5 Steps to a 5 AP Chemistry 2019

(^86) › STEP 4. Review the Knowledge You Need to Score High
❯ Free-Response Question
On the AP exam, there will be both long-answer and short-answer free-response questions.
The following is an example of a short-answer question.
You have 5 minutes to answer the following question. You may use a calculator and
the tables in the back of the book.
Question
An iron(III) nitrate, Fe(NO 3 ) 3 , solution is mixed with a potassium phosphate, K 3 PO 4 ,
solution, and a precipitate forms. What is the precipitate, and which ions, if any, are spectator
ions in this reaction? Explain how you arrived at your answers.
❯ Answer and Explanation
There are four ions present. These ions are K+, PO 4 3–, Fe^3 +, and NO 3 –. The starting materials
are in solution; therefore, they are soluble. The compounds that might form in the reaction
are KNO 3 and FePO 4. Since ammonium (and nitrate) salts are normally soluble, the pre-
cipitate must be FePO 4. The spectator ions are the nitrate ions (NO 3 – ) and the potassium
ions (K+ ), the ions not in the precipitate.
Total your points. There are 4 points possible. You get 1 point for correctly identifying
the precipitate. You get an additional 1 point for identifying the spectator ions and 2 points
for the explanation.
❯ Rapid Review
● Reaction questions will always appear in the free-response section of the AP exam. This
may not be true in the multiple-choice part.
● Energy may be released in a reaction (exothermic) or absorbed (endothermic).
● Chemical equations are balanced by adding coefficients in front of the chemical species
until the number of each type of atom is the same on both the right and left sides of the
arrow.
● The coefficients in the balanced equation must be in the lowest whole-number ratio.
● Water is the universal solvent, dissolving a wide variety of both ionic and polar substances.
● Electrolytes are substances that conduct an electrical current when dissolved in water;
nonelectrolytes do not.
● Most ions in solution attract and bind a layer of water molecules in a process called
solvation.
● Some molecular compounds, like acids, ionize in water, forming ions.
● In the molecular equation, the reactants and products are shown in their undissociated/
un-ionized form; the ionic equation shows the strong electrolytes in the form of ions;
the net ionic equation drops out all spectator ions and shows only those species that are
undergoing chemical change.
● Precipitation reactions form an insoluble compound, a precipitate, from the mixing of
two soluble compounds.