5 Steps to a 5 AP Chemistry 2019

146 ❯ STEP 4. Review the Knowledge You Need to Score High

❯ Free-Response Question

You have 15 minutes to answer the following question. You may use a calculator and the

tables in the back of the book.

Question

(a) The bond energy of fluorine is 159 kJ mol–1.

i. Determine the energy, in J, of a photon of light needed to break an F–F bond.

ii. Determine the frequency of this photon in s–1.

(b) Barium imparts a characteristic green color to a flame. The wavelength of this light is

551 nm. Determine the energy involved in kJ/mol.

❯ Answer and Explanation

(a) If you do not remember them, several of the equations given at the beginning of the

exam are necessary. In addition, the values of Planck’s constant, Avogadro’s number,

and the speed of light are necessary. These constants are also given on the exam.

i. This is a simple conversion problem:







 ×

















159 kJ

mol

1 mol

6.022   10

1, 000 J

(^23) 1 kJ

= 2.64 × 10 –19 J

Give yourself 1 point if you got this answer.

ii. This part requires the equation ΔE = hn. (This equation is given on the equation

page of the AP exam.)

n =

E

h

∆

=

×

×

−

−

2.64  10  J

6.63  10  Js

19

34 = 3.98 ×^10

(^14) s–1
Give yourself 1 point for this answer. If you got the wrong answer in the preced-
ing part but used it correctly here (in place of the 2.64 × 10 –19 J), you still get 1 point.
(b) This can be done as a one-step or two-step problem. The AP test booklet gives you the
equations to solve this directly as a two-step problem. This method will be followed
here. The two equations may be combined to produce an equation that will allow you
to do the problem in one step.
Using c = λn:
n = c
λ

=

 ×













− 

3.0  10  m/s

551 nm

1 nm

10  m

8

9 = 5.4 ×^10

(^14) s–1
Using ΔE = hn:
ΔE = (6.63 × 10 –34 Js)(5.4 × 1014 s–1)(1 kJ/1,000 J)(6.022 × 1023 /mol)
= 2.2 × 102 kJ/mol
Give yourself 1 point for each of these answers. If you did the problem as a one-step
problem, give yourself 2 points if you got the final answer correct or 1 point if you left
out any of the conversions.
Total your points. There are 4 points possible. Subtract 1 point if any answer does
not have the correct number of significant figures.