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7.9: Acid-Base and Gas Evolution Reactions

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    ⚙️ 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.


    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.

    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. 



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