Problem 54 - Chemistry: The Central Science in SI Units, Global Edition

Question

Write balanced molecular and net ionic equations for the reactions of (a) hydrochloric acid with nickel, (b) dilute sulfuric acid with iron, \((\mathbf{c})\) hydrobromic acid with magnesium, (d) acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH},\) with zinc.

Step-by-Step Solution

Answer

The balanced molecular and net ionic equations for the given reactions are: (a) Molecular: 2HCl + Ni -> NiCl₂ + H₂ Net ionic: 2H⁺ + Ni -> Ni⁺² + H₂ (b) Molecular: H₂SO₄ + Fe -> FeSO₄ + H₂ Net ionic: 2H⁺ + Fe -> Fe⁺² + H₂ (c) Molecular: 2HBr + Mg -> MgBr₂ + H₂ Net ionic: 2H⁺ + Mg -> Mg⁺² + H₂ (d) Molecular: 2CH₃COOH + Zn -> Zn(CH₃COO)₂ + H₂ Net ionic: 2CH₃COOH + Zn -> Zn⁺² + 2CH₃COO⁻ + H₂

Step 1: 1. Write unbalanced molecular equations for the reactions

For each reaction, we need to write the molecular equation by writing down the reactants and products. We can do this using the given information: (a) Hydrochloric acid (HCl) with nickel (Ni) HCl + Ni -> NiCl₂ + H₂ (b) Dilute sulfuric acid (H₂SO₄) with iron (Fe) H₂SO₄ + Fe -> FeSO₄ + H₂ (c) Hydrobromic acid (HBr) with magnesium (Mg) HBr + Mg -> MgBr₂ + H₂ (d) Acetic acid (CH₃COOH) with zinc (Zn) CH₃COOH + Zn -> Zn(CH₃COO)₂ + H₂

Step 2: 2. Balance the molecular equations

Now that we have the molecular equations for each reaction, let's balance each equation. (a) 2HCl + Ni -> NiCl₂ + H₂ (b) H₂SO₄ + Fe -> FeSO₄ + H₂ (c) 2HBr + Mg -> MgBr₂ + H₂ (d) 2CH₃COOH + Zn -> Zn(CH₃COO)₂ + H₂

Step 3: 3. Write unbalanced net ionic equations

Back to our reactions, we can represent these reactions as net ionic equations, which show only the species that participate in the reaction and are changed during it. (a) 2H⁺ + 2Cl⁻ + Ni -> Ni⁺² + 2Cl⁻ + H₂ (b) 2H⁺ + SO₄⁻² + Fe -> Fe⁺² + SO₄⁻² + H₂ (c) 2H⁺ + 2Br⁻ + Mg -> Mg⁺² + 2Br⁻ + H₂ (d) 2CH₃COOH + Zn -> Zn⁺² + 2CH₃COO⁻ + H₂

Step 4: 4. Write balanced net ionic equations

Finally, remove the spectator ions (ions that are unchanged in the reaction) and balance the equations. (a) 2H⁺ + Ni -> Ni⁺² + H₂ (b) 2H⁺ + Fe -> Fe⁺² + H₂ (c) 2H⁺ + Mg -> Mg⁺² + H₂ (d) 2CH₃COOH + Zn -> Zn⁺² + 2CH₃COO⁻ + H₂ These are the balanced molecular and net ionic equations for the given reactions.

Key Concepts

Molecular Equations

Molecular equations are used to represent chemical reactions in a straightforward way. These equations include all reactants and products as intact molecules. This format helps to visualize the entirety of the chemical change without dissecting it into individual ions or atoms.
For example, when hydrochloric acid reacts with nickel, we write:

• \( ext{HCl} + ext{Ni} \rightarrow ext{NiCl}_2 + ext{H}_2\)

Each substance is shown in its original chemical form, enabling us to see how molecules or compounds interact.
The above shows that molecules of hydrochloric acid (\( ext{HCl}\)) react with metallic nickel \( ext{(Ni)}\) to yield nickel chloride \( ext{(NiCl}_2)\) and hydrogen gas \( ext{(H}_2)\).
Molecular equations are typically the first step in understanding a reaction at a larger scale before we delve into more detailed ionic considerations.

Balancing Equations

The balancing of chemical equations is a crucial step to ensuring that the same number of each type of atom appears on both the reactant and product sides. It's based on the Law of Conservation of Mass, which states that matter is neither created nor destroyed.
To balance an equation, we adjust coefficients without altering the molecular formulas. For instance:

• Hydrochloric acid and nickel reaction:2HCl + Ni \rightarrow NiCl₂ + H₂

Here, the equation ensures that there are equal numbers of each atom on both sides. We have:
- 2 hydrogen atoms on both sides from the 2 molecules of \( ext{H}_2\).
- 2 chlorine atoms on both sides ensuring equal \( ext{Cl}\) atoms.
Balancing gives us the exact relationships between different quantities of reactants and products, which is vital for precise reactions in a lab or industry.

Net Ionic Equations

Net ionic equations simplify chemical reactions to their essence by showcasing only the entities that undergo change during the reaction. They exclude 'spectator ions' that don't participate directly in the chemical process.
Taking our previous example of hydrochloric acid reacting with nickel, the balanced net ionic equation is:

• \(2 ext{H}^+ + ext{Ni} \rightarrow ext{Ni}^{2+} + ext{H}_2\)

This illustrates only the transformation of hydrogen ions \((\text{H}^+)\) and nickel into nickel ions \((\text{Ni}^{2+})\) and hydrogen gas \((\text{H}_2)\).
Net ionic equations are powerful because they zoom in on the actual chemical changes, which is particularly useful in reactions involving solutions or where ions are prominent.
These equations provide clarity by stripping away unnecessary details and letting the core reaction shine through.