Question

Write a net ionic equation for the reaction that occurs when excess hydroiodic acid and chromium(II) carbonate (s) are combined.

Answer 1

Step 1: Write the balanced "molecular equation:

$2HI(aq) + CrCO₃ (s) → CrI₂ (s) + H₂O(l) + CO₂(g)$

Step 2: Carbon dioxide and water are written in molecular form. Consult the solubility and net ionic equation rules on the information page to determine which of the other substances will dissociate:

$2HI(aq) + CrCo₃ (s) → Crl₂ (s) +H₂O(l)+CO₂(g)$

yes no yes

Step 3: Dissociate all soluble salts, strong acids, and strong bases (except calcium hydroxide), Leave together all "not soluble" salts and weak acids or bases:

${2H₂O}^{ + } (aq) + {2I}^{ - } (aq) + CrCO₃ (s)→ {Cr}^{2 + } (aq) + {2I}^{ - }(aq) + 3H₂O(l) + CO₂(g)$

Step 4: Cross out "spectator lons that appear on both sides of the reaction (these lons do not participate in the chemistry) and rewrite the "net" reaction using the smallest possible coefficients:

${2H₂O}^{ + } (aq) + CrCO₃ (s) →3H₂O + CO₂ + {Cr}^{2 + }$