Download Chem 2 AP HW 19

Ch. 19.2-19.3: Galvanic Cells; Standard Reduction Potentials
Homework #19-2: Problems pg. 832-833 #4, 5, 10–13, 16, 17
4
Describe the basic features of a galvanic cell. Why are the two components of the cell separated
from each other?
Basic features of a galvanic cell include:
-- two half-cells: oxidation takes place in one half-cell; reduction in the other.
-- anode: electrode where the oxidation occurs (An-Ox)
-- cathode: the electrode where the reduction occurs (Red-Cat)
-- salt bridge: something that connects the two half-cells together and allows ions to flow. The salt
bridge keeps both cells electrically neutral.
-- voltmeter—measures the voltage of cell
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What is the function of a salt bridge? What kind of electrolyte should be used in a salt bridge?
The function is to connect the two half-cells together (this completes the circuit) by allowing ions to
flow between the two half-cells. This keeps both cells electrically neutral. The salt bridge must
consist of ions that do not react with other ions in the solutions or with the electrodes.
10
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Discuss the spontaneity of an electrochemical reaction in terms of its standard emf ( Ecell
)
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An electrochemical reaction is spontaneous (if at standard state conditions) if its standard emf or Ecell
is positive. (This means a voltage can be obtained.) This also means that at equilibrium, products will
be favored.
11
Calculate the standard emf of a cell that uses the Mg/Mg2+ and Cu/Cu2+ half-cell reactions at
25°C. Write the equation for the cell reaction that occurs under standard-state conditions.
Half-reaction
E°(V)
Mg2+(aq) + 2e– → Mg(s)
−2.37
2+
–
Cu (aq) + 2e → Cu(s)
+0.34
The overall equation is: Mg(s) + Cu2+(aq) → Mg2+(aq) + Cu(s)
E° = 0.34 V − (−2.37 V) = 2.71 V
12
Calculate the standard emf of a cell that uses Ag/Ag+ and Al/Al3+ half-cell reactions. Write the
cell reaction that occurs under standard-state conditions.
First write the standard reduction potentials of Al and Ag and determine which is the anode and
which is the cathode.
Ag+(1.0 M) + e– → Ag(s) E° = 0.80 V
Al3+(1.0 M) + 3e– → Al(s) E° = −1.66 V
Since Ag+/Ag is more positive than Al3+/Al, Ag+ will be reduced to Ag and oxidize Al to Al3+.
Anode (oxidation): Al(s) → Al3+(1.0 M) + 3e–
Cathode (reduction): 3[Ag+(1.0 M) + e– → Ag(s)]
Overall:
Al(s) + 3Ag+(1.0 M) → Al3+(1.0 M) + 3Ag(s)
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!
!
!
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Ecell = Ecathode − Eanode = E Ag + /Ag − E Al 3+ /Al
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Ecell
= 0.80 V − (−1.66 V ) = + 2.46 V
Check: The positive value of E° shows that the forward reaction is favored.
2
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Homework #19-2 Answer key
Predict whether Fe3+ can oxidize I– to I2 under standard-state conditions.
The appropriate half-reactions from Table 19.1 are
I2(s) + 2e− → 2I–(aq)
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Eanode
= 0.53 V
Fe3+(aq) + e− → Fe2+(aq)
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Ecathode
= 0.77 V
Since Fe3+/Fe2+ is more positive, it should oxidize I– to I2. This makes the I2/I– half-reaction the
anode. The standard emf can be found using Equation (19.1).
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!
!
Ecell
= Ecathode
− Eanode
= 0.77 V − 0.53 V = 0.24 V
(The emf was not required in this problem, but the fact that it is positive confirms that the reaction
should favor products at equilibrium.)
16
Predict whether the following reactions would occur spontaneously in aqueous solution at 25°C.
Assume that the initial concentrations of dissolved species are all 1.0 M.
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must be positive for a spontaneous reaction. In each case, we can calculate the standard cell emf
Ecell
from the potentials for the two half-reactions.
!
!
!
Ecell
= Ecathode
− Eanode
→ Ca2+(aq) + Cd(s)
(a) Ca(s) + Cd2+(aq) ⎯⎯
E° = −0.40 V − (−2.87 V) = 2.47 V. The reaction is spontaneous.
→ Br2(l) + Sn(s)
(b) 2Br–(aq) + Sn2+(aq) ⎯⎯
E° = −0.14 V − 1.07 V = −1.21 V. The reaction is not spontaneous.
→ 2Ag+(aq) + Ni(s)
(c) 2Ag(s) + Ni2+(aq) ⎯⎯
E° = −0.25 V − 0.80 V = −1.05 V. The reaction is not spontaneous.
→ Cu2+(aq) + Fe2+(aq)
(d) Cu+(aq) + Fe3+(aq) ⎯⎯
E° = 0.77 V − 0.15 V = 0.62 V. The reaction is spontaneous.
17
Which species in each pair is a better oxidizing agent under standard-state conditions?
From Table 19.1 of the text, we compare the standard reduction potentials for the half-reactions. The
more positive the potential, the better the substance as an oxidizing agent.
(a)
(b)
(c)
(d)
Br2 or Au3+? Au3+
H2 or Ag+? Ag+
Cd2+ or Cr3+? Cd2+
O2 in basic media or O2 is
acidic media? O2 in acidic
media.