CO₂ (g) + 2 KCl (aq)
sulfites and bisulfites	H₂SO₃	SO₂	2 HCl (aq) + K₂SO₃ (aq) → H₂SO₃ (aq) + 2 KCl (aq) Final Equation: 2 HCl (aq) + K₂SO₃ (aq) → H₂O (l) + SO₂ (g) + 2 KCl (aq)
ammonia	NH₄OH	NH₃	NH₄Cl (aq) + KOH (aq) → NH₄OH (aq) + 2 KCl (aq) Final Equation: NH₄Cl (aq) + KOH (aq) → H₂O (l) + NH₃ (g) + 2 KCl (aq)

The oxidation of metals by strong acids is another common example of a gas evolution reaction. This reaction will yield a metal salt and hydrogen gas.

2 HCl (aq) + Zn (s) → ZnCl₂ (aq) + H₂ (g)

Here, hydrochloric acid oxidizes zinc to produce an aqueous metal salt and hydrogen gas bubbles.

This page is shared under a CC BY 4.0 and CK-12 licenses and was authored, remixed, and/or curated by Boundless (www.boundless.com), Wikipedia, Paul Flowers (University of North Carolina - Pembroke), Klaus Theopold (University of Delaware) and Richard Langley (Stephen F. Austin State University) with contributing authors, Marisa Alviar-Agnew & Henry Agnew (OpenStax) and Lance S. Lund (Anoka-Ramsey Community College) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

LICENSED UNDER