Problem 40 - Chemistry The Central Science

Question

Write the balanced molecular and net ionic equations for each of the following neutralization reactions: (a) Aqueous acetic acid is neutralized by aqueous barium hydroxide. (b) Solid chromium(III) hydroxide reacts with nitrous acid. (c) Aqueous nitric acid and aqueous ammonia react.

Step-by-Step Solution

Answer

(a) Balanced molecular equation: \(2CH_3COOH + Ba(OH)_2 -> Ba(CH_3COO)_2 + 2H_2O\) Net ionic equation: \(2CH_3COOH + 2OH^- -> 2CH_3COO^- + 2H_2O\) (b) Balanced molecular equation: \(Cr(OH)_3 + 3HNO_2 -> Cr(NO_2)_3 + 3H_2O\) Net ionic equation: \(3OH^- + 3H^+ -> 3H_2O\) (c) Balanced molecular equation: \(HNO_3 + NH_3 -> NH_4^+ + NO_3^-\) Net ionic equation: \(HNO_3 + NH_3 -> NH_4^+ + NO_3^-\)

Step 1: (a) Acetic Acid and Barium Hydroxide Reaction

First, we need to write the chemical formulas for the reactants. Acetic acid has the formula CH₃COOH and barium hydroxide has the formula Ba(OH)₂. Since this is a neutralization reaction, the products will be a salt and water. The salt will result from the cation originating from the base (barium) and the anion originating from the acid (acetate): Ba(CH₃COO)₂. The other product is water (H₂O). Now we can write the unbalanced equation: CH₃COOH + Ba(OH)₂ -> Ba(CH₃COO)₂ + H₂O Next, we need to balance the equation. There are two acetic acid molecules and two hydroxide ions, so we need two water molecules in the product. Thus, the balanced equation is: 2CH₃COOH + Ba(OH)₂ -> Ba(CH₃COO)₂ + 2H₂O Finally, we can write the net ionic equation by breaking the compounds into their constituent ions: 2CH₃COOH + Ba²⁺ + 2OH⁻ -> Ba²⁺ + 2CH₃COO⁻ + 2H₂O Since barium ions (Ba²⁺) are present on both sides of the reaction, we can eliminate them as spectator ions. The net ionic equation is: 2CH₃COOH + 2OH⁻ -> 2CH₃COO⁻ + 2H₂O

Step 2: (b) Chromium(III) Hydroxide Reaction with Nitrous Acid

First, we need to write the chemical formulas for the reactants. Chromium(III) hydroxide has the formula Cr(OH)₃ and nitrous acid has the formula HNO₂. Following the same procedure as earlier, the salt formed will be chromium(III) nitrite (Cr(NO₂)₃) and water (H₂O): Cr(OH)₃ + HNO₂ -> Cr(NO₂)₃ + H₂O To balance the equation, we need 3 nitrous acid molecules and 3 water molecules: Cr(OH)₃ + 3HNO₂ -> Cr(NO₂)₃ + 3H₂O The full ionic equation is: Cr³⁺ + 3OH⁻ + 3H⁺ + 3NO₂⁻ -> Cr³⁺ + 3NO₂⁻ + 3H₂O Since chromium ions (Cr³⁺) and nitrite ions (NO₂⁻) appear on both sides, they are spectator ions. The net ionic equation is: 3OH⁻ + 3H⁺ -> 3H₂O

Step 3: (c) Nitric Acid Reaction with Ammonia

First, we need to write the chemical formulas for the reactants. Nitric acid has the formula HNO₃ and ammonia has the formula NH₃. The products will be a salt and water, with the resulting salt being ammonium nitrate (NH₄NO₃): HNO₃ + NH₃ -> NH₄NO₃ + H₂O However, on inspection, we see that the equation is not balanced and ammonia is not acting as a base in this reaction (no hydroxide ions are present). Since ammonia is a weak base and can accept a hydrogen ion (H⁺), we can rewrite this equation as: HNO₃ + NH₃ -> NH₄⁺ + NO₃⁻ This balanced equation represents the formation of an ammonium ion and a nitrate ion. The net ionic equation is the same since there are no spectator ions: HNO₃ + NH₃ -> NH₄⁺ + NO₃⁻

Key Concepts

Balanced Molecular Equations

Understanding balanced molecular equations is fundamental in chemistry, as these equations represent the stoichiometry of a chemical reaction using chemical formulas. Put simply, a balanced molecular equation ensures that the number of atoms for each element is equal on both sides of the equation, satisfying the law of conservation of mass.

Let's consider the neutralization of acetic acid with barium hydroxide. Initially, we have the unbalanced equation: CH₃COOH + Ba(OH)₂ → Ba(CH₃COO)₂ + H₂O. To balance it, we need to make sure the number of atoms of each element on the reactant side equals the number of atoms on the product side. After balancing, we get 2CH₃COOH + Ba(OH)₂ → Ba(CH₃COO)₂ + 2H₂O. This indicates that two molecules of acetic acid react with one molecule of barium hydroxide to produce one molecule of barium acetate and two molecules of water.

For students struggling to understand balanced equations, it's helpful to start with the most complex molecule and balance one element at a time. Use coefficients to multiply the number of molecules rather than changing the subscripts, which represent fixed ratios within compounds.

Net Ionic Equations

Net ionic equations are a more focused representation of a chemical reaction. They exclude spectator ions that don't participate in the actual chemical change, showcasing only the species that engage in the reaction. Spectator ions appear unchanged on both sides of a complete ionic equation.

In the example given, the reaction between acetic acid and barium hydroxide yields the following net ionic equation: 2CH₃COOH + 2OH⁻ → 2CH₃COO⁻ + 2H₂O. Here, the barium ion is omitted as it doesn't change during the reaction process.

Constructing Net Ionic Equations

To write net ionic equations, first identify the ions formed in solution and then eliminate the spectator ions. This often simplifies the equation, highlighting the acid-base interaction and aiding understanding. For students, memorizing common ion charges and states of substances in solutions can be beneficial for simplifying reaction equations efficiently.

Acid-Base Reactions

Acid-base reactions, also known as neutralization reactions, are a type of chemical reaction where an acid and a base react to form a salt and water. In the context of our problem, such reactions occur between aqueous acetic acid and barium hydroxide, chromium(III) hydroxide and nitrous acid, and between nitric acid and ammonia.

The essence of an acid-base reaction lies in the transfer of protons (H⁺) from the acid to the base. Specifically, hydroxide ions (OH⁻) from the base combine with hydrogen ions (from the acid) to produce water. Meanwhile, the remaining components form a salt.

In the case of ammonia reacting with nitric acid, ammonia acts as a base by accepting a hydrogen ion from the acid to form the ammonium ion (NH₄⁺). It's important for students to note that while ammonia doesn't have OH⁻ in its formula, it can still function as a base by increasing the concentration of OH⁻ in solution when it reacts with water.