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Temperature Dependence of emf

Teaching Assistant: TA Name

Chem 375-003

Group #5

2/6/96



Lab Partners:

Name 1

Name 2



Introduction & Theory

An electromotive force, or emf, may be generated in an electrochemical cell by separating a spontaneous reaction into two half-reactions and forcing the electrons involved to travel through a circuit to complete the reaction. The physical location of the half-reaction that produces excess electrons is called the anode, and the location lacking electrons is termed the cathode. In the cell used in this experiment, the half-reaction at the anode is

and the half-reaction at the cathode is

eq. (1) and eq. (2) leads to a net reaction of

where (s) implies pure crystalline solid, (aq,c) implies aqueous solution of concentration c, and (Hg,X2) implies a single phase liquid amalgam cadmium with a mole fraction of X2.

A cell such as that described above, when operating reversibly, isothermally, and isobarically with only electrical work and expansion work has an increase in free energy given by the following equation

where G is the increase in free energy brought about by the passage of n faradays of electricity through the cell, is the emf, and , the Faraday constant is equal to 96485.31 C equiv-1.

At constant temperature and pressure, the Gibbs-Helmholtz equation gives us the following relationship for a change in state

To find S, a combination of eqs. (4) and (5) yields

Finally, given G, S, and T, H can be found by

Goal

The goal of this experiment is to determine the change in free energy, entropy, and enthalpy in an electrochemical cell.

Apparatus

A precision voltmeter, standard cell (such as a Weston cell), 50 mL beaker, platinum electrode, J electrode, to leads suitable for connection to electrodes, two beakers, battery jar, 1 kg ice, 150 mL 0.1 M CdSO4, constant temperature baths at 15, 25, and 35 C, ring stand, large ring stirrer, large clamp and clamp holder will be used to take measurements.

To prepare the apparatus, 1 mL mercury, length of cadmium rod, eye dropper, and a 1.5 V dry cell will be used.









Figure 1 - Cell Apparatus 2

Experimental Observables

In this experiment, changes in voltage will be measured using a precision digital voltmeter and temperature will be measured with a thermometer.

Experimental Method

The temperature dependence of the emf of an electrochemical cell is to be measured with a precision voltmeter. The specific cell to be studied is

Experimental Procedure

Using dilute acid, remove the oxide coating from a thin rod of Cd. Then rinse the rod in distilled water, dry and weigh. Dissolve a portion of the rod in a known weight of triple distilled mercury to create an amalgam that is 2% cadmium by weight. After filling the cup of the J electrode with the amalgam, insert the electrode into a cell filled halfway with 0.1 M CdSO4.

Electroplate the platinum wire of the other electrode by placing it in a beaker containing 0.1 M CdSO4 and connecting it to the negative terminal of a 1.5 V dry cell battery. Connect the positive terminal of the batter to a cadmium rod in the solution until a heavy deposit of cadmium is seen on the electrode. Carefully transfer the electrode to the cell, taking care to avoid flaking of the coating and keeping the electrode wet.

Place the cell in a thermostated bath and allow to equilibrate for at least 10 minutes. Connect the negative connection from a digital voltmeter to the J electrode, and the positive connection to the straight electrode. Measure the emf at least three times with at least five minutes between measurements to verify that no systematic drift is occurring. Before each measurement, the voltmeter should be checked by measurement of a standard such as a Weston cell. The emf should be determined at a total of five points from 0 C to 40 C.

Safety(1)

Metallic cadmium is an anticipated carcinogen, strictly avoid direct contact. In the event of skin contact, quickly clean affected area with soap and water. Avoid uncontrolled exposure of cadmium to strong oxidants, nitrates, or nitric acid.

Mercury and mercury vapor are highly poisonous. Inhalation of vapor may lead to fever, nausea, vomiting, diarrhea, headache, chest pain, and possibly death. Skin contact may lead to a rash or allergic reaction, and if extensive may also cause the same effects as inhalation of vapor. Affected persons should be removed to fresh air and contaminated clothing removed. Necessary first aid techniques should be performed. Seek medical attention immediately.

Cadmium Sulfate is a carcinogen, avoid contact with skin, mucous membranes, or eyes. In the event of skin contact, quickly clean affected area with soap and water. Flush mucous membranes or eyes with copious amounts of water in the event of contact. Cadmium sulfate is poisonous, do not ingest.

References(2)

1. Safety data taken from the internet at "http://www.enviro-net.com/technical/msds",

"http://www-chem.uscd.edu/Courses/CoursePages/Uglabs/MSDS".

2. D.P. Shoemaker, C.W. Garland, J.W. Nibler, Experiments in Physical Chemistry, 6th ed., The McGraw-Hill Companies (1996).