Ionic and Net Ionic Equations (for double displacement reactions)
Recall double displacement reactions.
-Double displacement reactions occur when two bivalent ionic compounds exchange cations.
-These reactions occur in water and form aqueous solutions since water is the solvent.
-When this occurs, water is not part of the reaction but only acts as a medium in which the reaction can occur.
-Some of the ions will react chemically to form a precipitate (a new solid compound: an insoluble product in a reaction).
Possible products...
i) precipitate (consult solubility tables)
ii) gas (H2S, SO2, CO2, NH3 -- from sulfides, sulfites, and carbonates reacting with acid, and NH4OH reacting with base)
iii) water (neutralization)
-The other ions will simply interact with the water and dissolve in it. These ions do not participate in the chemical reaction and just like water, they are simply there.
-Spectator ions: Non-reacting ions in an aqueous solution (they dissolve in water because they remain as ions).
Example: combining a solution of silver nitrate and sodium chloride.
One solution contains Ag+ and NO3- ions.
The other solution contains Na+ and Cl- ions.
When the Ag+ and Cl- ions meet in the mixed solution, they bond together and form solid AgCl.
The solid AgCl precipitates and eventually settles to the bottom of the container.
The Na+ and NO3- ions stay dissolved in the
water. They did not react.
The entire reaction can be described by the reaction equation
AgNO3 (aq) + NaCl (aq) → AgCl (s) + NaNO3 (aq)
2AgNO3(aq) + Na2CrO4(aq) à Ag2CrO4(s) + 2NaNO3(aq)
Ionic Equation: replaces the formulas of soluble ionic compounds with the ions that these compounds form in water (you must know how to use solubility guideline tables).
2Ag+ (aq) + 2NO3- (aq) + 2Na+(aq) + CrO4-(aq) à Ag2CrO4(s) + 2Na+ (aq) + 2NO3- (aq)
Net ionic equation: an ionic equation that does not include the spectator ions (this is used to show you what is occurring during the reaction. It leaves all the unimportant aspects out. That is, it leaves all aspects that are not part of the reaction out).
2Ag+ (aq) + 2NO3- (aq) + 2Na+(aq) + CrO4-(aq) à Ag2CrO4(s) + 2Na+ (aq) + 2NO3- (aq)
2Ag+ (aq) + CrO4-(aq) à Ag2CrO4(s)
The chemical change that occurred is described by the net ionic equation
Ag+ (aq) + Cl- (aq) → AgCl(s)
NO3- and Na+ are called spectator ions because they don’t participate in the chemical reaction.
To write a net ionic equation for a reaction:
1. Write out the entire balanced chemical equation.
2. Use solubility guidelines to figure out which compounds are soluble in water.
3. Break the soluble compounds into their ions (be careful to get the number of ions and their charges correct).
4. The ions that are unchanged on both sides of the equation did not take part in the reaction. Eliminate these spectator ions from the equation.
5. Write the net ionic equation.
Guidelines for writing net ionic equations:
Solutions of all soluble salts are written as ions. (See solubility rules & table)
Solutions of all strong acids* are written as ions.
Solutions of all strong bases* are written as ions.
All covalent compounds are left in molecular form (with states indicated (s) (l) (g)Äie. do not write out as ions
Insoluble ionic compounds are written as formulas not ions.
Only ions and compounds that have reacted are included. (ie. no spectator ions)
Equation is balanced.
*Some common strong acids and bases for reference...
Strong Acids / Strong BasesHCl, HNO3, H2SO4, HClO4 / NaOH, KOH,
and Ba(OH)2
Write the ionic and the net ionic equations for each of the following chemical equations:
1. K2SO4(aq) + BaCl2(aq) ® BaSO4(S) + KCl(aq)
2. Ca(OH)2(s) + 2 HNO3(aq) ® Ca(NO3)2(aq) + H2O(l)
3. Cl2(g) + 2 NaBr(aq) ® Br2(aq) + 2 NaCl(aq)
4. Pb(NO3)2(aq) + 2 NaBr(aq) ® PbBr2(s) + 2 NaNO3(aq)
5. 3 NaOH(aq) + H3PO4(aq) ® Na3PO4(aq) + 3 H2O(l)
6. 3 Na2CO3(aq) + 2 H3PO4(aq) ® 2 Na3PO4(aq) + 3 H2O(l) + 3 CO2(g)
7. Pb(CH3CO2)2 + (NH4)2S(aq) ® PbS(s) + 2 NH4CH3CO2(aq)