Problem 54 - Chemistry The Central Science In Si Units

Question

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

Step-by-Step Solution

Answer

Molecular equations: (a) \(\text{Ni} + 2\text{HCl} \rightarrow \text{NiCl}_2 + \text{H}_2\), (b) \(\text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2\); Net ionic equations: (a) \(\text{Ni} + 2\text{H}^+ \rightarrow \text{Ni}^{2+} + \text{H}_2\), (b) \(\text{Fe} + 2\text{H}^+ \rightarrow \text{Fe}^{2+} + \text{H}_2\).

Step 1: Write the Molecular Equations

For each reaction, start by writing the molecular equation, which represents all reactants and products in their undissociated form. (a) For hydrochloric acid with nickel: \[ \text{Ni} + 2\text{HCl} \rightarrow \text{NiCl}_2 + \text{H}_2 \] (b) For sulfuric acid with iron: \[ \text{Fe} + \text{H}_2\text{SO}_4 \rightarrow \text{FeSO}_4 + \text{H}_2 \] (c) For hydrobromic acid with magnesium: \[ \text{Mg} + 2\text{HBr} \rightarrow \text{MgBr}_2 + \text{H}_2 \] (d) For acetic acid with zinc: \[ \text{Zn} + 2\text{CH}_3\text{COOH} \rightarrow \text{(CH}_3\text{COO)}_2\text{Zn} + \text{H}_2 \]

Step 2: Identify the Ionic Species

Recognize the strong acids/bases and soluble salts as sources of ions in aqueous solutions. (a) \(\text{HCl}\) fully dissociates: \[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \] (b) \(\text{H}_2\text{SO}_4\) partially dissociates in dilute solution, producing \(\text{H}^+\) and \(\text{HSO}_4^-\). Assume complete dissociation for simplicity: \[ \text{H}_2\text{SO}_4 \rightarrow 2\text{H}^+ + \text{SO}_4^{2-} \] (c) \(\text{HBr}\) fully dissociates: \[ \text{HBr} \rightarrow \text{H}^+ + \text{Br}^- \] (d) Acetic acid partially dissociates, but assume molecular form as it is a weak acid.

Step 3: Write the Full Ionic Equations

Show each ionic species in the reactions. (a) \[ \text{Ni} + 2\text{H}^+ + 2\text{Cl}^- \rightarrow \text{Ni}^{2+} + 2\text{Cl}^- + \text{H}_2 \] (b) \[ \text{Fe} + 2\text{H}^+ + \text{SO}_4^{2-} \rightarrow \text{Fe}^{2+} + \text{SO}_4^{2-} + \text{H}_2 \] (c) \[ \text{Mg} + 2\text{H}^+ + 2\text{Br}^- \rightarrow \text{Mg}^{2+} + 2\text{Br}^- + \text{H}_2 \] (d) \[ \text{Zn} + 2\text{CH}_3\text{COOH} \rightarrow \text{(CH}_3\text{COO)}_2\text{Zn} + \text{H}_2 \]

Step 4: Cancel Spectator Ions and Write Net Ionic Equations

Identify and cancel out spectator ions, which appear unaltered on both sides of the reaction.(a) Cancel \(\text{Cl}^-\): \[ \text{Ni} + 2\text{H}^+ \rightarrow \text{Ni}^{2+} + \text{H}_2 \] (b) Cancel \(\text{SO}_4^{2-}\): \[ \text{Fe} + 2\text{H}^+ \rightarrow \text{Fe}^{2+} + \text{H}_2 \] (c) Cancel \(\text{Br}^-\): \[ \text{Mg} + 2\text{H}^+ \rightarrow \text{Mg}^{2+} + \text{H}_2 \] (d) Remains unchanged as acetic acid is written in its molecular form.

Key Concepts

Molecular Equations

Molecular equations represent chemical reactions using complete formulæ of reactants and products, without breaking them down into ions. They show the substances as compounds in their molecular form rather than dissolved ions. This type of equation provides an overview of the reaction as a whole, which is helpful for understanding which compounds are involved. For example, when hydrochloric acid (HCl) reacts with nickel (Ni), the molecular equation is presented as \[ \text{Ni} + 2\text{HCl} \rightarrow \text{NiCl}_2 + \text{H}_2 \]. This showcases all reactants and products before any ionization occurs within the solution. Molecular equations are generally used as a primary step in analyzing chemical reactions before diving deeper into ionic details and dynamics.

Ionic Equations

Ionic equations delve deeper into the molecular equation by depicting the ions involved in a reaction that occur in aqueous solutions. Strong acids, bases, and soluble ionic compounds are shown dissociated into their ions. This type of equation provides more insight into the actual chemical processes taking place. For instance, hydrochloric acid (HCl) fully dissociates in water into hydrogen ions (\(\text{H}^+\)) and chloride ions (\(\text{Cl}^-\)). Therefore, the ionic equation for the reaction with nickel would look like \[ \text{Ni} + 2\text{H}^+ + 2\text{Cl}^- \rightarrow \text{Ni}^{2+} + 2\text{Cl}^- + \text{H}_2 \]. Here, each substance that can dissolve in water is split into its ionic components, giving a clearer picture of the reaction.

Net Ionic Equations

Net ionic equations focus on the species that actually participate in the chemical reaction, eliminating the spectator ions that remain unchanged throughout the process. Spectator ions do not contribute to the reaction itself and can be found on both sides of the ionic equation without altering. By crossing out these ions, we can see the essence of the transformation taking place. For example, in the equation involving nickel and hydrochloric acid, the chloride ions (\(\text{Cl}^-\)) are spectator ions. When they are cancelled, the net ionic equation simplifies to \[ \text{Ni} + 2\text{H}^+ \rightarrow \text{Ni}^{2+} + \text{H}_2 \]. This equation highlights that nickel is oxidized to \(\text{Ni}^{2+}\) while hydrogen ions are reduced to hydrogen gas, showing the core chemical changes.

Acid-Metal Reactions

Acid-metal reactions are a type of chemical reaction involving an acid and a metallic element, commonly leading to the formation of a salt and hydrogen gas. These reactions are not only fascinating but are also essential in various applications, from cleaning metals to generating hydrogen gas for fuel cell processes. The general form of such a reaction can be written as \[ \text{Metal} + \text{Acid} \rightarrow \text{Salt} + \text{Hydrogen gas} \]. For example, when zinc reacts with acetic acid \(\text{CH}_3\text{COOH}\), the products are zinc acetate and hydrogen gas, represented in the equation: \[ \text{Zn} + 2\text{CH}_3\text{COOH} \rightarrow \text{(CH}_3\text{COO)}_2\text{Zn} + \text{H}_2 \]. These reactions are driven by the exchange of electrons between the metal and hydrogen ions, making them a vital concept in both academic studies and industrial applications.