Problem 40 - Chemistry The Central Science In Si Units

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) 2CH3COOH + Ba(OH)2 -> Ba(CH3COO)2 + 2H2O. Net ionic: 2CH3COOH + 2OH- -> 2CH3COO- + 2H2O. (b) Cr(OH)3 + 3HNO2 -> Cr(NO3)3 + 3H2O, no net ionic. (c) HNO3 + NH3 -> NH4NO3; net ionic: H+ + NH3 -> NH4+.

Step 1: Write the Unbalanced Molecular Equation for Reaction (a)

Acetic acid (CH_3COOH) reacts with barium hydroxide (Ba(OH)_2). The unbalanced molecular equation is: \[ \text{CH}_3\text{COOH}_{(aq)} + \text{Ba(OH)}_2_{(aq)} \rightarrow \text{Ba(CH}_3\text{COO)}_2_{(aq)} + \text{H}_2\text{O}_{(l)} \]

Step 2: Balance the Molecular Equation for Reaction (a)

Balance the equation by checking the number of each type of atom on both sides: \[ 2\text{CH}_3\text{COOH}_{(aq)} + \text{Ba(OH)}_2_{(aq)} \rightarrow \text{Ba(CH}_3\text{COO)}_2_{(aq)} + 2\text{H}_2\text{O}_{(l)} \]

Step 3: Write the Net Ionic Equation for Reaction (a)

First write the ionic equation for the strong electrolytes:\[ 2\text{CH}_3\text{COOH}_{(aq)} + \text{Ba}^{2+}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow 2\text{CH}_3\text{COO}^-_{(aq)} + \text{Ba}^{2+}_{(aq)} + 2\text{H}_2\text{O}_{(l)} \]Removing the spectator ions (\(\text{Ba}^{2+}\)) results in:\[ 2\text{CH}_3\text{COOH}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow 2\text{CH}_3\text{COO}^-_{(aq)} + 2\text{H}_2\text{O}_{(l)} \]

Step 4: Molecular and Net Ionic Equations for Reaction (b)

Chromium(III) hydroxide (Cr(OH)_3) reacts with nitrous acid (HNO_2). The unbalanced molecular equation is:\[ \text{Cr(OH)}_3_{(s)} + \text{HNO}_2_{(aq)} \rightarrow \text{Cr(NO}_3)_3_{(aq)} + \text{H}_2\text{O}_{(l)} \]Balanced molecular equation (balancing OH and H from HNO_2):\[ \text{Cr(OH)}_3_{(s)} + 3\text{HNO}_2_{(aq)} \rightarrow \text{Cr(NO}_3)_3_{(aq)} + 3\text{H}_2\text{O}_{(l)} \]There is no net ionic equation since Cr(OH)_3 is not soluble and does not dissociate into ions.

Step 5: Molecular Equation for Reaction (c)

Nitric acid (HNO_3) reacts with ammonia (NH_3). The molecular equation is:\[ \text{HNO}_3_{(aq)} + \text{NH}_3_{(aq)} \rightarrow \text{NH}_4\text{NO}_3_{(aq)} \]

Step 6: Net Ionic Equation for Reaction (c)

The reaction involves nitric acid forming ammonium and nitrate ions. The net ionic equation is:\[ \text{H}^+_{(aq)} + \text{NH}_3_{(aq)} \rightarrow \text{NH}_4^+_{(aq)} \]

Key Concepts

Molecular Equations

Molecular equations represent the complete chemical formulas of the reactants and products involved in a chemical reaction. These equations show all the substances in their standard molecular forms, providing a straightforward overview of the reaction. For instance, when acetic acid reacts with barium hydroxide, the molecular equation would be written as:

• Unbalanced: \( \text{CH}_3\text{COOH}_{(aq)} + \text{Ba(OH)}_2_{(aq)} \rightarrow \text{Ba(CH}_3\text{COO)}_2_{(aq)} + \text{H}_2\text{O}_{(l)} \)
• Balanced: \( 2\text{CH}_3\text{COOH}_{(aq)} + \text{Ba(OH)}_2_{(aq)} \rightarrow \text{Ba(CH}_3\text{COO)}_2_{(aq)} + 2\text{H}_2\text{O}_{(l)} \)

In neutralization reactions, such as the reaction of acetic acid with barium hydroxide, the focus is on forming water and a salt as products. This setup helps students understand the overall changes happening during the reaction.
Writing molecular equations is the first crucial step in solving and balancing chemical reactions. It involves displaying all reactants and products upfront, giving a clear picture of all components involved before moving on to further analysis like balancing or simplifying to net ionic equations.

Net Ionic Equations

Net ionic equations simplify the molecular equations by showing only the species that are directly involved in the chemical change. Spectator ions, those ions that don't participate in the reaction but are present in the solution, are not included in net ionic equations.
As an example, consider the reaction between acetic acid and barium hydroxide. After writing the ionic forms, we remove \( \text{Ba}^{2+} \) as they don't change in the reaction:

• Molecular: \( 2\text{CH}_3\text{COOH}_{(aq)} + \text{Ba(OH)}_2_{(aq)} \rightarrow \text{Ba(CH}_3\text{COO)}_2_{(aq)} + 2\text{H}_2\text{O}_{(l)} \)
• Net Ionic: \( 2\text{CH}_3\text{COOH}_{(aq)} + 2\text{OH}^-_{(aq)} \rightarrow 2\text{CH}_3\text{COO}^-_{(aq)} + 2\text{H}_2\text{O}_{(l)} \)

For cases like the reaction of chromium(III) hydroxide with nitrous acid, if no ions are free to move around in the reaction mixture due to solid formation, the reaction doesn't lend itself to a net ionic equation. Therefore, understanding these net ionic equations is pivotal, as they highlight the essence of the reaction by focusing only on the gametic interaction and transformation steps.

Balancing Chemical Equations

Balancing chemical equations is a fundamental skill in chemistry, ensuring that the number of atoms of each element is the same on both sides of the equation. This practice stands on the law of conservation of mass, where matter cannot be created or destroyed, just rearranged.
For instance, when balancing the acetic acid and barium hydroxide reaction, start by carefully counting atoms of each element on both sides:

• Reactants: 2 \( \text{CH}_3\text{COOH}_{(aq)} \) (4 carbon, 8 hydrogen, 2 oxygen) + \( \text{Ba(OH)}_2 \) (1 barium, 2 oxygen, 2 hydrogen)
• Products: \( \text{Ba(CH}_3\text{COO)}_2_{(aq)} \)+ \( 2\text{H}_2\text{O}_{(l)} \) (4 carbon, 8 hydrogen, 4 oxygen, 1 barium)

In this neutralization, make sure each type of atom is balanced by adding the correct stoichiometric coefficients, showing the proportion in which elements and compounds react or are produced.
This balancing step ensures that your molecular and net ionic equations accurately reflect the actual chemical changes occurring, allowing for predictions and understanding of the process and results. Properly balanced equations provide the blueprint for interpreting and predicting chemical reactions.