Question

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

Short answer

The net ionic equations for the given reactions are: a) \(Ni(s) + 2H^{+}(aq) \rightarrow Ni^{2+}(aq) + H_{2}(g)\) b) \(Fe(s) + 2H^{+}(aq) \rightarrow Fe^{2+}(aq) + H_{2}(g)\) c) \(Mg(s) + 2H^{+}(aq) \rightarrow Mg^{2+}(aq) + H_{2}(g)\) d) \(Zn(s) + 2CH_{3}COOH(aq) \rightarrow Zn(CH_{3}COO)_{2}(aq) + H_{2}(g)\)

Step-by-step solution

1. Write a balanced molecular equation

The balanced molecular equation of hydrochloric acid (HCl) with nickel (Ni) is: \(Ni(s) + 2HCl(aq) \rightarrow NiCl_{2}(aq) + H_{2}(g)\)

2. Dissociate compounds into ions

The dissociation of the compounds in water is: \(HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)\)

3. Write the net ionic equation

The net ionic equation for the reaction is: \(Ni(s) + 2H^{+}(aq) \rightarrow Ni^{2+}(aq) + H_{2}(g)\) b) Reaction of dilute sulfuric acid with iron:

1. Write a balanced molecular equation

The balanced molecular equation of dilute sulfuric acid (H2SO4) with iron (Fe) is: \(Fe(s) + H_{2}SO_{4}(aq) \rightarrow FeSO_{4}(aq) + H_{2}(g)\)

2. Dissociate compounds into ions

The dissociation of the compounds in water is: \(H_{2}SO_{4}(aq) \rightarrow 2H^{+}(aq) + SO_{4}^{2-}(aq)\)

3. Write the net ionic equation

The net ionic equation for the reaction is: \(Fe(s) + 2H^{+}(aq) \rightarrow Fe^{2+}(aq) + H_{2}(g)\) c) Reaction of hydrobromic acid with magnesium:

1. Write a balanced molecular equation

The balanced molecular equation of hydrobromic acid (HBr) with magnesium (Mg) is: \(Mg(s) + 2HBr(aq) \rightarrow MgBr_{2}(aq) + H_{2}(g)\)

2. Dissociate compounds into ions

The dissociation of the compounds in water is: \(HBr(aq) \rightarrow H^{+}(aq) + Br^{-}(aq)\)

3. Write the net ionic equation

The net ionic equation for the reaction is: \(Mg(s) + 2H^{+}(aq) \rightarrow Mg^{2+}(aq) + H_{2}(g)\) d) Reaction of acetic acid with zinc:

1. Write a balanced molecular equation

The balanced molecular equation of acetic acid (CH3COOH) with zinc (Zn) is: \(Zn(s) + 2CH_{3}COOH(aq) \rightarrow Zn(CH_{3}COO)_{2}(aq) + H_{2}(g)\) Unlike strong acids, acetic acid does not fully dissociate in water. Therefore, we do not need to dissociate acetic acid in this case.

2. Write the net ionic equation

Since acetic acid does not fully dissociate, we don't need to separate the ions in the net ionic equation. The balanced molecular equation is the net ionic equation: \(Zn(s) + 2CH_{3}COOH(aq) \rightarrow Zn(CH_{3}COO)_{2}(aq) + H_{2}(g)\)

Key concepts

Molecular Equations

When describing chemical reactions, molecular equations provide a holistic view. They depict the reactants and products as compounds with their chemical formulas, as if they were undissociated, even when in a solution. For instance, in the reaction of hydrochloric acid with nickel, the molecular equation is written as:
\(Ni(s) + 2HCl(aq) \rightarrow NiCl_{2}(aq) + H_{2}(g)\).
This simple and straightforward equation does not show the ionic nature of the compounds in solution. Instead, it emphasizes the initial and final substances involved in the chemical reaction, which is essential for understanding the overall process before delving into its ionic details.

Acid-Base Reactions

Acid-base reactions, such as the ones you're studying, involve the transfer of protons (H+) from an acid to a base. In our examples, metals like nickel, iron, and magnesium act as bases that accept protons from acids like hydrochloric acid, resulting in the formation of hydrogen gas and a salt. The unique case of acetic acid with zinc demonstrates an acid that doesn't fully dissociate, showing that not all acid-base reactions can be generalized; they can have particular characteristics based on the nature of the acid or base involved.

Dissociation of Compounds

Dissociation in chemistry refers to the process in which a compound separates into its constituent ions when dissolved in a solvent like water. For strong acids, such as hydrochloric and sulfuric acids, this process is complete in aqueous solutions, as shown in the examples:
\(HCl(aq) \rightarrow H^{+}(aq) + Cl^{-}(aq)\),\(H_{2}SO_{4}(aq) \rightarrow 2H^{+}(aq) + SO_{4}^{2-}(aq)\).
Acetic acid, however, is a weak acid and does not fully dissociate, which greatly influences how we write its ionic equations. This is important to note because it affects the reaction's dynamics and the net ionic equations, which focus solely on the species that change during the chemical reaction.

Balancing Chemical Equations

Balancing chemical equations is akin to making sure that a mathematical equation has equal values on both sides. In chemistry, it implies that the number of each type of atom and the total charge is the same on both sides of the reaction. This ensures the law of conservation of mass is adhered to. In the reaction between dilute sulfuric acid and iron, for instance, the balanced equation:
\(Fe(s) + H_{2}SO_{4}(aq) \rightarrow FeSO_{4}(aq) + H_{2}(g)\)
shows that there are equal numbers of each atom before and after the reaction. This step is crucial for understanding chemical reactions since it directly impacts how we interpret the reaction stoichiometry and the quantities involved.