Problem 41 - Chemistry The Central Science In Si Units

Question

Write balanced molecular and net ionic equations for the following reactions, and identify the gas formed in each: (a) solid cadmium sulfide reacts with an aqueous solution of sulfuric acid; \((\mathbf{b})\) solid magnesium carbonate reacts with an aqueous solution of perchloric acid.

Step-by-Step Solution

Answer

(a) Gas: H₂S; (b) Gas: CO₂.

Step 1: Understanding the Reaction (a)

Identify the reactants: solid cadmium sulfide (CdS) and sulfuric acid (H₂SO₄). Cadmium sulfide is a sulfide that, when reacting with an acid, releases hydrogen sulfide gas (H₂S).

Step 2: Writing the Balanced Molecular Equation for (a)

Write the balanced chemical equation:\[ \text{CdS (s) + H}_2\text{SO}_4\text{ (aq)} \rightarrow \text{CdSO}_4\text{ (aq) + H}_2\text{S (g)} \] This represents cadmium sulfide reacting with sulfuric acid to produce cadmium sulfate and hydrogen sulfide gas.

Step 3: Writing the Net Ionic Equation for (a)

Disassociate aqueous compounds into ions and remove spectator ions (those which do not change).\[ \text{CdS (s) + 2H}^+\text{ (aq)} \rightarrow \text{H}_2\text{S (g) + Cd}^{2+}\text{ (aq)} \] CdS does not dissociate because it's a solid, and H₂S forms as a gas.

Step 4: Identify the Gas in Reaction (a)

The gas formed in this reaction is hydrogen sulfide \(\text{H}_2\text{S (g)}\).

Step 5: Understanding the Reaction (b)

Identify the reactants: solid magnesium carbonate (MgCO₃) and perchloric acid (HClO₄). Magnesium carbonate reacts with an acid to release carbon dioxide gas (CO₂).

Step 6: Writing the Balanced Molecular Equation for (b)

Write the balanced chemical equation:\[ \text{MgCO}_3\text{ (s) + 2HClO}_4\text{ (aq)} \rightarrow \text{Mg(ClO}_4\text{)}_2\text{ (aq) + H}_2\text{O (l) + CO}_2\text{ (g)} \] This shows magnesium carbonate reacting with perchloric acid to produce magnesium perchlorate, water, and carbon dioxide gas.

Step 7: Writing the Net Ionic Equation for (b)

Disassociate aqueous compounds into ions and remove spectator ions.\[ \text{MgCO}_3\text{ (s) + 2H}^+\text{ (aq)} \rightarrow \text{Mg}^{2+}\text{ (aq) + H}_2\text{O (l) + CO}_2\text{ (g)} \] The solid and gas remain as molecules without further dissociation.

Step 8: Identify the Gas in Reaction (b)

The gas formed in this reaction is carbon dioxide \(\text{CO}_2\text{ (g)}\).

Key Concepts

Molecular Equations

Molecular equations represent a chemical reaction in its entirety, showing the reactants and products as compounds. By writing these equations, we reveal the fundamental transformation processes in a reaction.
For example, consider the reaction where solid cadmium sulfide ( S (s)) reacts with sulfuric acid ( ₂SO₄ (aq)).

• The balanced molecular equation is: \[ S (s) + ₂SO₄ (aq) \rightarrow SO₄ (aq) + ₂S (g) \]
• This equation shows all substances involved and their states: cadmium sulfide as a solid, sulfuric acid in aqueous form, cadmium sulfate in solution, and hydrogen sulfide as a gas.

Writing a molecular equation helps you see the whole picture of the chemical reaction, allowing you to visualize and understand how substances change and interact.

Net Ionic Equations

Net ionic equations focus on the essentials of a chemical reaction. By highlighting only the ions and molecules directly involved in the reaction, net ionic equations throw "spectator ions" out of the mix.
Consider the example of cadmium sulfide reacting with sulfuric acid:

• The net ionic equation is: \[ S (s) + 2H^+ (aq) \rightarrow ₂S (g) + ^{2+} (aq) \]
• Here, only cadmium sulfide, hydrogen ions, cadmium ions, and hydrogen sulfide are represented. Spectator ions like sulfate are eliminated as they don't participate in the reaction.

By focusing on the core components, net ionic equations offer clarity, helping you comprehend the specific particles involved in the transformation.

Gas Formation

In many chemical reactions, particularly those involving acids and carbonates or sulfides, gases can be products. This formation of gas is often a key indicator of a chemical reaction taking place.
For instance, in the reaction between magnesium carbonate and perchloric acid:

• The gas produced is carbon dioxide ( ₂ (g)).

The formation of gases such as ₂ or ₂S can cause noticeable bubbling or fizzing, a clear sign that a reaction is happening. Understanding gas production helps you predict and identify the substances generated during chemical reactions. It also underscores aspects of reactions like changes in volume and pressure.

Balanced Equations

Balanced equations ensure that the number of atoms of each element is equal on both sides of the reaction. This respects the law of conservation of mass, which states that matter cannot be created or destroyed in a chemical reaction.
To illustrate, take the balanced molecular equation for magnesium carbonate reacting with perchloric acid:

• The balanced equation is: \[ CO_3 (s) + 2HClO_4 (aq) \rightarrow (ClO_4)_2 (aq) + H_2O (l) + ₂ (g) \]
• In this reaction, two hydrogen ions are needed to react completely with magnesium carbonate to produce carbon dioxide gas and water, balancing the atoms involved.

Balanced equations matter because they accurately reflect the conservation principles and predict the amount of products formed in a reaction, providing a framework for quantitative analysis.