Problem 53 - Chemistry The Central Science In Si Units

Question

Write balanced molecular and net ionic equations for the reactions of (a) manganese with dilute sulfuric acid, (b) chromium with hydrobromic acid, (c) tin with hydrochloric acid, (d) aluminum with formic acid, HCOOH.

Step-by-Step Solution

Answer

(a) Mn + 2H₂SO₄ → MnSO₄ + H₂; Mn + 2H⁺ → Mn²⁺ + H₂ (b) Cr + 2HBr → CrBr₂ + H₂; Cr + 2H⁺ → Cr²⁺ + H₂ (c) Sn + 2HCl → SnCl₂ + H₂; Sn + 2H⁺ → Sn²⁺ + H₂ (d) 2Al + 6HCOOH → 2Al(HCOO)₃ + 3H₂; 2Al + 6H⁺ → 2Al(HCOO)₃ + 3H₂

Step 1: Write Molecular Equations

For each metal reacting with an acid, we need to write the balanced molecular equation. (a) Manganese with dilute sulfuric acid:\[ \text{Mn} + \text{H}_2\text{SO}_4 \rightarrow \text{MnSO}_4 + \text{H}_2 \](b) Chromium with hydrobromic acid:\[ \text{Cr} + 2\text{HBr} \rightarrow \text{CrBr}_2 + \text{H}_2 \](c) Tin with hydrochloric acid:\[ \text{Sn} + 2\text{HCl} \rightarrow \text{SnCl}_2 + \text{H}_2 \](d) Aluminum with formic acid, HCOOH:\[ 2\text{Al} + 6\text{HCOOH} \rightarrow 2\text{Al(HCOO)}_3 + 3\text{H}_2 \]

Step 2: Dissociate the Ionic Compounds

Next, we need to dissociate any ionic compounds in aqueous solution into their ions. For simplicity, assume all salts are soluble and dissociate completely, and the acid is in aqueous solution.(a) Manganese sulfate:\[ \text{MnSO}_4 \rightarrow \text{Mn}^{2+} + \text{SO}_4^{2-} \](b) Chromium bromide:\[ \text{CrBr}_2 \rightarrow \text{Cr}^{2+} + 2\text{Br}^- \](c) Tin chloride:\[ \text{SnCl}_2 \rightarrow \text{Sn}^{2+} + 2\text{Cl}^- \](d) Aluminum formate does not dissociate easily and is kept as \( \text{Al(HCOO)}_3 \).

Step 3: Identify Spectator Ions

Spectator ions are ions that appear the same on both sides of the equation. These ions do not participate in the overall reaction and will be removed in the net ionic equation. (a) There are no spectator ions in this reaction. (b) Again, there are no spectator ions. (c) No spectator ions. (d) No spectator ions, as we assume aluminum formate stays intact in the reaction.

Step 4: Write Net Ionic Equations

Now we write the net ionic equations by removing the spectator ions and focusing on the chemical species that change.(a) Manganese with sulfuric acid:\[ \text{Mn} + 2\text{H}^+ \rightarrow \text{Mn}^{2+} + \text{H}_2 \](b) Chromium with hydrobromic acid:\[ \text{Cr} + 2\text{H}^+ \rightarrow \text{Cr}^{2+} + \text{H}_2 \](c) Tin with hydrochloric acid:\[ \text{Sn} + 2\text{H}^+ \rightarrow \text{Sn}^{2+} + \text{H}_2 \](d) Aluminum with formic acid:\[ 2\text{Al} + 6\text{H}^+ \rightarrow 2\text{Al(HCOO)}_3 + 3\text{H}_2 \]

Key Concepts

Molecular Equations

Molecular equations provide a complete overview of a chemical reaction. They include all reactants and products, depicted together as intact molecules. These equations help visualize what happens when these molecules interact.
Let's take a simple example: When manganese (Mn) reacts with dilute sulfuric acid (H\(_2\)SO\(_4\)), the equation looks like this:

• Reactants: Mn and H\(_2\)SO\(_4\)
• Products: Manganese sulfate (MnSO\(_4\)) and hydrogen gas (H\(_2\))

The balanced equation is:\[ \text{Mn} + \text{H}_2\text{SO}_4 \rightarrow \text{MnSO}_4 + \text{H}_2 \] The molecular equation shows you every element involved and ensures the number of each type of atom is the same on both sides. Balancing molecular equations is crucial since it respects the law of conservation of mass.

Net Ionic Equations

The net ionic equation focuses only on the ions and molecules directly involved in the chemical change—those atoms or molecules that actually participate in the reaction.
Why do we need net ionic equations? They help simplify understanding of reactions by removing all other entities that don't change, i.e., spectator ions.For example, consider the reaction of tin (Sn) with hydrochloric acid (HCl), which results in tin chloride (SnCl\(_2\)) and hydrogen gas (H\(_2\)).
The net ionic equation for this reaction focuses on the tin ions gaining a charge and the release of hydrogen gas:\[ \text{Sn} + 2\text{H}^+ \rightarrow \text{Sn}^{2+} + \text{H}_2 \] This equation highlights the essential transformation from neutral tin atoms to positively charged ions and hydrogen molecules splitting to form hydrogen gas. Net-ionic equations are more focused and are perfect for grasping the substance of a reaction.

Spectator Ions

Spectator ions are the ions that remain unchanged on both sides of a complete ionic equation. They do not participate in the actual chemical change, hence the term 'spectator.' These ions can often be ignored when considering the essential parts of a reaction.

No spectator ions were noted in the presented reactions, meaning each ion in the reactants and products all partake directly in the reaction. For instance, in the reaction of chromium (Cr) with hydrobromic acid (HBr):

• While bromide (Br\(^-\)) is present in the complete ionic equation, it does not appear in the net ionic version.
• This is because bromide ions stay the same before and after the reaction.

Identifying spectator ions simplifies the reaction and aids in balancing chemical equations more effectively. It provides a clear, concise view of what is actually changing at the molecular level.