5 Steps to a 5 AP Chemistry 2019

AP Chemistry Practice Exam 2 ❮ 361

Question 2

2 NO(g) + Cl 2 (g) → 2 NOCl(g)

Thermodynamic values related to the above reaction are given in the table below.

SUBSTANCE DHFç (kJ/mol) Sç (J/mol K) BONDS

BOND ENERGIES

(kJ/mol)

NO(g) 90 210 N–O 170

Cl 2 (g) 0 220 N=O 610

NOCl(g) 50 260 Cl–Cl 240

N–Cl 200

(a) Determine the enthalpy change for the above reaction.

(b) Assuming that the NO bond in NOCl is a double bond, estimate the NO bond energy in NO.

(c) Does the NO bond energy in NO agree with any of the values in the table? What is there about the

electronic structure of NO that might be the cause of this observation?

(d) Calculate the entropy change for the reaction.

(e) Is this reaction spontaneous or nonspontaneous at 25°C? Justify your prediction.

Question 3

5 Fe^2 +(aq) + MnO 4 - (aq) + 8 H+(aq) → Mn^2 +(aq) + 5 Fe^3 +(aq) + 4 H 2 O(l)

The above reaction is to be used in the analysis of a sample containing iron metal. A sample is dissolved in

acid and the iron reduced to iron(II) ions with a little solid zinc. Excess zinc is filtered from the solution

and a titration with standard potassium permanganate is performed immediately before air has had time

to oxidize any of the iron(II) ions. The presence of a permanent pink color of excess permanganate ion

indicates the endpoint of the titration.

(a) It is possible to standardize the potassium permanganate solution using solid iron(II) ammonium sul-

fate hexahydrate, Fe(NH 4 ) 2 (SO 4 ) 2 · 6H 2 O. You are given a potassium permanganate solution. Outline

the general experimental procedure (not the calculations) for standardizing this solution.

(b) Show how to calculate the concentration of the potassium permanganate solution.

(c) Show how to calculate the percent iron in an unknown.

(d) If some of the excess zinc was not filtered from the solution, would the reported percentage of iron in

the sample be higher, lower, or the same as when the zinc is removed? Explain.

(e) Standardization with iron(II) ammonium sulfate hexahydrate has the advantage of employing the

same reaction as in the iron determination. However, there is a problem with using this compound to

standardize the potassium permanganate. What might this problem be?

(f ) Give the electron configuration of iron(II) ions, Fe^2 +.

Question 4

Ca(CN) 2 (s) + H 2 SO 4 (aq) + 2 H 2 O(l) → 2 HCN(g) + CaSO 4 · 2H 2 O(s)

It is possible to generate HCN gas by the above reaction. The gas is extremely toxic and great care is neces-

sary when using this compound or related cyanides in any form.

(a) Calculate the number of moles of HCN present in 1.00 L of this gas at 273 K and 1.00 atm.

(b) What volume of HCN gas would the reaction of 1.00 g of Ca(CN) 2 with excess H 2 SO 4 form? The

volume is measured at 298 K and 1.00 atm. The molar mass of Ca(CN) 2 is 92.1 g mol-^1.

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