One of the free-response questions on the AP* Chemistry exam is a reactions question with eight reactions, such as the example sets.  Of the eight, you are to chose any five that you wish to include as the final answer.  If more than five answers are included in the final answer, only the first five will be graded.  Unless a reaction says otherwise, it can be assumed that all solution are in water, that is, aqueous.  Final answers should be in the form of a net ionic equation, meaning compounds that are soluble in aqueous solution should be written as the ions into which they dissociate, or ionize.  Ions or molecules that do not change in the reaction (e.g. spectator ions) should be omitted.  The reactions do not need to be balanced.

To understand the concept of a net ionic equation, we first must be familiar with the solubility rules.  See solubility rules notes.

For example, consider the following reaction:

Solutions of sodium chloride and lead (II) nitrate are mixed.

The regular balanced equation for the reaction with state symbols would be:

If you don't understand why each of the compounds is in the state that it is (e.g.  aq = aqueous, s = solid), then you probably need to brush up on your solubility rules.  See solubility rules notes.

Re-writing the aqueous solutions as their respective ions gives us the following.  State symbols are not required, so we can remove them one we know which compounds are aqueous and will dissociate and which ones are not.

But remember that we cannot have molecules or ions that are on both sides of the reaction because they are unchanged by the reaction.  The spectator ions in this reaction are Na⁺ and NO₃^-.  When these are cancelled out, we are left with:

Our final answers on the reactions question do not need to be balanced, so the final answer for this particular reaction would be:

NOTE:  You should only be working with ions if you are working in solution.  Compounds outside of solution will not spontaneously ionize.

Keywords

There are a few keywords which should alert you that something may be different in the reaction.

• SOLUTION or AQUEOUS SOLUTION - If it is stated that the reactants are solutions, remember to double check which ones are soluble and which ones are not.  The soluble ones should be written as their respective ions and any spectator ions should be cancelled out.
• EXCESS - If it is stated that there is excess of some reactant, the product(s) of the reaction will most likely include as much of that reactant as possible.  In other words, the atom that combines with the excess reactant will react with more of the excess reactant than normal and will generally take on a higher oxidation state.

Consider the following reaction:

Excess chlorine gas is passed over heated powdered iron.

If there were no excess chlorine gas, the reaction would be written as:

However, since there is excess chlorine gas, the iron will take on an oxidation state of +3 instead of +2 and 3 chlorine will bond to the iron instead of 2:

This may seem like a minor difference, but writing FeCl₂ instead of FeCl₃ will most likely result in a point deduction.

Scoring

The reactions question is scored on a 15-point scale in which each of the five reactions in the final answer is worth no more than 3 points a piece.  One point can be earned for the correct reactants, and an additional 2 points can be earned for the correct products for a total of 3 points for a completely correct answer.  The reactions question is worth 15% of the free-response grade.

Example Reactions Question Sets

On the example sets provided, it would probably be best to attempt all eight of the questions for the purpose of practicing to write these type of reactions.  It is much nicer come exam time to know all eight and have to choose which ones to leave out than to only know three and not have a clue on two more that you need.  When choosing the five that you want to include in your final answer, try to pick the reactions that you believe are the most correct or accurate.  Remember that there are point deductions for incorrect charges, failure to write soluble compounds as their respective ions, and failure to omit ions or molecules that are unchanged.

If you think you are ready to tackle some of the example reactions, go to the Reactions Questions Menu.

There are, however, more notes about the types of reactions that could appear on the reactions question . . .

Next:  "Overview of Reactions with Water"

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