Experiment 2: Electrolytic Cell-Electrolysis of Aqueous Solution

Experiment 2: Electrolytic Cell-Electrolysis of Aqueous Solution

Introduction

Electrolysis is the process of electrical energy is used to cause a nonspontaneous chemical reaction to occur. The electrolytic cell as shown in Figure 1 is an apparatus for performing electrolysis. The electrolytic cells consist of power supply such as battery to provide energy to transfer electrons from anode to cathode. In electrolysis cell, reduction occurs at cathode while oxidation occurs at anode. In Molten Sodium Chloride, the cations are Na +  and the anions are Cl- ion. This process is used to produce chlorine gas. The reaction occurs are;

Anode (oxidation)

: 2Cl-1(l)  Cl2(g) + 2e 

Cathode (reduction) : 2Na+(l) + 2e Overall reaction

2Na(l)



: 2Na+(l) + 2Cl -1(l)  Cl2(g) + 2Na(l) 

However, aqueous sodium chloride solution contains water which indicates several species that cloud be oxidized and reduced. The main purpose of this experiment is to know which ions will oxidize or reduce with the present of water based on it electrochemical negativity and also others factor such as application of over voltage.

Figure 1 : Electrolysis Cell Objectives

The intentions of this experiment are:i.

to identify the reactions occurring at the anode and cathode during the electrolysis of various aqueous salt solutions

ii.

to observe the electrolysis of water and the gases liberated at the electrodes and

iii.

to study the suitable electrode and aqueous solution for water splitting process.

Materials and Apparatus

The lists of materials and apparatus used are:i.

Carbon/Graphite rods

ii.

Copper plates

iii.

100 mL beaker

iv.

Power supply

v. vi. vii.

Two electrical wires (different color) that attached to crocodile clips Cardboard with two holes Litmus paper (blue)

Chemicals

The lists of chemicals used are:i.

 NaCl solution

ii.

 NaBr solution

iii.

 Na2SO4 solution

iv.

Phenolphthalein indicator

Procedure

Procedure Overview

The resulted products from the electrolysis of various salt solutions was observed and identified.

1. Set Up the Electrolysis Apparatus

The Electrolytic Cell apparatus was set up as in Figure 1. The crocodile clips were connected to the responding electrodes, listed in Table 1.

2. Electrolyze the solutions

A solution from Table 1 was filled in beaker until three-froths full and 10 drops of  phenolphthalein were added in the solutions. The suggested electrode in Table 1 was immersed into the solutions. The solutions were electrolyzed at 6V for 5 minutes. The changes at the solutions, electrodes and the gas evolved were observed and recorded.

No.

Solution

Electrodes(Anode Electrodes(Anode and Cathode)

1.

Distilled Water

Carbon(Graphite)

2.

0.001M NaCl

Carbon(Graphite)

3.

0.5M NaCl

Carbon(Graphite)

4.

0.5M NaBr

Carbon(Graphite)

5.

0.5M Na2SO4

Polished Copper Metal Strips

6.

0.5M Na2SO4

Carbon(Graphite) Table 1

Results and Analysis

Electrodes No. Solution

Observations

(Anode and Cathode)

1

2

Distilled Water

Carbon/Carbon

0.001 M  NaCl

Carbon/Carbon

Anode

No change

Cathode

No change

Anode

No change

Cathode Litmus paper

Light purple color appear No change The colorless solution turned to light



 purple upon the addition addition of

Solution 3

 phenolphthalein

0.5 M  NaCl



Carbon/Carbon Anode

No change

Cathode

Gas evolve

Litmus paper Blue 4

0.5 M

Carbon/Carbon

Light purple

Solution



 purple



red



The colorless solution turned to light

 NaBr

pink upon the addition addition of  phenolphthalein 

Anode

Light pink

 purple



Solutions changes to light brown

Cathode

Gas evolve

Odor

Pungent smell 

The colorless solution turned to  purple upon the addition addition of

5

0.5M  Na2SO4

 phenolphthalein

Carbon/Carbon 

 purple

 purple (no change)



Anode

Bubbles slower

Cathode

Bubbles faster 

The colorless solution turned to  purple upon the addition addition of

6

0.5M  Na2SO4

 phenolphthalein

Copper/Copper 

 purple

Anode

No change

Cathode

No change

 purple (no change)



Table 2

Electrolyte

Dissociation of

Solution

Electrolytes

Expectation Expectation of selected ion and product

Ions migrating to cathode: Na +, H+ Ions discharged at cathode: H +  Na+ + e- → Na  NaCl (Dilute)

Product at cathode: H2 gas

Cl2( g   g ) + 2 e→2Cl O2 + 4 H+ + 4 e- →2 H2O

Ions migrating to anode: Cl -, OH-

2H2O + 2e → H 2 + 2OH-

Ions discharged at anode: OH Product at anode: O 2 gas

 NaCl (Concentrated)

Ions migrating to cathode: Na + , H+  Na+ + e- → Na

Ions discharged at cathode: Na +

Cl2( g   g ) + 2 e→2Cl -

Product at cathode: H2 gas

O2 + 4 H+ + 4 e- →2 H2O 2H2O + 2e → H 2 + 2OH-

Ions migrating to anode: Cl-, OHIons discharged at anode: Cl Product at anode: Cl 2 gas Ions migrating to cathode: Na +, H+ Ions discharged at cathode: Na +

 Na+ + e- → Na  NaBr

Product at cathode: H2 gas

Br 2 + 2 e- → 2 Br O2 + 4 H+ + 4 e- →2 H2O

Ions migrating to anode: Br - ,OH-

2H2O + 2e → H 2 + 2OH-

Ions discharged at anode: Br Product at anode: Br 2 Ions migrating to cathode: Na + , H+ Ions discharged at cathode: H +

 Na+ + e- → Na  Na2SO4

Product at cathode: H2

S2O82- + 2 e- →2 SO42O2 + 4H+ + 4 e- →2 H2O

Ions migrating to anode: SO 42- ,OH-

2H2O + 2e → H 2 + 2OH-

Ions discharged at anode: OH Product at anode: O 2

Discussions:

In the electrolysis process, the ions dissociated would determine which ions will be discharged according to the electrochemical series. From the observation, blue litmus paper change to red color in the electrolysis of concentrated aqueous sodium chloride indicated that gas produced at anode was Cl 2 gas not O 2. This phenomenon happens due to application of overvoltage during performing this analysis. Higher voltage was applied because Cl 2  gas is more easily reduced than O2.that it would be more difficult to oxidize Cl - than H2O. The  pungent smell produced in the electrolysis of an aqueous Sodium bromide indicates the  product at anode is bromine gas. In the electrolysis elect rolysis of an aqueous sodium sulfate, halide ions are reducing at cathode and OH- is oxidizing at anode. SO 42- is the best anion that the most difficult anion to oxidize. The most suitable electrode and aqueous solutions for water splitting process are Copper/Copper and Na 2SO4 respectively.

Conclusions

1. The reactions occurring at the anode and cathode during the electrolysis of various aqueous salts was identified. Water is present in aqueous solution; the ions dissociated would determine which ions will be discharged according to the electrochemical series. 2. Due to the electrochemical series, the most suitable electrode and aqueous solutions for water splitting process are Copper/Copper and Na 2SO4 respectively.

Questions

1. Would solid NaCl conduct electricity? Why did the salt make the water more conductive to electricity? 

 No, Solid NaCl (salt) do not have free electron to allow electric current to pass through the substances. In the presence of water and when a voltage is applied, it aids the salt to let the ions to be free, thus they are more in response to an electric field which makes NaCl to be able to conduct electricity.

2. What gases were formed at anode and cathode in beaker containing 0.5M NaCl? Write the half-reaction that occurred in this beaker. 

Anode: 2H 2O + 2e → H 2 + 2OH-, gas evolved: H2 gas.



Cathode: Cl2(g) + 2 e→2Cl-, gas evolved: Cl 2.

3. What was the purpose of adding phenolphthalein to the solution? 

As an indicator which enables any changes to be observed.

4. If copper electrodes had been used instead of the carbon electrodes for the electrolytic electrolytic cells, the observed reactions may have been different. Why? 

Carbon electrode is an inert electrode, thus it will not compete with the ions present in the solutions. By replacing carbon with copper which is not an inert electrode, it will undergo erosion or electrodeposition instead of producing gas at the electrode.

5. Why does more gas form at one electrode than at the other electrode for the electrolysis of 0.5M Na 2SO4 solution? 

Anode: O2 + 4 H+ + 4 e- →2 H2O



Cathode: 2H2O + 2e → H2 + 2OH-



At cathode, reduction occurs and the H 2 gas is evolved while at anode, oxidation takes  place.

-

6. In the electrolysis of an aqueous salt solution, the nitrite ion NO 3 , migrates to the -

anode. Considering that the nitrogen in the NO 3 (aq) is at its maximum oxidation state (of +5), what electrolysis products(s) might you expect to observe at the anode? 

Based on the electrochemical series, NO-3 won’t be discharge because it is located at the upper top while OH- located at the bottom. Hence, expected product would be oxygen gas.



O2 + 4 H+ + 4 e- →2 H2O

7. Calculate the volume of gases that will be collected at anode and cathode when an aqueous solution of Na 2SO4 is electrolyzed for 2 hours with a 10A current.

Cathode (reduction): 2H 2O + 2e → H 2 + 2OH-  (x2) Anode (oxidation) : 2 H 2O →O2+ 4H+ 4 e Overall: 2H2O → 4H2 + O2

From oxidation: 1 mol O2 = 4e Q = It = (10)(2x60x60) = 72000C Q = mnF = (4)(n)(96500) m = 0.1865 mol volume of gas = no. of mol x 24L = 0.1865 x 24 = 4.4L

From reduction Q = (2)(n)(96500) m = 0.3731 mol volume of gas = no. of mol x 24L = 0.3731 x 24 = 8.95 L

8. Describe in your own words the whole process of electrolysis of water from your findings (including the electrochemical reactions occur at both electrodes, the suitable electrode and electrolyte used). 

The electrolysis of water is considered a well-known principle to produce oxygen and hydrogen gas, There must be no unwanted gas evolved such as bromine or chlorine gas occurs. Due to the electrochemical series, the most suitable electrode and aqueous solutions for water splitting process are Copper/Copper and Na 2SO4  respectively. Halide ions are reducing at cathode and OH - is oxidizing at anode. SO 42- is the best anion that the most difficult anion to oxidize.

References

C.Raymon, , M.S.A.M.S.Azyze.,G.F.Chuen.,M.A.Salwana., M.S.A.M.S.Azyze.,G.F.Chuen.,M.A.Salwana., Ghazali.N., Ramli.S., (2005)Physical Chemistry for Matriculation, revised ed., McGraw-Hill Malaysia, page 359365