# 5.7: Acid–Base and Gas Evolution Reactions

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:

$\text{acid + base} → \text{water + salt}$

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:

$\ce{NaOH (aq) + HCl (aq) \rightarrow NaCl (aq) + H_2O (ℓ)} \label{Eq2}$

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

$\ce{2NaOH (aq) + H_2SO_4 (aq) \rightarrow Na_2SO_4(aq) + 2H_2O (ℓ)} \label{Eq3}$

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

Steps Explanation Equation

Write the unbalanced equation.

This is a double displacement reaction, so the cations and anions swap to create new products.

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

Balance the equation.

Because there are two OH ions in the formula for Ca(OH)2, we need two moles of HNO3 to provide H+ ions

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

Additional step: identify the salt.   The salt formed is calcium nitrate.

Exercise $$\PageIndex{1}$$

Hydrocyanic acid ($$\ce{HCN(aq)}$$) can be neutralized by potassium hydroxide ($$\ce{KOH(aq)}$$). Write a balanced chemical equation for the reaction between these two compounds and identify the salt that it produces.

$\ce{KOH (aq) + HCN(aq) → KCN (aq) + H2O(ℓ)} \nonumber$

## Gas Evolving Reactions

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. For example, nitric acid reacts with sodium carbonate to form sodium nitrate, carbon dioxide, and water (Table $$\PageIndex{1}$$):

$​\ce{2HNO3(aq)+Na2CO3(aq)→2NaNO3(aq)+CO2(g)+H2O(l)​}$

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

$\ce{H2SO4(aq) + CaCO3(aq) → CaSO4(aq) + CO2(g)+H2O(l)}$

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

$​\ce{2HCl(aq) + CaCO3(aq) → CaCl2(aq) + CO2(g) + H2O(l)}$​

Figure $$\PageIndex{1}$$ demonstrates this type of reaction: Figure $$\PageIndex{1}$$: Reaction of acids with carbonates. In this reaction setup, lime water (water + calcium hydroxide) 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.

In this reaction setup, lime water, a dilute calcium hydroxide ($$Ca(OH)_2$$) solution, 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.

Table $$\PageIndex{1}$$: Types of Compounds That Undergo Gas-Evolution Reactions
Reactant Type Intermediate Product Gas Evolved Example
sulfide none $$\ce{H2S}$$ $$\ce{2HCl(aq) + K2S \rightarrow H2S (g) + 2KCl (aq)}$$
carbonates and bicarbonates $$\ce{H2CO3}$$ $$\ce{CO2}$$ $$\ce{2HCl(aq) + K2CO2 \rightarrow H2O (l) + CO2(g) + 2KCl (aq)}$$
sulfites and bisulfites $$\ce{H2SO3}$$ $$\ce{SO2}$$ $$\ce{2HCl(aq) + K2SO2 \rightarrow H2O (l) + SO2(g) + 2KCl (aq)}$$
ammonia $$\ce{NH4OH}$$ $$\ce{NH3}$$ $$\ce{NH4Cl(aq) + KOH \rightarrow H2O (l) + NH3(g) + 2KCl (aq)}$$

The gas-evolving experiment lime water is illustrated in the following video:

Video $$\PageIndex{1}$$: Carbon Dioxide ($$CO_2$$) & Limewater (Chemical Reaction). As the reaction proceeds, the limewater on the 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.

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. Another method to chemically generate gas is the oxidation of metals in acidic solutions. This reaction will yield a metal salt and hydrogen gas.

$\ce{2HCl (aq) + Zn(s) \rightarrow ZnCl_2 (aq) + H_2 (g)}$

Here, 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 $$Zn^{2+}$$, and the acid, $$H^+$$, is reduced to $$H_2(g)$$. The oxidation of metals by strong acids is another common example of a gas evolution reaction.

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