IA on effect of concentration on emf produced by Mn2+/Cu2+ voltaic cell, measured using energy sensor.

1. Mn2+ Cu2+
Mn Cu
Cu2+
Cu
Mn
Mn2+
-
-
-
-
+
+
+
+
Metal
pair
Conc
MnSO4
Conc
CuSO4
E cell/V Current
/A
Mn/Cu 1.00 1.00 ? ?
Mn/Cu 1.00 0.66 ? ?
Mn/Cu 1.00 0.50 ? ?
Mn/Cu 1.00 0.40 ? ?
Mn/Cu 1.00 0.33 ? ?
Procedure/Method
• Cut metal into (1cm x 1cm) for Mn and Cu
• Pipette 5ml 1M CuSO4 into (+) side of well
• Insert Cu to CuSO4 and connect to (+) side of voltmeter
• Salt bridge by soaking cotton string in saturated NaCI
• Pipette 5ml 1M MnSO4 into (-) side of well
• Measure emf and current of Mn/Cu cell
•Repeat with diff conc CuSO4 shown
1 M 1 M 1 M 0.66 M
Raw data - changing conc of Cu2+
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Expt/V
1 1.00 1.0 0.00 2.253
1 0.66 1.5 0.41 2.245
1 0.50 2.0 0.69 2.240
1 0.40 2.5 0.92 2.231
1 0.33 3.0 1.10 2.214
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Theoretical/V
1 1.00 1.0 0.00 1.520
1 0.66 1.5 0.41 1.515
1 0.50 2.0 0.69 1.511
1 0.40 2.5 0.92 1.508
1 0.33 3.0 1.10 1.506
How changing cathodic conc Cu2+ ion affect cell potential in Mn2+ /Cu2+

2. Mn half cell (-ve)
Oxidation
Cu half cell (+ve)
Reduction
Mn/Cu Cell
-e -e
Mn/Cu half cells
Std electrode potential as std reduction potential
Find Eθ
cell (use reduction potential)
Mn + Cu2+ → Mn2+ + Cu
Mn 2+ + 2e ↔ Mn Eθ = -1.18V
Cu2+ + 2e ↔ Cu Eθ = +0.34V
Mn ↔ Mn2+ + 2e Eθ = +1.18V
Cu2+ + 2e ↔ Cu Eθ = +0.34V
Mn + Cu2+ →Mn 2+ + Cu Eθ = +1.52V
Oxidized sp ↔ Reduced sp Eθ/V
Li+ + e- ↔ Li -3.04
K+ + e- ↔ K -2.93
Ca2+ + 2e- ↔ Ca -2.87
Na+ + e- ↔ Na -2.71
Mg 2+ + 2e- ↔ Mg -2.37
Al3+ + 3e- ↔ AI -1.66
Mn2+ + 2e- ↔ Mn -1.18
H2O + e- ↔ 1/2H2 + OH- -0.83
Zn2+ + 2e- ↔ Zn - 0.76
Fe2+ + 2e- ↔ Fe -0.45
Ni2+ + 2e- ↔ Ni -0.26
Sn2+ + 2e- ↔ Sn -0.14
Pb2+ + 2e- ↔ Pb -0.13
H+ + e- ↔ 1/2H2 0.00
Cu2+ + e- ↔ Cu+ +0.15
SO4
2-
+ 4H+ + 2e- ↔ H2SO3 + H2O +0.17
Cu2+ + 2e- ↔ Cu + 0.34
1/2O2 + H2O +2e- ↔ 2OH- +0.40
Cu+ + e- ↔ Cu +0.52
1/2I2 + e- ↔ I- +0.54
Fe3+ + e- ↔ Fe2+ +0.77
Ag+ + e- ↔ Ag +0.80
1/2Br2 + e- ↔ Br- +1.07
1/2O2 + 2H+ +2e- ↔ H2O +1.23
Cr2O7
2-+14H+ +6e- ↔ 2Cr3+ + 7H2O +1.33
1/2CI2 + e- ↔ CI- +1.35
MnO4
-
+ 8H+ + 5e- ↔ Mn2+ + 4H2O +1.51
1/2F2 + e- ↔ F +2.87
+1.52 V
Cu2+
-
-
-
-
Mn Cu
+
+
+
+
Q
nF
RT
E
E ln

 
Mn +Cu2+→Mn2++Cu E = ?
1M 1M
Mn2+
1 M 1 M
Using Nernst Eqn
E0 = Std condition (1M) – 1.10V
R = Gas constant (8.31)
n = # e transfer (2 e)
F = Faraday constant (96 500C mol -1 )
𝐸 = 𝐸𝑜 −
8.314 298
𝑛(96500)
lnQ
𝐸 = 1.52 −
8.314 298
𝑛(96500)
ln1
E = +1.52 V
Q =
[𝑀𝑛2
+
]
[𝐶𝑢2
+
]

3. Mn half cell (-ve)
Oxidation
Cu half cell (+ve)
Reduction
Mn/Cu Cell
-e -e
Mn/Cu half cells
Cu2+
-
-
-
-
Mn Cu
+
+
+
+
Q
nF
RT
E
E ln

 
Mn + Cu2+→ Mn2++Cu
1M 0.66M
Mn2+
1 M 0.66 M
Using Nernst Eqn
𝐸 = 𝐸𝑜 −
8.314 298
𝑛(96500)
lnQ
𝐸 = 1.52 −
8.314 298
2(96500)
x0.41
E = +1.51 V
Raw data - changing conc of Cu2+
Theoretical value for E
Expt value for E
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Theoretical/V
1 1.00 1.0 0.00 1.520
1 0.66 1.5 0.41 1.515
1 0.50 2.0 0.69 1.511
1 0.40 2.5 0.92 1.508
1 0.33 3.0 1.10 1.506
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Expt/V
1 1.00 1.0 0.00 2.253
1 0.66 1.5 0.41 2.245
1 0.50 2.0 0.69 2.240
1 0.40 2.5 0.92 2.231
1 0.33 3.0 1.10 2.214
y = -0.0321x + 2.2567
R² = 0.8715
y = -0.0129x + 1.5201
R² = 0.9989
0
0.5
1
1.5
2
2.5
0 0.2 0.4 0.6 0.8 1 1.2
E/V
ln Q
ln Q vs E/V
Theoretical fit
-intercept at 1.52V when lnQ = 0
Expt fit
-intercept at 2.25V when lnQ =0
% error =
(𝑬𝒙𝒑𝒕 𝒗𝒂𝒍𝒖𝒆 −𝑷𝒓𝒆𝒅𝒊𝒄𝒕𝒆𝒅 𝒗𝒂𝒍𝒖𝒆)
𝑬𝒙𝒑𝒕 𝒗𝒂𝒍𝒖𝒆
x 100%
% error =
(𝟏.𝟓𝟐−𝟐.𝟐𝟓)
𝟏.𝟓𝟐
x 100% = 48%
Q =
[𝑀𝑛2
+
]
[𝐶𝑢2
+
]

4. y = -0.0321x + 2.2567
R² = 0.8715
y = -0.0129x + 1.5201
R² = 0.9989
0
0.5
1
1.5
2
2.5
0 0.2 0.4 0.6 0.8 1 1.2
E/V
ln Q
ln Q vs E/V
Theoretical value for E
Expt value for E
Theoretical fit
-intercept at 1.52V when lnQ = 0
Expt fit
-intercept at 2.25V when lnQ =0
Better plot using ln Q vs E/V as the conc Cu2+.
Possible plot using Q vs E/V as the conc Cu2+changes
Q
nF
RT
E
E ln

 
𝐸 = 𝐸𝑜 −
8.314 298
𝑛(96500)
lnQ
expt done at 25C
𝐸 = 𝐸𝑜 − 0.0128 lnQ
n=2 for Cu2+
y = 1.5201 -0.0129x
Compared to
theoretical fit
Eo =1.5201V
slope = 0.0129
Compared to
expt fit y = 2.256 - 0.0321x
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Theoretical/V
1 1.00 1.0 0.00 1.520
1 0.66 1.5 0.41 1.515
1 0.50 2.0 0.69 1.511
1 0.40 2.5 0.92 1.508
1 0.33 3.0 1.10 1.506
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Expt/V
1 1.00 1.0 0.00 2.253
1 0.66 1.5 0.41 2.245
1 0.50 2.0 0.69 2.240
1 0.40 2.5 0.92 2.231
1 0.33 3.0 1.10 2.214
y = -0.0184x + 2.2734
R² = 0.9476
2.2
2.22
2.24
2.26
0 0.5 1 1.5 2 2.5 3 3.5
E/V
Q
E/V vs Q
y = 0.0321ln(x) + 2.2567
R² = 0.8751
2.2
2.22
2.24
2.26
0 0.2 0.4 0.6 0.8 1 1.2
E/V
conc Cu2+/M
Conc Cu2+/M vs E/V
Possible plot using conc Cu2+ vs E/V

5. Theoretical value for E
Expt value for E
Poor plot using conc Cu2+ vs E/V. Conc change for Cu2+ is
too small. At least 10 fold change needed.
Q
nF
RT
E
E ln

 
𝐸 = 𝐸𝑜 −
2.303𝑅𝑇
𝑛𝐹
lgQ
𝐸 = 𝐸𝑜 −
2.303 8.314 298
𝑛(96500)
lgQ
Convert ln Q to log Q
expt done at 25C
𝐸 = 𝐸𝑜 −
0.059
𝑛
lgQ
E change by 0.059V per 10-fold change in conc of a substance in a one-elec transfer.
for two elec transfer, change is 0.028V per 10 fold change in conc (Cu2+).
n=2 for Cu2+
Need at least a 10 fold change in conc (Q)
to get a significant voltage change
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Theoretical/V
1 1.00 1.0 0.00 1.520
1 0.66 1.5 0.41 1.515
1 0.50 2.0 0.69 1.511
1 0.40 2.5 0.92 1.508
1 0.33 3.0 1.10 1.506
Mn2+ Cu2+
Q
Mn2+/Cu2+ ln Q Expt/V
1 1.00 1.0 0.00 2.253
1 0.66 1.5 0.41 2.245
1 0.50 2.0 0.69 2.240
1 0.40 2.5 0.92 2.231
1 0.33 3.0 1.10 2.214
y = -0.0321x + 2.2567
R² = 0.8715
y = -0.0129x + 1.5201
R² = 0.9989
0
0.5
1
1.5
2
2.5
0 0.2 0.4 0.6 0.8 1 1.2
E/V
ln Q
ln Q vs E/V
Better plot using ln Q vs E/V as the conc Cu2+.
y = 0.0321ln(x) + 2.2567
R² = 0.8751
2.21
2.22
2.23
2.24
2.25
2.26
0 0.2 0.4 0.6 0.8 1 1.2
E/V
conc Cu2+/M
Conc Cu2+/M vs E/V