Net ionic equations

Some ions appear in solution as both products and reactants as seen in the reaction below. Potassium and nitrate ions remain dissolved in solution throughout the reaction. A spectator ion is unchanged in its same dissolved, aqueous form before and after the reaction. The potassium and nitrate ions are considered "spectators" because both are present but neither participate in the reaction. Both ions remain dissolved in solution while the two remaining ions, lead(II) and iodide, combine to form a solid precipitate. Read the text aloud

A net ionic equation showing all spectator ions and precipitate

When working with solutions it is common to see only actively reacting ions in the chemical equation. This is called a net ionic equation. Double replacement reactions are frequently written as net ionic equations as well as neutralization and redox reactions you will see in future chapters. The net ionic equation ignores spectator ions and shows only the ions that react to form a new product. Let's write the net ionic equation for the reaction above: Pb(NO₃)₂(aq) + 2KI(aq) → 2KNO₃(aq) + PbI₂(s).

Break apart the aqueous compounds into ions but leave solids, liquids and gases intact. Carry states and coefficients to each ion when present. When subscripts indicate the number of ions, bring the number in front of the ion as a coefficient as seen on nitrate below.

Pb²⁺(aq) 2NO₃⁻(aq) + 2K⁺(aq) 2I⁻(aq) → 2K⁺(aq) 2NO₃⁻(aq) + PbI₂(s)
Note: Make sure the equation is balanced.
Cancel all ions that appear in the same state on both sides of the equation.

Pb²⁺(aq) ${2NO}_{3}^{-} (aq)$ + ${2K}_{}^{+} (aq)$ 2I⁻(aq) → ${2K}_{}^{+} (aq)$ ${2NO}_{3}^{-} (aq)$ + PbI₂(s)
Rewrite the equation with the remaining substances.

Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)

Write the net ionic equation for: MgCl₂(aq) + Na₂CO₃(aq) → MgCO₃(s) + 2NaCl(aq).

Relationships

The net ionic equation shows only substances that react.

Solve

Break apart aqueous substances and leave solids intact. Include states and coefficients.

Mg²⁺(aq) 2Cl⁻(aq) + 2Na⁺(aq) CO₃^2-(aq) → MgCO₃(s) + 2Na⁺(aq) 2Cl⁻(aq)

Cancel ions that appear the same on both sides

Mg²⁺(aq)

{2Cl}_{}^{−} (aq)

{2Na}_{}^{+} (aq)

CO₃^2-(aq) → MgCO₃(s) +

{2Na}_{}^{+} (aq)

{2Cl}^{-} (aq)

Rewrite the reaction with the remaining substances.
Mg²⁺(aq) + CO₃^2-(aq) → MgCO₃(s)

Answer

The net ionic equation is: Mg²⁺(aq) + CO₃^2-(aq) → MgCO₃(s)


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