Question Video: Identifying the Chemical Equation for the Ionization of Nitric Acid Chemistry

In a solution, nitric acid (HNO₃) ionizes completely to form an acidic solution. Which of the following equations shows the ionization of HNO₃? [A] HNO₃ (aq) + H⁺ (aq) ⟶ H₂NO₃ (aq) [B] HNO₃ (aq) ⟶ H⁺ (aq) + NO₃^− (aq) [C] HNO₃ (aq) ⟶ NO₂ (aq) + OH⁻ (aq) [D] HNO₃ (aq) ⟶ HNO₂ (aq) + O⁻ (aq) [E] 2HNO₃ (aq) ⟶ 2H⁺ (aq) + N₂ (g) + 3O₂ (g)

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Video Transcript

In a solution, nitric acid, HNO3, ionizes completely to form an acidic solution. Which of the following equations shows the ionization of HNO3? (A) HNO3 aqueous plus H+ aqueous react to form H2NO3 aqueous. (B) HNO3 aqueous reacts to form H+ aqueous plus NO3− aqueous. (C) HNO3 aqueous reacts to form NO2 aqueous plus OH− aqueous. (D) HNO3 aqueous reacts to form HNO2 aqueous plus O− aqueous. (E) Two HNO3 aqueous reacts to form two H+ aqueous plus N2 gas plus three O2 gas.

In this question, we are told that when nitric acid forms an acidic solution, it ionizes completely. By glancing at the answer choices, we see that the chemical formula of nitric acid is followed by the state symbol aq. Recall that aq is the state symbol that represents an aqueous solution. An aqueous solution contains one or more solutes dissolved in water. In this problem, the solute is the acid HNO3.

When an acid dissolves in water, the solution is said to be an acidic solution. Some acids, like nitric acid, ionize completely when they dissolve in water. This means that during the dissolving process, all HNO3 molecules dissociate into separate ions. To help us solve the problem, let’s create a basic equation to represent the ionization of HNO3. We know that nitric acid ionizes when dissolved in a solution. In chemical equations representing ionization, water is typically included as a reactant or written above the reaction arrow. However, in the answer choices, aq is used instead, which represents the solution containing water.

Let’s write the chemical formula HNO3 followed by the state symbol aq to represent the reactant of our ionization equation. The products in our ionization equation will be the dissolved ions. One of these ions, ion A, will have a positive charge, and the other ion, ion B, will have a negative charge. So, what ions would HNO3 produce when dissolved in water? According to Arrhenius theory, an acid is a substance that produces or increases H+ ions when dissolved in water. And according to Brønsted–Lowry theory, an acid is a substance that donates an H+ ion during a reaction.

If we wrote an equation for the ionization of HNO3 using the Arrhenius definition, one of the ions produced is the H+ ion. If we wrote an equation using the Brønsted–Lowry theory, instead, one of the ions produced is the H3O+ ion. According to this theory, a nitric acid molecule donates a hydrogen ion to a water molecule, forming the H3O+ ion.

When scanning the answer choices, we notice that none of them include water as a reactant or H3O+ as a product. We’ll need to use the first equation that we wrote to help us select the correct answer choice.

Let’s begin by looking at answer choice (A). In this equation, nitric acid is reacting with an H+ ion. However, we know that during the ionization process an acid produces H+ ions. We also notice that no ions appear at all on the right side of the equation. H2NO3 does not have a charge, so it is a neutral molecule. For these reasons, we can eliminate answer choice (A).

Now, let’s look at answer choices (C) and (D). In answer choice (C), the products are NO2, a neutral molecule, and the OH− ion. Because the OH− ion is produced instead of the H+ ion and only one ion is formed, answer choice (C) is not a correct answer choice. In answer choice (D), the products formed are HNO2, a neutral molecule, and the O− ion. Like answer choice (C), the equation indeed does not include H+ as a product, and it also does not show two ions formed during the ionization process. So, we can eliminate answer choice (D).

This leaves only answer choices (B) and (E). Both of these answer choices include the H+ ion as a product. In answer choice (B), the second product is an ion. And in answer choice (E), there are two additional products, nitrogen gas and oxygen gas, which are both composed of neutral molecules. We know that when an acid ionizes, one of the ions is the H+ ion, and the second ion is an oppositely charged ion. Although answer choice (E) does include the H+ ion as a product, it does not include the negative ion that also forms. For this reason, we can eliminate answer choice (E).

When a nitric acid molecule dissociates into separate ions, one of the ions is the H+ or hydrogen ion, and the other ion is the NO3− or nitrate ion. Answer choice (B) includes both of these ions as the products and is therefore the correct equation for the ionization of HNO3.

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