A student writes the net ionic equation for HCl reacting with NaOH and includes Na⁺ and Cl⁻ on both sides. Their partner tells them to cancel those ions. Why?
ANa⁺ and Cl⁻ react with each other to form table salt, so they cancel out
BNa⁺ and Cl⁻ are spectator ions — they appear unchanged on both sides and play no role in the reaction
CNa⁺ and Cl⁻ are too large to react in aqueous solution
DThey cancel because strong acids and bases neutralize all dissolved ions
Spectator ions are present in solution but do not participate in the actual chemical reaction. Because Na⁺ and Cl⁻ appear identically on both sides of the ionic equation, they cancel to give the net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l). Option A is a common misconception — NaCl does form in solution, but as dissolved ions, not as a product of Na⁺ reacting with Cl⁻.
Question 2 Multiple Choice
Acetic acid (CH₃COOH, a weak acid) reacts with NaOH. Which is the correct net ionic equation?
AH⁺(aq) + OH⁻(aq) → H₂O(l)
BCH₃COOH(aq) + OH⁻(aq) → CH₃COO⁻(aq) + H₂O(l)
CCH₃COO⁻(aq) + Na⁺(aq) → CH₃COONa(aq)
DCH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l)
Because acetic acid is a weak acid, it does not fully dissociate in water — it remains mostly as CH₃COOH molecules in solution. Therefore it cannot be broken into ions in the ionic equation; it appears as the intact molecule. Only strong acids and bases are split into their constituent ions. The net ionic equation shows the undissociated acid reacting with the hydroxide ion. Option A is the net ionic equation for strong acid + strong base neutralization only. Option D is the molecular equation, not the net ionic equation.
Question 3 True / False
The net ionic equation for HCl reacting with NaOH is identical to the net ionic equation for HBr reacting with KOH.
TTrue
FFalse
Answer: True
For any strong acid + strong base pair, the molecular spectator ions (Na⁺, K⁺, Cl⁻, Br⁻, etc.) cancel out, and the net ionic equation is always H⁺(aq) + OH⁻(aq) → H₂O(l). This universality is the key insight of the net ionic equation: regardless of which strong acid or strong base is chosen, the actual chemistry is identical — a proton transfer forming water.
Question 4 True / False
Mixing equal moles of acetic acid and sodium hydroxide produces a solution at pH 7.
TTrue
FFalse
Answer: False
At the equivalence point of a weak acid + strong base reaction, the solution contains sodium acetate (CH₃COO⁻), which is the conjugate base of the weak acid. Because acetate partially accepts protons from water (hydrolysis), the resulting solution is slightly basic — pH greater than 7, not equal to 7. A neutral solution at pH 7 is only expected when a strong acid reacts with a strong base in equal moles, where the resulting salt does not hydrolyze.
Question 5 Short Answer
Why does the net ionic equation for a weak acid reacting with a strong base look different from the net ionic equation for a strong acid reacting with a strong base?
Think about your answer, then reveal below.
Model answer: A weak acid does not fully dissociate in water, so it cannot be represented as separated ions — it must appear as the intact molecule in the ionic equation. A strong acid dissociates completely, so it is written as H⁺(aq) in solution and cancels into the simple H⁺ + OH⁻ → H₂O equation. The weak acid net ionic equation shows the molecular acid reacting with OH⁻ and leaving behind a conjugate base, not just a water molecule.
The core distinction is dissociation extent. Strong acids are essentially 100% dissociated — no intact HA molecules remain. Weak acids are only partially dissociated — most molecules remain intact. Net ionic equations only break apart species that actually exist as separated ions in solution, so weak acids and weak bases always appear as neutral molecules. This also explains why weak acid + strong base reactions produce a buffer or basic solution rather than a neutral one.