Question # 4 on the Free Response AP Exam is a predicting products question. The exam gives the reactants in word form and asks for an unbalanced, net-ionic complete reaction.
Net Ionic reactions
To write net ionic reactions, separate all aqueous, soluble, ionic compounds into ions. Only subscripts within polyatomic ions remain—all other subscripts are changed to coefficients. Cross out all spectator ions and re-write the equation with remaining ions. The AP Exam will always have “reactions”—all the ions will never cancel out.
Do not dissociate any covalent compound, solid, liquid or gas, weak acid or base (even when in solution). Do dissociate soluble ionic compounds and strong acids and bases when in solution.
The question awards 1 point for correctly writing the reactants in formula form and for dissociating any that should. Common phrases used include:
Solid, powdered, metal, filings, strip = solid (don’t dissociate)
Solution = dissociate any soluble ionics or strong acids/bases
Suspension = solid precipitate spread throughout water. Don’t dissociate
If there is a solution or water in the reactants—assume that the water (solution) is still there in the products and break up any soluble ionic compounds or strong acid/bases in the products.
Acidified solution = make H+ one of the reactants
Types of reactions:
Double replacement—precipitation: 2 ionic compounds reacting together
- Switch cations with anions of the reactants. Write the new compounds, balancing charges with subscripts.
- Dissociate any soluble compound that is currently in solution.
- Cross out spectator ions & write net-ionic reaction
Double replacement—acid/base: An acid and a base reacting together
- Remove the proton from the acid and give it to the base.
- Dissociate only strong acids and bases and only if they are in solution.
- Cross out spectator ions & write net-ionic reaction
Decomposition: Only one reactant
- Break the reactant into more than one product.
- Carbonate compounds form CO2 as a product. Ammonium salts form NH3 as a product.
- Dissociate any soluble compound or strong acid/base that is currently in solution.
- Cross out spectator ions & write net-ionic reaction
Composition: More than one reactant forms one product. Often something is added to water or two things are heated together.
- Metals and metal oxides added to water make bases
- Nonmetals and nonmetal oxides added water make acids
- Dissociate any soluble compound or strong acid/base that is currently in solution.
- Cross out spectator ions & write net-ionic reaction
Complex Ion Formation: When something reacts with “excess” hydroxide, cyanide or ammonia.
- Know the common complex ions on the AP exam: Ag(CN)2-1, Fe(SCN)+2, Cu(NH4)3+2, Zn(NH4)3+2, Ag(NH3)2+1, Ni(NH3)6+2, Al(OH)4-1, Zn(OH)4-2
- Dissociate any soluble compound or strong acid/base that is currently in solution.
- Cross out spectator ions & write net-ionic reaction
Combustion: Something reacting with O2
When hydrocarbons are added to O2, they always produce CO2 and H2O
Redox: Charges are changing.
All single replacement reactions are redox reactions.
Other common oxidizers are NO3-1, MnO4-1, Cr2O7-2, H2O2
Dissociate any soluble compound or strong acid/base that is currently in solution.
Cross out spectator ions & write net-ionic reaction
