Question

(a) The net ionic equation for the aqueous neutralization reaction between acetic acid and sodium hydroxide is different from that for the reaction between hydrochloric acid and sodium hydroxide. Explain by writing balanced net ionic equations. (b) For a solution of acetic acid in water, list the major species in decreasing order of concentration.

Short answer

Net ionic equation for acetic acid: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l). Net ionic equation for hydrochloric acid: H⁺(aq) + OH⁻(aq) → H₂O(l). Major species in acetic acid solution: H₂O, CH₃COOH, CH₃COO⁻, H⁺.

Step-by-step solution

Write the molecular equations

Start by writing the molecular equations for the reactions. For acetic acid (CH₃COOH) and sodium hydroxide (NaOH): CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l) For hydrochloric acid (HCl) and sodium hydroxide (NaOH): HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Write the complete ionic equations

Write the complete ionic equations by separating all the strong electrolytes into ions. Acetic acid is a weak electrolyte and remains in molecular form.CH₃COOH(aq) + Na⁺(aq) + OH⁻(aq) → CH₃COO⁻(aq) + Na⁺(aq) + H₂O(l)H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)

Write the net ionic equations

Eliminate the spectator ions (ions that appear on both sides of the equation) from the complete ionic equations.For acetic acid:CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l)For hydrochloric acid:H⁺(aq) + OH⁻(aq) → H₂O(l)

Major species in acetic acid solution

Identify and list the major species in a solution of acetic acid (CH₃COOH) in water, in order of decreasing concentration. Acetic acid partially dissociates in water:CH₃COOH ⇌ CH₃COO⁻ + H⁺The species in the solution, in order of decreasing concentration, are:1. H₂O (water)2. CH₃COOH (acetic acid)3. CH₃COO⁻ (acetate ion)4. H⁺ (hydrogen ion)

Key concepts

acetic acid neutralization

Neutralization reactions involve an acid and a base reacting to form water and a salt.
In the case of acetic acid (CH₃COOH) neutralizing sodium hydroxide (NaOH), the molecular equation is: CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l).
Acetic acid is a weak acid, meaning it does not completely dissociate into ions in water.
Upon reacting with NaOH, which is a strong base and completely dissociates, the reaction produces water and sodium acetate (CH₃COONa).
The net ionic equation strips out the spectator ions, leaving: CH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l).
Only the components actively involved in the reaction are shown, simplifying the process.

hydrochloric acid neutralization

Unlike acetic acid, hydrochloric acid (HCl) is a strong acid that completely dissociates in water.
The molecular equation for HCl reacting with NaOH is: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l).
In the complete ionic equation, HCl fully dissociates: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l).
Spectator ions (ions that do not change during the reaction) are removed in the net ionic equation.
For HCl neutralization by NaOH, the net ionic equation simplifies to: H⁺(aq) + OH⁻(aq) → H₂O(l).
This shows that the neutralization principally involves hydrogen ions from the acid and hydroxide ions from the base forming water.

major species in solution

When acetic acid (CH₃COOH) dissolves in water, it partially dissociates.
The major species in the solution are in order of their concentration:
1. Water (H₂O), because it is the solvent.
2. Undissociated acetic acid (CH₃COOH) since it is a weak acid and does not fully dissociate.
3. Acetate ions (CH₃COO⁻) from the partial dissociation of acetic acid.
4. Hydrogen ions (H⁺), also from the partial dissociation.
Therefore, the concentration of water is highest, followed by acetic acid, and then smaller amounts of acetate ions and hydrogen ions.

complete ionic equations

Complete ionic equations step away from the molecular form to present how compounds dissociate into ions in aqueous solutions.
For acetic acid reacting with NaOH, the complete ionic equation is: CH₃COOH(aq) + Na⁺(aq) + OH⁻(aq) → CH₃COO⁻(aq) + Na⁺(aq) + H₂O(l).
Note that since acetic acid is weak, it remains mostly in molecular form.
For HCl reacting with NaOH, the complete ionic equation is: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l).
Both ions of HCl and NaOH completely dissociate because they are strong electrolytes.
Overall, complete ionic equations provide a clearer picture of what is happening at the ionic level in the reaction.