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How to Correctly Write Net Ionic Equations
1. Write the correct molecular equation. (a) Be sure all substances participating in the reaction are included. All reactants on left All products on right (b) Be sure all substances are written with their correct formulas e.g., Barium hydroxide is Ba(OH)2 Chlorine gas is Cl2 Sodium sulfide is Na2S (c) Be sure the equation is properly balanced. (d) Include state labels, writing (aq) after substances in aqueous solution, (s) after any material that starts out solid (on left) or that form a precipitate (on right), (l) after liquids (including H2O), and (g) after gases.
2. Re-write the equation as a total ionic equation. (a) Write solids, liquids, and gases exactly as they were written in the molecular equation. NO CHARGES SHOULD BE WRITTEN WITH THE FORMULAS OF NEUTRAL ELEMENTS AND COMPOUNDS. (E.g., H2O, not H2O+) (b) If substances in aqueous solution are ionic, write their formulas as separate ions. Be sure that charges ARE written for individual separated ions. e.g., HBr(aq) is written as H+(aq) + Br-(aq) BaCl2(aq) is written as Ba2+(aq) + 2 Cl-(aq) NaHCO3(aq) is written as Na+(aq) + HCO3-(aq) Al2(SO4)3(aq) is written as 2 Al3+(aq) + 3 SO42-(aq) (c) If substances in aqueous solution are primarily molecular, do NOT write their formulas as separate ions. e.g., H2O(l) is written as H2O(l) -- exists almost entirely as molecule, not as ions. H3PO4(aq) is written as H3PO4(aq) -- weak electrolyte, mostly unionized in sol=n. NH3(aq) is written as NH3(aq) -- also a weak electrolyte. HC2H3O2(aq) is written as HC2H3O2(aq) -- another weak electrolyte. (d) Learn to distinguish between molecules and ions when they have similar formulas. e.g., NO2(g) is the neutral molecule nitrogen dioxide; NO2-(aq) is the nitrite ion. SO3(g) is the neutral molecule sulfur trioxide; SO32-(aq) is the sulfite ion.
3. Cancel species appearing on both sides of the equation to produce the net ionic equation for the reaction. Include state labels. (a) You must have the same species to cancel. e.g., 2 H+(aq) does not cancel H2(g) Zn(s) does not cancel Zn2+(aq) (b) Partial cancellation is allowed. e.g., 4 Cl-(aq) on left and 2 Cl-(aq) on right are equivalent to 2 Cl-(aq) on left.
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