Question

Write balanced ionic equations for the following reactions: (a) Aqueous perchloric acid is neutralized by aqueous calcium hydroxide. (b) Aqueous sodium hydroxide is neutralized by aqueous acetic acid.

Short answer

(a) \( H^+(aq) + OH^-(aq) \rightarrow H_2O(l) \); (b) \( OH^-(aq) + CH_3COOH(aq) \rightarrow CH_3COO^-(aq) + H_2O(l) \).

Step-by-step solution

Write the Molecular Equations

First, write the balanced molecular equations. For equation (a), the equation is:\[ HClO_4(aq) + Ca(OH)_2(aq) \rightarrow Ca(ClO_4)_2(aq) + 2H_2O(l) \]For equation (b), the equation is:\[ NaOH(aq) + CH_3COOH(aq) \rightarrow CH_3COONa(aq) + H_2O(l) \]

Break Down Into Ionic Equations

Convert each reactant and product that is a strong electrolyte into its ions. For equation (a): \[ 2H^+(aq) + 2ClO_4^-(aq) + Ca^{2+}(aq) + 2OH^-(aq) \rightarrow Ca^{2+}(aq) + 2ClO_4^-(aq) + 2H_2O(l) \]For equation (b):\[ Na^+(aq) + OH^-(aq) + CH_3COOH(aq) \rightarrow CH_3COO^-(aq) + Na^+(aq) + H_2O(l) \] Note that acetic acid (\(CH_3COOH\)) is a weak acid and should not be split into ions.

Cancel Spectator Ions

Identify and cancel the ions that appear on both sides of the ionic equations. For equation (a), the \(Ca^{2+}\) and \(ClO_4^-\) ions are spectators, so:\[ 2H^+(aq) + 2OH^-(aq) \rightarrow 2H_2O(l) \]For equation (b), the \(Na^+\) ion is a spectator, so:\[OH^-(aq) + CH_3COOH(aq) \rightarrow CH_3COO^-(aq) + H_2O(l) \]

Net Ionic Equations

Write the net ionic equations simplifying where possible. For equation (a), the net ionic equation is:\[ H^+(aq) + OH^-(aq) \rightarrow H_2O(l) \]For equation (b), the net ionic equation is:\[ OH^-(aq) + CH_3COOH(aq) \rightarrow CH_3COO^-(aq) + H_2O(l) \]

Key concepts

Molecular Equations

To begin understanding balanced ionic equations, we first consider molecular equations. These equations capture a chemical reaction in its entirety by showing all compounds in their complete, non-ionized forms. For instance, when perchloric acid (HClO₄) reacts with calcium hydroxide (Ca(OH)₂), the molecular equation is:

HClO₄(aq) + Ca(OH)₂(aq) → Ca(ClO₄)₂(aq) + 2H₂O(l)

Similarly, if sodium hydroxide (NaOH) is neutralized by acetic acid (CH₃COOH), the molecular equation reads:

NaOH(aq) + CH₃COOH(aq) → CH₃COONa(aq) + H₂O(l)

Molecular equations are pivotal because they help visualize the reaction's overall chemistry before delving into the details of ion exchange.

Ionic Equations

Delving deeper into the nature of reactions, ionic equations break down the strong electrolytes present in solution into their respective ions. In doing so, they provide a closer look at the real players in the chemical reaction.
For example, considering our first reaction, we convert the molecular representation into an ionic one:

2H⁺(aq) + 2ClO₄⁻(aq) + Ca²⁺(aq) + 2OH⁻(aq) → Ca²⁺(aq) + 2ClO₄⁻(aq) + 2H₂O(l)

In converting to ionic equations, it's crucial to recognize strong electrolytes which fully dissociate and weak electrolytes, such as acetic acid (CH₃COOH), that don't dissociate completely. This gives an ionic view limited to the truly active ions without disrupting the reaction's overall balance.

Net Ionic Equation

Through the construction of net ionic equations, we focus solely on the ions that influence the reaction's chemistry. By eliminating ions that do not take part in the chemical change, we highlight the essence of the reaction.
Looking at the earlier examples, for the first reaction, the net ionic equation emerges as:

H⁺(aq) + OH⁻(aq) → H₂O(l)

In this form, unimportant ions, known as spectator ions, are omitted. The emphasis rests on the crucial participants — in this case, hydrogen (H⁺) and hydroxide (OH⁻) combining to form water (H₂O). This net ionic portrayal is central to understanding the driving force of the reaction.

Spectator Ions

In a chemical reaction, spectator ions are those ions that appear unchanged on both the reactant and product sides of the equation. They play no direct role in the transformation happening in the reaction.

• Looking at the first example, both calcium ions (Ca²⁺) and perchlorate ions (ClO₄⁻) appear as spectators.
• The second example spotlights sodium ions (Na⁺) as spectators.

By disregarding these ions when forming a net ionic equation, we streamline the reaction, which allows students to concentrate on the productive parts of the chemical process and reinforcing the balance and dynamics between the active players.