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Concept Version 11
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Gas Evolution Reactions

A gas evolution reaction is a chemical process that produces a gas, such as oxygen or carbon dioxide.

Learning Objective

  • Identify when a reaction will evolve a gas.


Key Points

    • Acids react with carbonates to produce a salt, carbon dioxide, and water.
    • Limewater can be used to detect the presence of carbon dioxide evolved in a reaction.
    • Hydrogen gas and a metal salt are produced when acids react with metals.

Terms

  • gas evolution reaction

    a chemical process that produce a gas, such as oxygen or carbon dioxide

  • limewater

    an aqueous solution of Ca(OH)2; a common indicator used to detect the presence of carbon dioxide gas


Full Text

A gas evolution reaction is a chemical process that produces a gas, such as oxygen or carbon dioxide. In the following examples, an acid reacts with a carbonate, producing salt, carbon dioxide, and water, respectively.

Nitric acid reacts with sodium carbonate to form sodium nitrate, carbon dioxide, and water:

$2HNO_{3}(aq)+Na_2CO_{3}(aq) \rightarrow 2NaNO_{3}(aq)+CO_{2}(g)+H_{2}O(l)$

Sulfuric acid reacts with calcium carbonate to form calcium sulfate, carbon dioxide, and water:

$H_{2}SO_{4}(aq)+CaCO_{3}(aq) \rightarrow CaSO_{4}(aq)+CO_{2}(g)+H_{2}O(l)$

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

$2HCl(aq)+CaCO_{3}(aq) \rightarrow CaCl_{2}(aq)+CO_{2}(g)+H_{2}O(l)$

The following setup demonstrates this type of reaction:

Reaction of acids with carbonates

In this reaction setup, lime water is poured into one of the test tubes and sealed with a stopper. A small amount of hydrochloric acid is carefully poured into the remaining test tube. A small amount of sodium carbonate is added to the acid, and the tube is sealed with a rubber stopper. The two tubes are connected. As a result of the acid-carbonate reaction, carbon dioxide is produced and the lime water turns milky.

The test tube on the right contains limewater (a solution of calcium hydroxide, Ca(OH)2). On the left, a solution of hydrochloric acid has been added to a solution of sodium carbonate to generate $CO_2(g)$. The test tubes are sealed with rubber stoppers and connected with a delivery tube. As the reaction proceeds, the limewater on the right turns from clear to milky; this is due to the $CO_2(g)$ reacting with the aqueous calcium hydroxide to form calcium carbonate, which is only slightly soluble in water. The entire experiment is illustrated in the following video:

Science Experiment : Carbon Dioxide (Co₂) & Limewater (Chemical Reaction)

Carbon dioxide (formed from bicarbonate) and an acid react with limewater, turning it from clear to milky.

When this experiment is repeated with nitric or sulfuric acid instead of HCl, it yields the same results: the clear limewater turns milky, indicating the production of carbon dioxide.

Oxidation of Metals in Acidic Solution

The oxidation of a metal in acidic solution will yield a metal salt and hydrogen gas:

Acid and metal reaction

Hydrochloric acid oxidizes zinc to produce an aqueous metal salt and hydrogen gas bubbles.

Recall that oxidation refers to a loss of electrons, and reduction refers to the gain of electrons. In the above redox reaction, neutral zinc is oxidized to Zn2+, and the acid, H+, is reduced to H2(g). The oxidation of metals by strong acids is another common example of a gas evolution reaction.

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