Answers and Explanations
First Free-Response Question
a. 4 OH−+Al^3 +→Al(OH) 4 −
The aluminum ion behaves as the Lewis acid, because it is accepting electron pairs from
the hydroxide anion.
You get 1 point for the correct formulas for the reactants and products and 1 point for
the correct coefficients in the balanced equation. Answering the associated question
correctly is worth 1 point.
b. 5 Fe^2 ++8 H++MnO 4 −→5 Fe^3 ++Mn^2 ++4 H 2 O
The purple color of the permanganate ion slowly disappears as the reaction proceeds.
You get 1 point for the correct formulas for the reactants and products and 1 point for
the correct coefficients in the balanced equation. Answering the associated question
correctly is worth 1 point.
c. Cl 2 O 7 +H 2 O →2 H++2 ClO 4 −
The oxidation number of the chlorine in the perchlorate ion is +7.
You get 1 point for the correct formulas for the reactants and products and 1 point for
the correct coefficients in the balanced equation. Answering the associated question
correctly is worth 1 point.
There is a maximum of 9 points possible.
Second Free-Response Question
a. Sr +2 H 2 O →Sr^2 ++2 OH−+H 2
The resulting solution is basic owing to the presence of the hydroxide ion. The
phenolphthalein indicator is pink in basic solutions, so the solution would be pink.
You get 1 point for the correct formulas for the reactants and products and 1 point for
the correct coefficients in the balanced equation. Answering the associated question
correctly is worth 1 point.
b. MnO 2 +4 H++2 Cl−→Mn^2 ++Cl 2 +2 H 2 O
Chlorine is being oxidized from −1 in the chloride ion to 0 in diatomic chlorine gas.
You get 1 point for the correct formulas for the reactants and products and 1 point for
the correct coefficients in the balanced equation. Answering the associated question
correctly is worth 1 point.
c. Pb^2 ++SO 42 −→PbSO 4
The spectator ions are the nitrate ions (NO 3 −) and the ammonium ions (NH 4 +).
You get 1 point for the correct formulas for the reactants and products and 1 point for
the correct coefficients in the balanced equation. Answering the associated question
correctly is worth 1 point.
There is a maximum of 9 points possible.
Reactions and Periodicity 85