Harvard University - Experiment 11 Electrochemistry: Voltaic Cells. Construct voltaic cells, gauge t

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LABORATORY REPORT Experiment 11 Electrochemistry: Voltaic Cells Name: ____________________________________________

Date: ___________________

Lab Partner’s Name: _________________________________

Score: _________________

I.

Introduction

The main purpose of this experiment is to construct voltaic cells, gauge the corresponding cell potentials and compare the calculated cell potentials with the theoretical cell potentials. The experiment is also designed to discover the properties of electrochemical cells. A voltaic cell is a system that is specially prepared where oxidation-reduction (redox) reaction occurs impulsively.

The oxidation reaction occurs in the anode half-cell while reduction reaction occurs in the cathode half-cell. The oxidation and reduction reactions are separated to enable current run through an external wire producing an easily gauged electrical potential. The electrical potential is predicted by use of standard reduction potentials and also can be calculated combining the potentials of half reactions. The half-reaction which is more negative is oxidation hence the reduction reaction is one that is tabulated. The reduction potentials are gauged at standard conditions; I atmosphere, 25 degree Celsius and IM concentration of solutions.

E⁰cell= E⁰reduction +E⁰oxidation The cell potential of a cell operated on non-standard conditions is calculated using Nernst equation.

Ecell = E⁰ -(0.05916/n)log Q 

E = potential at non-standard conditions

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

E⁰ = potential at standard conditions



n = number of moles of electrons being transferred



Q = reaction quotient

In the part I of this experiment, the potential of the voltaic cell with Cu-Pb electrode will be determined using a voltmeter. Two voltaic cells of unknown metal electrodes will also be tested paired with lead (Pb). In part II, the cell potentials of the unknown voltaic cells with be determined through careful measurements. In part III, the potential of a concentration cell (a special voltaic cell) will be measured where two electrodes of the metal with different concentrations will be made. In the first concentration cell, observation will be made on how a voltaic cell maintains spontaneous redox reaction with identical lead electrodes. Measure the potentials of the other two other cells and use Nernst equation to calculate the concentration of the ions to allow the calculation of equilibrium constant values.

II. Results Analysis A. Data and Calculations

Metal

Average Cell Potential

Electrode

(V)

Cu/Pb

0.4969

X/Pb

0.4709

Y/Pb

1.220

Potential reading for Cu/ Pb = 0.4969

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Theoretical reading for Cu/ Pb = 0.47V Difference/ error = (0.4969 - 0.47)V Cell Potential difference/ error = 0.0269 v

No. 1 2 3 4 5

LED Color Red Yellow Ultraviolet Blue green

Voltage (V) On Connection 1.760 1.842 2.618 2.523 2.260

On Disconnection 3.417 3.401 3.394 3.386 3.351

 When the voltage increase, the light intensity increases turning on the LED  Increasing voltage also increases the intensity of light source  The voltage increase upon connection to the Vernier  The voltage drops when the LED is disconnected

B. Discussion In part I and II, the resulting average cell potential for Cu/ Pb is 0.4969, 0.4709 for X/ Pb and 1.22 for Y/Pb. The average calculated cell potential for Cu/ Pb is 0.4969 while the theoretical potential value is 0.47V. The difference/ error may have resulted from the salt bridge not

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soaking long enough or the voltage probes being immersed in the solution along with wires.

The unknown X is Zn whereas Y is magnesium. This is because only PB is only reduced by Zn and Mg because they have higher reducing power. In part III, the voltage reading for battery cells were 0.840 V, 0.877 V, 0.827 V and 0.808 V hence the average voltage of an individual battery cell will be 0.838 volts, and the voltage increases with an increase in the number of battery cells connected.

III. Conclusion From the experiment, it is clear that from every voltaic cell high concentration, and low cell potential along with low cell concentration with high cell potential. The experimental errors in the experiment resulting from the salt bridge not soaking enough in the solution can be prevented by coming up with way to ensure that it is completely immersed and soaked.

References (APA Style) AspenCore, Inc. (2019). The light emitting diode. Retrieved November 1, 2019, from https://www.electronics-tutorials.ws/diode/diode_8.html Randall, J. and Volz, D. (2017). Advanced Chemistry with Vernier, 3rd edition. Electrochemistry: voltaic cells. Beaverton, OR: Vernier Software and Technology.

Tro, N. (2017). Chemistry: A Molecular Approach. 4th Edition. Boston: Pearson.

Wikipedia. (2019). Light-emitting diode. Retrieved November 1, 2019, from https://en.wikipedia.org/wiki/Light-emitting_diode

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DATA AND CALCULATIONS

Parts I and II Results

Metal

Average Cell Potential

Electrode

(V)

Cu/Pb

0.4969

X/Pb

0.4709

Y/Pb

1.220

DATA ANALYSIS 1. (Part I) Compare the average cell potential, for your Cu/Pb cell, with the E°cell that you calculated in the pre-lab exercise. Explain why your cell potential is different from the text value.

The average cell potential was 0.47V while the calculated is 0.4969V. The difference may be because the IM KNO3 salt bridge might have not been soaked enough when inserted in the PB and Cu 2. (Part II) The unknown metals X and Y were either magnesium, silver, or zinc. Use the text value for the reduction potential of Pb and the measured cell potentials for the unknowns to identify X and Y electrodes.

The unknown X is Zinc while the unknown Y is magnesium. This is because lead is only

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reduced by Mg and Zinc because they have high reducing power than Pb, but silver cannot reduce Pb.

Part III

Making of Soil Battery and Light LED Lamps

Table III.1

Cell Potential of Individual Soil Battery 1, 2, 3, and 4

Cell

1

2

3

4

Voltage (V)

0.840

0.877

0.827

0.808

Table III.2

Cell Potential of Soil Batteries in Series and LED Light

Activation/Intensity

Battery

1

1-2

1-2-3

1-2-3-4

Voltage (V)

0.893

1.762

2.630

3.430

LED

Observation of Light Activation and Intensity Ligh Light

No.

Color

t

Intensit

Light

Intensit

(Y/N

Light

y

(Y/N)

y

(Y/N)

Intensity (Y/N)

Intensity

) 1

Red

N

n/a

Y

D

Y

M

Y

I

2

Yello

N

n/a

N

n/a

Y

M

Y

D

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w Ultra 3

N

n/a

N

n/a

N

n/a

Y

D

violet 4

Blue

N

n/a

N

n/a

N

n/a

Y

I

5

green

N

n/a

N

n/a

N

n/a

y

i

Note: Y - Yes; N - No; Intensity: D – Dim glow; M – Moderate glow; I - Intense glow

Table III.3

Cell Potentials of Battery 1-2-3-4 as LED is Connected and

Disconnected

Observe and record the voltage of battery 1-2-3-4 as LED is connected and disconnected:

No. 1 2 3 4 5

LED Color Red Yellow Ultraviolet Blue green

Voltage (V) On Connection 1.760 1.842 2.618 2.523 2.260

On Disconnection 3.417 3.401 3.394 3.386 3.351

DATA ANALYSIS 1. In Part III, what is the voltage reading of individual soil battery cell, and what happens to the

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voltage reading when you connect two or more soil battery cells?

Average reading = (0.840 + 0.877 + 0.827 + 0.808) ÷ 4 =3.352 ÷ 4 = 0.838 volts The voltage reading increases when you connect two or more soil battery cells 2. What is the purpose of connecting the battery in series when trying to light the individual LED? To divide the total power supply between LEDS hence ensuring that each LED receives the total output voltage from the power supply 3. What happens to the LED and the intensity of its light as the voltage increases?

Increasing the voltage will turn on the LED Increasing voltage also increases the intensity of light source 4. What happens to the voltage of battery 1-2-3-4 connected to the Vernier as you connect and disconnect the LED? The voltage drops upon connection and increases after the LED is disconnected

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