Td of Electrochemical Cells Lab Report

Thermodynamics of electrochemical cells- Lab report Electrochemistry studies the relationship between electrical energy and chemical conversions. Electron transfers in the spontaneous redox reactions occurring at the electrodes is at the root of energy changes, and is reflected in the enthalpy changes. The Gibb’s free energy measures the amount of useful work that could be extracted from an electrochemical cell, while entropy change indicates the change in randomness of the system. The purpose of this experiment is to study the behavior of EMF of a galvanic cell at changing temperatures and calculating thermodynamic parameters there from. Primary Data: The experiment was performed as outlined in the lab manual. Two 2+¿/Cu +¿/ Ag electrodes, (anode) and (cathode) were prepared by C u¿ A g¿ dipping the metallic electrodes into a solution of their soluble sulfate salts. Coupling the two electrodes through a salt bridge affords a 0 complete electrochemical cell, having E value of 0.46 V. Keeping the electrolyte concentrations and other physical factors nearly constant, the EMF of this cell was measured and studied at three different temperatures. Resulting electrode potential values are listed as a function of time and temperature. The change in free energy, enthalpy and entropy were calculated from these data utilizing the following thermodynamic relations: ∆ G=−nFE

∆ S=nF

( ∂∂ ET )

P

∆ H=∆ G+T ∆ S

( ∂∂ ET )

P

value was calculated from the slope of the potential vs.

temperature graph.

Table 1: Cell potential vs. time Temperature

Cold = 5.10 ℃

Med = 25.1 ℃

Hot = 68.6 ℃

Time (min) 0 5 10 15 16 17 18 19 20 21 22

Potential (V) 0.3586 0.3722 0.3751 0.37623 0.37647 0.3766 0.3767 0.37676 0.3768 0.37686 0.3769

Potential (V) 0.358 0.35655 0.3534 0.35023 0.35065 0.3510 0.3506 0.34995 0.35045 0.3502 0.35025

Potential (V) 0.323 0.300 0.294 0.28451 0.28094 0.27737 0.27466 0.27219 0.2709 0.26943 0.26807

Table 2: Cell potential vs. temperature

E° (V) Absolute Temp (K)

0.3765

0.3508

0.2769

278.25

298.15

341.75

Theoretical Values at 25°C for the reaction, from literature values:

T (K) 298.15

∆ G(J /mol)

E(V )

∆ S(J /mol . K )

∆ H ( J /mol)

−88766

0.46 00

−192.5

−146131

Results:

From the experimental results, the following graphs were obtained:

Figure 1: Plot of EMF vs. Time at three temperature values

EMF vs. time 0.4 0.35 0.3

EMF (V)

0.25

potential(5.1)

0.2

potential(25.1)

0.15

potential(68.6)

0.1 0.05 0 0

5

10

15

Time (min)

20

25

Figure 2: Plot of EMF vs. Temperature

EMF vs. Temperature 0.40000 0.35000

f(x) = - 0x + 0.82 R² = 1

0.30000 0.25000 EMF (V)

0.20000 0.15000 0.10000 0.05000 0.00000 270

280

290

300

310

320

Temperature (K)

Calculations: From the graph of variation of EMF with temperature, E0298 =−0.00159× 298.15+0.821 3=0.347 2 V ∆ G=−nF E0=−2 ×96485 ×0.34 72V =−6 7007 J /mol

330

340

350

∆ S=nF

( ∂∂ ET ) =2× 96485 ×−0.00159=−307 K .Jmol P

∆ H=∆ G+T ∆ S=−67007

% Absolute error in ¿

calculations

∆S

calculations

307−192.5 × 100 =59.5 192.5

% Absolute error in ¿

∆G

88766−6 7007 ×100 =24.5 88766

% Absolute error in ¿

(−307 ) J J J +298.15 × =−158539 mol mol mol

∆H

calculations

158 539−146131 ×100 =8. 5 146131

Discussion: The equilibrium values of EMF were used. So, the average potential data after about 10 minutes from the beginning of the experiment, when the EMF values stabilized, were considered for relevant calculations. The literature values of ∆ G , ∆ S

and ∆ H

were taken from Table of Constants, Physical Chemistry, 8e by Atkins, P. ; De Paula, J. Oxford University Press, Oxford (2006).

The EMF of an electrochemical cell varies with temperature since the cell reaction involves a change in entropy, ∆S. A rise in temperature causes a rise in ∆S. Assuming ∆H to remain constant, it lowers the ∆G, and hence the potential value. Addition of heat near lower temperatures increased the entropy to a greater degree than at higher temperatures. The purpose of this experiment was to study how changes of temperature affect electromotive force of the electrochemical cell. The variation was nicely demonstrated from the results of this experiment. Sources of errors could be from insufficient stabilization of EMF, error in the assumption that the electrolyte concentration was constant throughout and from the electronic measuring devices. In addition, the errors could also have come from inadequate cleansing of the voltmeter that read the cell. Conclusion The experiment demonstrated the variation of EMF of a

Cu/CuSO 4− A g2 S O4 / Ag

galvanic cell, with variation in temperature and established that as temperature increases, the cell potential decreases. The percent error in the final results of ∆ G , ∆ H∧∆ S based on the experimental data was determined. The absolute error values ranged from 8.5% in ∆ H

measurement to a huge 59% in ∆ S measurement. This may be due to

insufficient homogenization of the electrolyte chambers during heating, or due to some procedural error.