Determination of Primary Cell's Electromotive Force

Determination of Primary Cell's Electromotive

Force

Contents

Purposes and Demands Principle Apparatus and ReagentProcedure Data Records and Processing QuestionAttentionsDemonstrated

Purposes and Demands

Determine electromotive force of Cu-Zn cell and potential for both electrodes with Cu and Zn.

Understand what’s the meaning of reversible cell, reversible electrodes and salt bridge.

Learn to prepare electrodes and some other dealing methods.

Command the principle and technique of the method which using potential to determine electromotive force. potentiometer

Principle

A cell is composed of positive and negative electrodes. In the discharge process, deoxidize reaction takes place in positive electrode and oxidation reaction occurs in another electrode.

Beside considered as electricity source, a cell is also used to do researches about its properties of thermodynamics.

Thermochemistry said, provided certain temperature, pressure and reversible state, there is an equation from the reaction that is expressed:

△G=-nFE (1)

Where △G is the free energy change, F is the Faraday constant(F=96500C), n is the molar of electrons which transfer through the electrochemistry reaction, and E is the electromotive force.

E which has been determined will be taken into account the free energy change △G and any other thermochemistry functions in the experiment.

It is important that the condition should be controlled in constant temperature, pressure and reversible.

Firstly, the reaction in cell must be reversible. It is to say that both the process of charging and discharge occur in an equilibrium state and that it allows hardly any current runs through the cell.

Makeup of batteries ： Zn│ZnS04(C1)‖CuS04 (C2)│Cu

Reaction in the negative electrode Zn→Zn2+ （ аZn2+ ） +2е

Reaction in the positive electrode Cu2+(аCu2+)+2е→Cu

The general reaction is Zn+Cu2+

(аCu2+)→Zn2+(аZn 2+)+Cu

Free energy change

△G =△G°+RTlnаZn2+ *аCu/аCu2+ *аZn (2) Activity of pure solid is 1.

аZn =аCu =1 (3) Combining Eq. (2) with Eq. (3), we

obtain△G=△G°+RTlnаZn2+/аCu2+ (4)

Making use of Eq. (1), we find the relationship between electromotive force and activity

E=E°-RT/2F×lnаZn2+/аCu2+ (5)

Where E°is the electromotive force when both activity of cuprum and zinc ion are equal to 1.

Since the general reaction contains two reactions in two electrodes, the electromotive force can be expressed as the potential difference by the two electrodes.

Given potential of positive electrode

is φ+ and potential of negative

electrode is φ- , thus it can be stated as the following equation:

E=φ+ -φ- (6)

To Cu-Zn battery, we obtain two equations for φ+ and φ-

φ+=φ°Cu2+-RT/2F×lnаCu/а Cu2+(7)

φ-=φ°Zn2+-RT/2F×lnаZn/а Zn2+(8)

At 25℃,the two electrode potential determined in condition which presents two equilibrium processes Cu- Cu2+ and Zn- Zn2+ when activity both of Cu2+ and Zn2+ are equal to 1 are called standard potential.

Apparatus and ReagentPotentiometer 1Standard cell 1Calomel electrode 1Zinc electrode 1Beaker (50ml) 1

ZnSO4 ( 0.1000M)

CuSO4 (0.01000M)

CuSO4 (0.1000M)KCl (saturated)Sheet copper (used for plating)Milliammeter (0－25－50)

Procedure

1. Preparation of electrodes 1)    Zinc electrode It should be burnished with sand paper

and soaked in diluted vitriol in order to remove oxide on surface. Then it was washed clean for standby.

(2)    Cuprum electrode Soak the sheet copper instead of

diluted nitric acid and burnish it. Wash and clean it for standby.

(3)    Assemble the battery.

Make a salt bridge. Dissolve 3g of agar-agar in 20-30 ml hot KCl saturated-solution and immediately transfer it to U tube then allow it stand for several min. Assemble two electrodes and the salt bridge to form a Cu-Zn battery.

2. Determination of electromotive force(1) Connect the electrocircuit with instruments

of potentiometer and galvanometer.

(2) According to the following temperature proofread formula

Et/v=1.0186-4.06×10-5(t/℃-20)-9.5×10- 7×(t/℃-20)2

determine electromotive force of standard battery at room temperature.

(3) Demarcate electric current of potentiometer, base on the value calculated above.

(4) Determine electromotive force in terms of battery below

Zn│ZnS04(0.1000M)‖CuS04 (0.1000M)│Cu （ A ）

Cu│CuS04 (0.1000M)‖CuS04 (0.1000M)│Cu （ B ）

Zn│ZnS04 (0.1000M)‖KCl(sat)│Hg2Cl2│Hg （ C ）

Hg│Hg2Cl2│KCl(sat)‖CuS04 (0.1000M)│Cu （ D ）

Sketch-maps of Equipments

Data Records and Processing

1. Calculate potential of calomel electrode by the temperature proofread formula written in attaching table V-16 at room temperature.

2.Figure out academic value of electromotive force of calomel battery by Eq. (6), (7) and (8).

3.Fill in the table with true values of electromotive force that are determined from experiment.

battery （ A ） （ B ）Etrue    

Eacademic    

True Academi

c φZn 2+  

φCu2+    

Questions

1.Did voltage can be used to determine electromotive force?

2.Why did a galvanometer points to one direction when determine electromotive force?

Attentions

Click the big-small key rapidly, and stay no more time.Don’t move the galvanometer in the determination process. Regulate sensibility in the lowest switch for start that turn from 0.01X to 1X

Determination of electromotive force

1.Connect the electrocircuit. Adjust the galvanometer and U-25

type potentiometer to zero value. Go on after teacher’s examination.

2.     Proofread the standard battery.

Calculate electromotive force of standard battery at room temperature as stated above.

The process of regulate galvanometer to zero:

Turn knob K to N.

(1) Regulate knobs of ‘big’ and ‘middle ’to make the galvanometer point to zero value.

(2) Regulate knobs of ‘small’ and ‘little’ to adjust the galvanometer to zero.

3.Determination of electromotive force. Turn knob K to X1.

Connect electrode with ‘unknown’ port. Turn knobs from ‘big’ to ‘little’ and

adjust the keyboard Ux to make galvanometer to zero value.

Mark down the value of Ux.