FACULTY OF APPLIED SCIENCES DEPARTMENT

FACULTY OF APPLIED SCIENCES
DEPARTMENT: Analytical Chemistry
DIPLOMA: Analytical Chemistry
Physical chemistry III
Experiment 2
Group D
Date Performed: March 2018
TOPIC: THERMODYNAMICS OF AN ELECTROCHEMICAL CELL: FREE ENERGY , ENTHALPY AND ENTROPY OF REACTION.

NAME: YANGA
SURNAME: NTULI
STUDENT NO. : 216060230
LECTURER: DR T. Oosthuyse

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AIM: Is to determine the enthalpy and entropy changes for a chemical reaction from the temperature dependence of a reversible cell potential.
THEORY
The electrochemical cell is a device that generates a potential difference between electrodes using chemical reaction.
In the experiment the redox reaction involve the transfer of electrons between zinc and lead. The redox reaction consist of two parts, a reduced half and an oxidised half. The reduced forms of metals are present as liquid solution in mercury. The oxidised form lead, Pb2+ (aq), is in equilibrium with solid PbSO4, sparingly solute salt. The oxidised form of zinc exist as ZnSO4 (aq), which is very soluble in water, and this property allows the Zn2+ (aq) composition to be an experiment variable.
The overall reaction is: Zn (Hg) (s) + PbSO4(s) ? Zn2+ (1M) + SO42-(1M) + Pb(Hg)(s)
The direct measurements of the free change associated with a chemical reaction that can be used to do work. The relationship between the standard free energy change for a process conditions, ?G0, and the equilibrium constant, K, is : ?G0 = -RTInK
The relationship between the free energy associated with electron transfer reaction and the cell potential is obtained as follows. The small change in the Gibbs in the free energy for the a process in which electrical work is done is: dG = -SdT+ VdP +dWrev
Where dWrev represent the infinitesimal amount of useful, the non P-V work that can be obtained from the process. The rev subscript indicates that the work is carried out reversibly. In the case of a charge transfer reaction in which an amount of charge dq is transferred under an electrical potential Erev, therefore dWrev = Erevdq
When the electron-transfer reaction takes place takes place at constant temperature and pressure, and if the process takes place so slowly that the cell potential is constant throughout, the resulting change in free energy is ?G = ErevQ, where Q is the total amount of charge transferred, because ?G corresponds to the free energy change when n is the number of moles of electrons flow. The equation can be used ?G = -nFErev
Where F is equal to the charge in coulombs associated with 1 mole of electrons; F= 96485 coulombs mol-1. That equation holds only when the cell voltage represents the reversible process. The previously equation can be combined with the Gibbs equation, ?G=-?H+T?S to obtain, nFE = ?G=-?H+T?S
If the expression is differentiated with respect temperature at constant pressure, P and n, the equation obtained is: nF(?E/?T)P = -(??H/?T)P + T(??S/?T)P+?S
The enthalpy change for the reaction for the reaction,?H, can also be expressed in terms of cell potential measurements by combining nFE = ?G=-?H+T?S and nF(?E/?T)p, therefore ?H=nF(?E/?T)P –E.

APPARATUS
Thermometer
Voltmeter
Water bath
H- cell
Stop watch

PROCEDURE
A diagram of the cell was available in the practical.
It is an H cell in which a fine-porosity glass plug separates the two half-cell compartments.
The amalgams and the solution to be put into the two compartments of H cell will be prepared and putted into the cell in the laboratory.
The cell was clamped securely in the 0? bath and waited for 10minutes for the cell to equilibrate.
The cell leads were connected to a digital voltmeter, only during a measurement.
The cell potential was measured and recorded 8 times with one minute interval to be sure that it has stabilised.
When there is a problem in stability, the electrodes can be manipulated gently in the amalgams, and tapping the cell slightly also helps.
The potential was recorded after a stable, the cell moved to the next temperature and same procedure repeated.
The cell potential measured at about 10 degrees intervals up to 50? .

CALCULATION AND RESILTS
Table 1: Temperature and cell potential
Temperature(?) Cell potential , E (mV)
0 0.718
10 0.715
20 0.712
30 0.709
40 0.706
50 0.703

a) Mean of the cell potential = (0.718+0.715+0.712+0.709+0.706+0.703)/8
= 0.5329mV

b) (0;0.718) ; (20;0.712)

?S = (y2-y1)/(x2-x1)
= ((0.718-0.712)/(0-20)
= -0.003mVK-1
?H= c- intercept
y= mx +c
(0;0.718):
0.178 = -0.003(0) + c
c= 0.718
Therefore ?H = 0.178Mvk-1
Difference in temperature = (0+10+20+30+40+50)/8
= 18.75? = 291.75K
F = 96485 constant
Mean E = 0.5329mV

nF(?E/?T) = ?S
1×96485(0.5329/291.75) = ?S
176.2360Mvk-1 = ?S

At 0? = 273K ; E = 0.718mV
?H = nFE – T?S
= 1 × 96485× 0.718– 273×176.2360
= 21163,8020 mV

At 10? = 283K ; E = 0.715mV
?H = nFE – T?S
= 1 × 96485× 0.715– 283×176.2360
= 19111.9870 mV

At 20? = 293K ; E = 0.712mV
?H = nFE – T?S
= 1 × 96485× 0.712– 293×176.2360
= 17060,1720 mV

At 30? = 303K ; E = 0.709mV
?H = nFE – T?S
= 1 × 96485× 0.709– 303×176.2360
= 15008.3570 mV

At 40? = 313K ; E = 0.706mV
?H = nFE – T?S
= 1 × 96485× 0.706– 313×176.2360
= 12667.0870 mV

At 50? = 323K ; E = 0.703mV
?H = nFE – T?S
= 1 × 96485× 0.703– 323×176.2360
= 10904.727 mV

?H appears to be temperature dependent, because it decrease as the temperature increases.
At 25?
T = 25? between 20? (E=0.712mV) and 30? (E= 0.709mV)
Therefore E at 25? = (0.712+0.709)/2
= 0.7105Mv
?G= -nFE
= -(1 ×96485 ×0.7105)
= -68552.5925mV

DISCUSSION
During the experiment the cell provides the electrical potential energy difference to drive the electrons through the circuit. The cell does work on a charge to move it from the low energy terminal to the high energy terminal. The energy of the cell that is transferred in form of work comes from the chemical potential
The spontaneous redox reaction is characterised by a negative value of ?G which correspond to a positive value of E°Cell. The free energy depends on temperature while entropy and enthalpy are constant over a small temperature range. In the experiment the change in voltage should be determined over a small temperature changes.
Since is impossible to measure directly the potential of an isolated half-cell, the standard hydrogen used as a reference at potential of 0,00V.
The plot of cell voltage versus temperature formed a straight line graph. The slope of the line related to the entropy change of the reaction and the intercept related to the enthalpy change of the reaction.

CONCLUSION

REFERENCE
Haber, S., Salciccioli, K. and Sanader, M. 2011.Chemistry 2. Toronto: Nelson Education.
The South Africa Oxford School Dictionary. 1996. Cape Town: Oxford.

Q-ANSWERS
a) Galvanic cell
b) Electrolytic cell

The digital voltmeter is an instrument used for measuring the potential difference between two points in an electrical circuit. It has a high resistance to ensure that its connection do not change flow of the current in the circuit. The cell leads connected to the digital voltmeter only during measurements in order to get accurate results.

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