5 Steps to a 5 AP Chemistry

1. D—You may wish to review the Kinetics chapter if
   you have forgotten what the activation energy is.


2. A—The free energy is the minimum energy
   required for a non-spontaneous reaction and the
   maximum energy available for a spontaneous
   reaction.


3. C—This is a basic postulate of kinetic molecular
   theory.


4. B—This is the reverse of the lattice energy
   definition.


5. D

2[C(s) +O 2 (g) →CO 2 (g)] 2(−393.5 kJ)

H 2 (g) +(1/2) O 2 (g) →H 2 O(l) −285.8 kJ

2 CO 2 (g) +H 2 O(l) →−(−1299.8 kJ)

C 2 H 2 (g) +(5/2) O 2 (g)

2 C(s) +H 2 (g) →C 2 H 2 (g) 227.0 kJ

6. B—The system is insulated and no work can be
   done on or by the system (rigid container); thus,
   the energy is constant. At the melting point,
   some of the gallium will melt and some will
   freeze, the entropy change of these two processes
   cancel each other so there is no net entropy
   change.


7. E—The process is endothermic (the ammonium
   chloride is absorbing heat to cool the water).
   Endothermic processes are “helped” by higher
   temperatures. Answers A and C, and possibly D
   would give an increase in temperature. There is
   insufficient information about answer B.


8. E—The reaction showing the greatest increase in
   the number of moles of gas will show the greatest
   entropy increase. If no gases are present, then the

greatest increase in the number of moles of liquid

would yield the greatest increase.

9. D—If the reaction occurs readily, it must be spon-
   taneous. Spontaneous reactions require ΔG°<0.
   A negative free energy leads to a large K(>1).


10. A—Heat is required to melt something (ΔH>0).
    A transformation from a solid to a liquid gives an
    increase in entropy (ΔS).


11. E—Equations I and III both have an overall
    decrease in the amount of gas (high entropy)
    present. Equation II produces more gas (increases
    entropy).


12. C—Nonspontaneous →ΔG> 0

Since the reaction becomes spontaneous, the sign

must change. Recalling: ΔG=ΔH−TΔS. The

sign change at higher temperature means that the

entropy term (with ΔS>0) must become more

negative than the enthalpy term (ΔH>0).

13. C— [2(436 kJ) +499 kJ] −{2[2(464 kJ)]}
    =−485 kJ


14. A—This is the definition of ionization energy.


15. A—The reaction that produces the most gas will
    have the greatest increase in entropy; the one
    losing the most gas would have the greatest
    decrease.


16. A

CO(g) +2 H 2 (g) →CH 3 OH(g) ΔH =−91 kJ

CH 3 OH(g) →CH 3 OH(I) ΔH =−37 kJ

CO(g) +2 H 2 (g) →CH 3 OH(l) ΔH =−128 kJ

17. B—Dissolving almost always has: ΔS>0. A decrease
    in temperature means the process has: ΔH>0.

134  Step 4. Review the Knowledge You Need to Score High

 Answers and Explanations

 Free-Response Questions

Answer the following questions. You have 10 minutes, and you may not use a calculator.

You may use the tables at the back of the book.

Xe(g) +3F 2 (g) XeF 6 (g)

Under standard conditions, the enthalpy change for the reaction going from left to right

(forward reaction) is: ΔH°=−294 kJ.