clr,p
AP@
cHEMrsrRY
2003 SCORING GUIDELINES (Form B)
Question 6
Total Score 9 points
Answer the following questions about electrochemistry.
l
(a) Several different electrochemical cells can be constructed
using the materials shown below. Write
reaction
that
occurs
in the cell that would have the greatest
for
the
the balanced net-ionic equation
positive value
of E|rn
Hil
1.0 M
n
Al
Metal
Strip
.
Al(Nol)1
1.0 M Cu(NOr)z
Cu Metal
Strip
Materials fbr
Salt Bridge
Fe Metal
Strip
2
+2e- +
Solution to
Fill Salt Bridge
Cu(s)
Al(s) + 3 Cuz+(aq)
(b) Calculate the
-->
2
Al3+(aq)
+
I point for correctly
3 Cu(s)
balanced net ionic
equation
standard cell potential, Eo rrl, for the reaction written in part (a).
E =-1.66Y
Al3+(aq)+3e- + Al(s)
Cuz+(aq)*Te- -+ Cu(s) E" : +0.34V
E|ru :
Voltmeter
with Wire
1 point for selection of correct two
redox couples
Al(s) -+ Al3*(aq) + 3 eCuz+(aq)
M Fe(NO-,)2
mH
n
n
1.0
Elanoa, - Ekoa" :
1
point for correct
E"""n
(Must be consistent with part (a))
+0.34
V-(-1.66V) = +2.00V
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{3
AP@ CHEMISTRY
2003 SCORING GUIDELINES (Fornn B)
Question 6 (cont'd.)
(c) A cell is constructed
based on the reaction in part (a) above. Labellthe metal used for the anode on
the cell shown in the figure below.
Voltmeter
Salt Bridge
Cathode
(Anode Metal)
The metal is aluminum solid.
I point for correct metal
(Must be consistent vrith part (a))
(d) Of the compounds NaOH, CuS, and NaNO3 , which one is appropriate to use in a salt bridge?
Briefly explain your answer, and for each of the other compounds, include a reason why it is not
appropriate.
NaOH is not appropriate. The anion, OH-, would migrate
towards the anode. The OH- would react with the Al3+ ion in
solution.
CUS is not appropriate. It is insoluble in water, so no ions would
be available to migrate to the anode and cathode comparhnent to
balance the charge.
i
point for correctly
whether each compound is
appropriate, along with an
explanation (3 points total)
NaNO3 is appropriate. It is soluble in water, and neither the
cation nor the anion will react with the ions in the anode or
cathode compartment.
Coppight O 2003 by College Entrance Examination Board. All rights reserved.
Available at apcentral.collegeboard.com.
14
AP@ CHEMISTRY
2003 SCORING GUIDELINES (Form B)
Question 6 (cont'd.)
(e) Another
standard cell is based on the following reaction.
zn + Pt2+ -> znz* +
If the concentration of h?+ is decreased from
potential? Justiff your answer.
E,,,
1.0
Pb
M to 0.25 M,whateffect
: ELu -0.0s91n (rtr#J
rf fznz+1is reduced, then the *,"
h(tf#i)
does this have on the cell
I point for correctly indicating
how E"nn is affected
(t3^o11od
<
t, therefore
< 0' rhus 8""11 increases (becomes more positive).
I point for explanation in terms
ofNernst equation and Q
Copyright O 2003 by College Entrance Examination Board. All rights reserved.
Available at apcentral,collegeboard.com.
l5
AP@ CHEMISTRY
2004 SCORING GUIDELINES (Form B)
Question 6
Voltmeter
f,lr
lrlr
:
a
-
MAgNO..,
:
-i MZn(NO.*),
The following questions refer to the electrochemical cell shown in the diagram above.
(a) Write a balanced net ionic equation for the spontaneous reaction that takes place in the cell.
Znz+(aq)
* 2 e- --> Zn(s)
Ag*(aq)
+ e- -+
I point for
Ag(s)
1
Zn(s)
+
ZAg+(aq)
the balanced net-ionic equation
point for correct direction (reactants and products)
-+ Zr?*@q; + 2Ag(s)
(b) Calculate the standard cell potential, E, for the reaction in part
+ 2e -+ Zn(s)
Ag*(oq) + e- -+ Ag(s)
Znz+(aq)
E""ru
=
Elo*oa"
-
(c) In the diagram
(i)
(a).
tr :
-0.76Y
E" = *0.80 V
Ekoa" = +0.80V
-
(-0.76
1
V)
:
point for the correct, positive 8"""n
+1.56V
above,
label the anode and the cathode on the dotted lines provided, and
The anode is the zinc metal electrode, the cathode is
the silver metal electrode.
I point
for the correct metal for anode and
cathode
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12
I
AP@ GHEMISTRY
2004 SCORING GUIDELINES (Form B)
Question 6 (cont'd.)
(ii)
indicate, in the boxes below the half-cells, the concentration
of AgNO3
and the concentration
of
Zn(NO3)2 that are needed to generate E"
[Ag].IOrl
:
[Zn(NO3)rl
=IM
(d) How will the cell potential
A precipitate will form
as
I-
1
point for the correct concentration for the anodic chamber
I point for the correct concentration for the cathodic chamber
be affected
ions react with
if KI is added to the silver half-cell?
Ag'
rons
in solution in cathode compartment. [Ag*] will be
reduced, causing cell potential to decrease.
Justify your answer.
I point for correctly indicating a reaction occurs
I point for indicatingthe cell potential decreases
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l
{
CHEMISTRY
2007 SCORING GUIDELINES (Form B)
AP@
Question 3
ZH2G) + Oz(s)
*t
2IjIO(I)
In a hydrogen-oxygen fuel cell, energy is produced by the overall reaction represented above.
(a) When the fuel cell operates at25"C and
Calculate the volume
of
1.00 atm for 78.0 minutes, 0.0746 mol
of
Oz(S) is consumed.
H2(S) consumed during the same time period. Express your answer in liters
measured at25oC and 1.00 atm.
(0.0746mo1o).#A
n-
RT
=
0.14e mol
H,
One point is earned for
calculation of moles of H,
(0.149 mol H2X0.0821L atm mol-r K-1X298 K)
1.00atm
P
One point is earned for
substitution into PV = nRT
,
.
One point is earned for the
answer.
= 3.65LHz
(b) Given that the fuel cell reaction
(i)
write the two half reactions that occur as the cell operates,
o2
+ 4H*
Hz
(ii)
-)
* 4e- --+ zHzO
One point is earned for each
of the two half reactions.
2H+ + 2e-
\\
identify the half reaction that takes place at the cathode, and
O, + 4H* + 4 e- -+
(iii)
takes place in an acidic medium,
2H,zO
One point is eamed for either the equation of the correct half
reaction, or for indicating "the reduction half reaction" if the
correct equation is given in (bXi).
determine the value of the standard potential, E., of the cell.
E=
1.23Y
+0.00V = 1.23V
One point is eamed for the standard potential.
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CHEMISTRY
2A07 SCORING GUIDELINES (Form B)
AP@
Question 3 (continued)
(c)
Calculate the charge, in coulombs, that passes through the cell during the 78.0 minutes of operation as
described in part (a).
One point is earned for the
4 mole'x
Q.0746 mol O" )
lmolO,
x
96,500 C
lmol e-
= 2.88xtdC
@ 2007 The College Board.
stoichiometry.
One point is earned for the
answer.
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reserved.
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