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; (b) solid magnesium carbonate reacts with an aqueous solution of perchloric acid.

Short answer

The balanced molecular and net ionic equations for the given reactions are as follows: (a) $\text{CdS (s)}+{\text{H}}_2{\text{SO}}_{4\text{ (aq)}}\to {\text{CdSO}}_{4\text{ (aq)}}+{\text{H}}_2\text{S (g)}$; net ionic: $\text{CdS (s)}+2{\text{H}}^{\text{+ (aq)}}+{\text{SO}}_4^{2-\text{ (aq)}}\to {\text{Cd}}_{\text{(aq)}}^{2+}+{\text{SO}}_4^{2-\text{ (aq)}}+{\text{H}}_2\text{S (g)}$. The gas formed is hydrogen sulfide, ${\text{H}}_2\text{S (g)}$. (b) \(\text{MgCO}_{3\text{ (s)}} + 2\text{HClO}_{4\text{ (aq)}} \to \text{Mg(ClO}_{4\text{ )}_{2\text{ (aq)}} + \text{H}_{2}\text{O (l)} + \text{CO}_{2\text{ (g)}\}\); net ionic: \(\text{MgCO}_{3\text{ (s)}} + 2\text{H}^{\text{+ (aq)}} + 2\text{ClO}_{4}^{-\text{ (aq)}} \to \text{Mg}^{2+}_{\text{(aq)}}+2\text{ClO}_{4}^{-\text{ (aq)}} + \text{H}_{2}\text{O (l)} + \text{CO}_{2\text{ (g)}\). The gas formed is carbon dioxide, \(\text{CO}_{2\text{ (g)}\).

Step-by-step solution

Write the balanced molecular equation

$\text{CdS (s)}+{\text{H}}_2{\text{SO}}_{4\text{ (aq)}}\to {\text{CdSO}}_{4\text{ (aq)}}+{\text{H}}_2\text{S (g)}$

Write the balanced net ionic equation

Since solid cadmium sulfide ($\text{CdS (s)}$) and aqueous cadmium sulfate (${\text{CdSO}}_{4\text{ (aq)}}$) are not dissociated into ions, they remain unchanged. The aqueous sulfuric acid dissociates into hydrogen cations (${\text{H}}^{+}$) and sulfate anions (${\text{SO}}_4^{2-}$). The formed hydrogen sulfide is a gas and will not dissociate in water either. Now, let's write the balanced net ionic equation by canceling the common spectator ions: $\text{CdS (s)}+2{\text{H}}^{\text{+ (aq)}}+{\text{SO}}_4^{2-\text{ (aq)}}\to {\text{Cd}}_{\text{(aq)}}^{2+}+{\text{SO}}_4^{2-\text{ (aq)}}+{\text{H}}_2\text{S (g)}$

Identify the gas formed

In this reaction, hydrogen sulfide (${\text{H}}_2\text{S (g)}$) is the gas formed. ## Reaction (b): Solid magnesium carbonate reacts with an aqueous solution of perchloric acid ##

Write the balanced molecular equation

\( \text{MgCO}_{3\text{ (s)}} + 2\text{HClO}_{4\text{ (aq)}} \to \text{Mg(ClO}_{4\text{ )}_{2\text{ (aq)}} + \text{H}_{2}\text{O (l)} + \text{CO}_{2\text{ (g)}} \)

Write the balanced net ionic equation

Since solid magnesium carbonate (\(\text{MgCO}_{3\text{ (s)}\)) and aqueous magnesium perchlorate (\(\text{Mg(ClO}_{4\text{ )}_{2\text{ (aq)}}\)) are not dissociated into ions, they remain unchanged. The aqueous perchloric acid dissociates into hydrogen cations (${\text{H}}^{+}$) and perchlorate anions (${\text{ClO}}_4^{-}$). The formed water (${\text{H}}_2\text{O (l)}$) and carbon dioxide (${\text{CO}}_{2\text{ (g)}}$) will not dissociate in water either. Now, let's write the balanced net ionic equation by canceling the common spectator ions: ${\text{MgCO}}_{3\text{ (s)}}+2{\text{H}}^{\text{+ (aq)}}+2{\text{ClO}}_4^{-\text{ (aq)}}\to {\text{Mg}}_{\text{(aq)}}^{2+}+2{\text{ClO}}_4^{-\text{ (aq)}}+{\text{H}}_2\text{O (l)}+{\text{CO}}_{2\text{ (g)}}$

Identify the gas formed

In this reaction, carbon dioxide (\(\text{CO}_{2\text{ (g)}\)) is the gas formed.

Key concepts

Understanding Molecular Equations

Molecular equations provide a bird's-eye view of chemical reactions by showing the reactants and products in their undissociated form. These equations help emphasize the substances that are reacting and the new compounds formed. For a clear and accurate representation, it's crucial to balance molecular equations, meaning the number of atoms for each element must be the same on both sides of the equation.

Take, for example, the reaction of solid cadmium sulfide with sulfuric acid. The balanced molecular equation is as follows:
$$\text{CdS (s)}+{\text{H}}_2{\text{SO}}_{4\text{ (aq)}}\to {\text{CdSO}}_{4\text{ (aq)}}+{\text{H}}_2\text{S (g)}$$This equation clearly shows the reactants: cadmium sulfide and sulfuric acid, and the products: cadmium sulfate and hydrogen sulfide gas. Balancing this type of equation is essential for students to understand the overarching process and conservation of mass in the reaction.

The Role of Net Ionic Equations

Net ionic equations drill down to the heart of a chemical reaction, showing only the species that actually change during the process. They omit spectator ions, which do not partake in the reaction, to provide a clearer picture of the actual chemical changes taking place.

For example, when we look at the net ionic equation of the reaction between cadmium sulfide and sulfuric acid, it simplifies to:
$$\text{CdS (s)}+2{\text{H}}^{\text{+ (aq)}}+{\text{SO}}_4^{2-\text{ (aq)}}\to {\text{Cd}}_{\text{(aq)}}^{2+}+{\text{SO}}_4^{2-\text{ (aq)}}+{\text{H}}_2\text{S (g)}$$This equation emphasizes the change cadmium sulfide undergoes to form cadmium ions and hydrogen sulfide gas, while excluding the sulfate ions that remain unchanged. By focusing on the active participants, net ionic equations enhance a student's comprehension of the reactions at the ionic level and help them understand ionic interactions in solutions.

Identification of Reaction Products

Identifying reaction products is a key skill in chemistry as it involves predicting the substances that will form during chemical reactions. Recognizing a gas evolution is particularly important, as it is often an indication of a reaction taking place.

In the exercises provided, two types of gases are formed: hydrogen sulfide (${\text{H}}_2\text{S (g)}$) and carbon dioxide (\(\text{CO}_{2\text{ (g)}\)). These gases can be identified by their unique properties, such as odor or the bubbling visible during a reaction. Knowing how to identify gas production and other reaction products enables students to understand and predict the outcomes of chemical reactions. This skill can be enhanced further by learning about common types of reactions, such as acid-base neutralizations and precipitation reactions, which often result in the formation of gases or other identifiable products.