Chapter 7 · Chemical Reactions
7.8Acid–Base and Gas-Evolution Reactions
12 min · two checks
Predict
What gas forms when hydrochloric acid meets solid sodium hydrogen carbonate?
The idea
- Write a neutralization as acid plus base → salt plus water.
- Predict CO₂, SO₂, or H₂S from the common gas-evolution patterns.
In the Arrhenius picture used here, an acid produces H⁺ in water and a base produces OH⁻. A neutralization is H⁺ + OH⁻ → H₂O, with the leftover ions forming a salt. HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). The net ionic equation for a strong acid and a strong base is always H⁺(aq) + OH⁻(aq) → H₂O(l). You will refine what H⁺ really is — H₃O⁺ — in the acids chapter.
Gas-evolution reactions often look like acid plus a salt of a unstable acid. Carbonates and hydrogen carbonates plus acid give CO₂. Sulfites give SO₂. Some sulfides give H₂S. The intermediate acid (H₂CO₃, H₂SO₃) is not isolated; write the gas and water as products. These are still double-replacement patterns followed by decomposition of the unstable product.
Keep these
- Strong acid + strong base → salt + water. Net: H⁺ + OH⁻ → H₂O.
- Acid + carbonate → salt + CO₂ + H₂O.
- Acid + sulfite → salt + SO₂ + H₂O.