8/2/2019 Experiment 5- Thermodynamics of an Electrochemical Cell

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 CHM122L

Experiment No. 5

Thermodynamics of an Electrochemical Cell

DISCUSSION:

The change in Gibbs free energy (the maximum amount of work that can be obtained from a system) of 

a system is related to the changes in enthalpy and entropy by the following equation,

ΔG = ΔH ‐ T ΔS

where G = Gibbs free energy, H = enthalpy, S = entropy and T = temperature.

For an electrochemical cell, the Gibbs free energy can also be defined as

ΔG = ‐nFE 

where n = the number of moles of electrons, F = Faraday’s constant, and E is the electromotive force

(EMF) (voltage).

Measuring the electrochemical potential of a cell at a particular temperature will allow the calculation of 

the maximum work obtainable (DG) but not DH or DS. Using the equations above,

‐nFE = ΔH ‐ T ΔS

so therefore,

The change in entropy ΔS can be obtained from the slope of a plot of E vs T. With ΔG and ΔS now known

for a given temperature, DH can be calculated.

Procedure

Part 1: The Daniell Cell

Place 1.0 M ZnSO4 solution in a 25-mL beaker and 1.0 M CuSO4 solution in another beaker of the samesize. The liquid levels should be the same. In the zinc solution, place a clean strip of zinc, and in the

copper solution, a clean strip of copper. The zinc strip may be cleaned by dipping it in a beaker of about

2 M HCl. The copper strip can be cleaned by dipping it in a beaker of 2 M HNO3. Rinse the electrodes

with deionized water thoroughly before using them.

8/2/2019 Experiment 5- Thermodynamics of an Electrochemical Cell

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Obtain a multimeter along with one red and one black wire. Set up the meter to measure 2V DC Voltage.

Attach the red wire to the V port and the black wire to the COM port. Note the sign of the voltage; it

must be positive (+) or reverse which electrode the red and black probes are touching. When E o cell is

positive, the black electrode is the anode. The cells are connected with a cotton length soaked in 0.5 M

potassium chloride (see Figure One above). Electrons will flow from the zinc electrod e to the copper

electrode because of the reactions occurring in the half-cells:

The overall reaction for the cell is:

Measure the voltage of this cell. Also, measure the voltage of a Daniell cell with 0.10 M ZnSO4 and 1.0 M

CuSO4, and with 1.0 M ZnSO4 and 0.10 M CuSO4.

Starting at near 0 ºC, measure the voltage of your electrochemical cell. Ensure that the electrical wires

that connect your cell to the voltmeter are only attached for the measurement of the voltage. Do not

connect these wires for longer than is absolutely necessary. Slowly warm the cell so that it remains nearto equilibrium and repeat voltage measurements every 5 ºC up to about 50 ºC. Take three voltage

readings at each temperature. MAKE SURE TO WARM SLOWLY TO REMAIN NEAR EQUILIBRIUM.

From your EMF readings, calculate ΔG298K. Plot EMF vs. temperature and determine ΔS.