ELECTROLYSIS

Week: THREE                    Date:     27-31/05/2019                                                   Time:

Period:                                 Duration: 1 HR 20 MIN.                                                 Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                        Class: SS TWO

Topic:                                    ELECTROLYSIS

Sub topic:  Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials:

Entry behavior: The students have been taught chemical reaction

Behavioural objective: At the end of the lesson the students should be able to:

i.                     Define electrolysis

ii.                   Differentiate between metallic conductor and electrolytic conductor

iii.                  State the factors that affect preferential discharge of ion

iv.                 Explain electrolysis of some electrolytes

CONTENT

ELECTROLYSIS

Electrolysis is the process of decomposing (breaking down) an ionic substance, called an electrolyte, into simpler substances using electricity. The chemical reaction of electrolysis occurs when an electric current is passed through a solution containing ions (ions are charged atoms, they have more or less electrons than protons which causes an imbalance, the overall charge can be negative or positive).
Conductors and Non Conductors

Substances around us can be divided into two classes based on their ability of conduct electricity:

Non-Conductors: Those substances which do not allow electric current to pass through them are called non-conductors or insulators. Example: - wood, plastic glass, rubber etc.

Conductors: Those substances which allow electric current to flow through tem are called conductors. Examples: Copper, Iron, Gold, Silver, Graphite, salt solution etc.

Conductors can further be divided into two groups

I.Metallic Conductors: These conductors conduct electricity or electric current by movement of electrons without undergoing any chemical change during the process. These conduct electricity in both solid as well as molten state. Example: All the metals and Graphite

II.Electrolytes: Those substances which conduct electricity only when they are present in aqueous solution and not in solid form are called electrolytes. These conduct electricity by movement of ions in solutions.

Comparison of Electrolytic and Metallic Conduction

S.No	Metallic Conduction	Electrolytic Conduction
1	Electric current flows by movement of electrons.	Electric current flows by movement of ions.
2	No chemical change occurs.	Ions are oxidized or reduced at the electrodes.
3	It does not involve the transfer of any matter.	It involves transfer of matter in the form of ions.
4	Ohm's law is followed.	Ohm's low is followed.
5	Resistance increases with increase of temperature.	Resistance decreases with increase of temperature.
6	Faraday law is not followed.	Faraday law is followed.

 

TERMS IN ELECTROLYSIS

Electrolysis	decomposition of compound using electricity
Electrolyte	an ionic compound which conducts electric current in molten or aqueous solution, being decomposed in the process.
Electrode	a rod or plate where electricity enters or leaves electrolyte during electrolysis.  Reactions occur at electrodes.
Discharge	the removal of electrons from negative ions to form atoms or the gain of electrons of positive ions to become atoms.
Anode	positive electrode connected to positive terminal of d.c. source.  Oxidation occurs here.  Anode loses negative charge as electrons flow towards the battery, leaving anode positively charged.  This causes anion to discharge its electrons here to replace lost electrons and also, negative charge are attracted to positive charge.
Cathode	negative electrode connected to negative terminal of d.c. source.  Reduction occurs here.  Cathode gains negative charge as electrons flow from the battery towards the cathode, making cathode negatively charged.  This causes cation to be attracted and gains electrons to be an atom.
Anion	negative ion attracted to anode.
Cation	positive ion attracted to cathode.

 

TYPES OF ELECTROLYTES

Non-electrolytes	Weak electrolytes	Strong electrolytes
Organic liquids or solutions	Weak acids and alkalis	Strong acids, alkalis and salt solutions
ethanol C2H5OH tetrachloromethane CCl4 trichloromethane CHCl3 pure water H2O sugar solution C12H22O11 molten sulphur S	limewater Ca(OH)2 ammonia solution NH3 aqueous ethanonoic acid CH3COOH aqueous sulphurous acid H2SO3 aqueous carbonic acid H2CO3	aqueous sulphuric acid H2SO4 aquous nitric acid HNO3 aquous hydrochloric acid HCl aqueous potassium hydroxide KOH aqueous sodium hydroxide NAOH copper(II) sulphate solution CuSO4

For the electrolyte to conduct electricity, it must be:
An ionic compound (I) In molten (II)  In aqueous state

The process of Electrolysis: Electrolysis of an ionic substance

i.                    The ionic substance is heated until it melts.

ii.            The ions are able to move freely after the ionic substance is melted.

iii.          The power supply is connected and the electrodes are charged.

iv.           The ions move to the oppositely charged electrode (the negative electrons move to the positive electrode, called the anode, and the positive electrons move to the negative electrode, called the cathode).

v.             The electrodes give/take electrons from the ions which makes the ions neutral.

vi.                 The ions become atoms (because they are neutral) and are deposited onto the electrode. At the cathode, the ion gain electrons to become atom, this is reduction while at the anode the ions loose electrons to become atom, this is oxidation.

Electrolysis of Molten Compounds

Molten/aqueous ionic compounds conduct electricity because ions free to move. 

In solid state, these ions are held in fixed position within the crystal lattice. 

Hence solid ionic compounds do not conduct electricity.

When molten binary compound is electrolysed, metal is formed on cathode while non-metal is formed on anode.

Example

Electrolysis of molten PbBr₂
To make molten lead(II) bromide, PbBr₂, we strongly heat the solid until it melts. To electrolyse it, pass current through the molten PbBr₂.
What happens:
Ions present: Pb²⁺ and Br^-
Reaction at Anode
Br- loses electrons at anode to become Br atoms. Br atoms created form bond together to make Br2 gas.
2Br^-(aq) --> Br₂(g)+ 2e^-
Reaction at Cathode
Pb²⁺ gains electrons at cathode to become Pb atoms becoming liquid lead (II).
Pb²⁺(aq) + 2e^- --> Pb(l)
Overall equation
PbBr₂(l) --> Pb(l) + Br₂(g)
Electrolysis of Aqueous Solution

Aqueous solutions contain additional H⁺ and OH^- ions of water, totaling 4 ions in the solution : 

2 from electrolyte, 2 from water.  Only 2 of these are discharged.

FACTORS THAT AFFECTS THE PREFERENTIAL DISCHARGE OF ION

Thus, Selection of ions for preferential discharge is based on the following three factors:

I.                    POSITION OF IONS IN THE ELECTROCHEMICAL SERIES

For the electrolyte to conduct electricity, it must be Electrolysis of aqueous solutions use the theory of selective discharge.

 

 

 Cation  K⁺, Na⁺, Ca⁺, Mg²⁺, Al³⁺, Zn²⁺, Fe²⁺, Sn²⁺, Pb²⁺, H⁺, Cu²⁺, Ag⁺, Hg⁺

 

Anion    SO₄^2-, NO₃^-, Cl^-, Br^-, I^-, OH^-

                    Increasing ease of discharge                                                                                                                                                                               [NOTE: KING NOSIRU CAN MAKE A ZOO IN SAJE OR PANSEKE HAVING COLLECTED SOLID GOLD.]          

If two or more positive ion migrate to the cathode, it is the ion lowest in the electrochemical series (e.c.s) that is discharged preferentially.

Example: Cu2+ are discharge in preference to H+ while H+ are discharge in preference to Na+. It will be recalled that the reactivity series gives the order of the tendency of metals to form ions. Metals at the top of the series have the greatest tendency to form ions and are therefore the most difficult to discharge at the cathode.

If two or more negative ions migrate to the anode, the ion lower in e.c.s is discharged preferentially.

II.                  CONCENTRATION OF THE IONS IN THE ELECTROLYTE

At cathode
In CONCENTRATED solutions of nickel/lead compound, nickel/lead will be discharged instead of hydrogen ions of water which is less reactive than nickel/lead.

In VERY DILUTE solutions, hydrogen, copper and silver ions are preferable to be discharged, according to its ease to be discharged.

Reactive ions (potassium, sodium, calcium, magnesium, aluminium) will NEVER BE DISCHARGED in either concentrated or dilute condition. Instead, hydrogen ions from water will be discharged at cathode.

At anode
In CONCENTRATED solutions, iodine/chlorine/bromine ions are preferable to be discharged, although it’s harder to discharged compared to hydroxide ions.

In VERY DILUTE solutions containing iodide/chloride/bromide ions, hydroxide ions of water will be discharged instead of iodide/chloride/bromide, according to ease of discharge.

Sulphate and nitrate are NEVER DISCHARGED in concentrated/dilute solutions.

Examples

A. Concentration Solutions: - Electrolysis of Concentrated NaCl
What happens:
Ions Present: Na⁺, H⁺, OH^- and Cl^-
Reaction at Anode
Cl^- loses electrons at anode to become Cl atoms, although OH^- is easier to discharge. 

Cl atoms created form covalent bond together to make Cl₂ gas.

2Cl^- (aq) --> Cl₂ (g) + 2e^-

Reaction at Cathode
H⁺ gains electrons at cathode to become H atoms becoming hydrogen gas

2H⁺ (aq) + 2e^- --> H₂ (l)

Overall Equation
2HCl(l) --> H₂(l) + Cl₂(g)
Note: any cation and anion left undischarged in solution forms new bonds between them. 
E.g. in above, leftovers Na⁺ and OH^- combine to form NaOH.
B. Very Dilute Solutions:-  Electrolysis of Dilute H2SO4
What happens:
Ions Present: H⁺, OH^- and SO₄^2-
Reaction at Anode
OH- loses electrons at anode to become O₂ and H₂O.

4OH^- (aq) --> O₂ (g) + 2H₂O (l) + 4e^-

Reaction at Cathode
H⁺ gains electrons at cathode to become H atoms becoming hydrogen gas.

2H⁺(aq) + 2e^- --> H₂ (g)

Overall Equation
Both equations must be balanced first. 

The cathode equation is short 2 electrons. Hence, we should first even them by multiplying cathode equation by 2.

(2H⁺(aq) + 2e^- --> H₂ (g)) x 2 = 4H ⁺(aq) + 4e^- --> 2H₂ (g)

Now we can combine the equations, forming: 

4H⁺ (aq) + 4OH⁺ (aq) --> 2H₂ (g) + O₂ (g) + 2H₂O (l)

4H⁺ and 4OH⁺ ions, however, combine to form 4H₂O molecules. 

Hence: 4H₂O (l) --> 2H₂ (g) + O₂ (g)+ 2H₂O (l)

H₂O molecules are formed on both sides. 

Therefore, they cancel the coefficients: 2H₂O (l) --> 2H₂ (g) + O₂ (g)

Since only water is electrolysed, the sulfuric acid now only becomes concentrated.

III.                NATURE OF THE ELECTRODE

Electrolysis using different types of electrodes

Inert Electrodes are electrodes which do not react with electrolyte or products during electrolysis.

Eg. platinum and graphite.

Active Electrodes are electrodes which react with products of electrolysis, affecting the course of electrolysis. 

Eg. copper.

A. Electrolysis of CuSO₄ Using Inert Electrodes (e.g. carbon)
What happens:
Ions Present: Cu²⁺, H⁺, OH^- and SO₄^2-
Reaction at Anode
OH^- loses electrons at anode to become O₂ and H₂O.

4OH^- (aq) --> O₂ (g) + 2H₂O (l) +4e^-

Reaction at Cathode
Cu²⁺ gains electrons at cathode to become Cu atoms becoming liquid copper. 

Hydrogen ions are not discharged because copper is easier to discharge.

Cu²⁺ (aq) + 2e^- --> Cu (s)

Overall Equation
Both equations must be balanced first. 

The cathode equation is short 2 electrons. Hence, we should first even them by multiplying cathode equation by 2.

(Cu²⁺ (aq) + 2e^- --> Cu (s)) x 2 = 2Cu²⁺ (aq) + 4e^- --> 2Cu (s)

Now we can combine the equations, forming:

2Cu(OH)₂ (aq) --> 2Cu (s) + O₂ (g) + 2H₂O (l)

Since copper ions in solution are used up, the blue colour fades. 

Hydrogen and sulphate ions left forms sulphuric acid.

B. Electrolysis of CuSO₄ Using Active Electrodes (e.g. copper)
Ions Present: Cu²⁺, H⁺, OH^- and SO₄^2-
Reaction at Anode
Both SO₄^2- and OH^- gets attracted here but not discharged. Instead, the copper anode discharged by losing electrons to form Cu²⁺. So, the electrode size decreases.

Cu (s) --> Cu²⁺ (aq) + 2e^-

Reaction at Cathode
Cu²⁺ produced from anode gains electrons at cathode to become Cu atoms becoming copper. Hence, the copper is deposited here and the electrode grows.

Cu²⁺ (aq) + 2e^- --> Cu (s)

Overall Change
There is no change in solution contents as for every lost of Cu²⁺ ions at cathode is replaced by Cu²⁺ions released by dissolving anode. 

Only the cathode increases size by gaining copper and anode decreases size by losing copper. 

We can use this method to create pure copper on cathode by using pure copper on cathode and impure copper on anode. Impurities of anode falls under it.

ELECTROLYSIS OF COMMON ELECTROLYTES

ELECTROLYTE	CATHODE	PRODUCT DISCHARGE AT CATHODE	ANODE	PRODUCT DISCHARGE  AT ANODE	EFFECTS ON ELECTROLYTE
Acidifed water (dil. H2SO4)	Platinum	Hydrogen gas (2 volume)	Platinum	Oxygen gas (1 volume)	Conc. Of H+ and SO42- increase as water is removed making the electrolyte more acidic
Conc. Copper(ii)chloride solution	Copper	Copper deposited	Carbon	Chlorine gas	Removal of Cu2+ and Cl- dil. electrolyte
dil. CuCl2 solution	Copper	Copper deposited	Carbon	oxygen gas	Conc. Of H+ and Cl- increase making electrolyte acidic
Conc. CuCl2 or  CuSO4 solution	Copper	Copper deposited	Copper	Copper anode dissolves	Conc. Of electrolyte remain unchanged
Conc. CuSO4 solution	Copper	Copper deposited	Platinum	Oxygen gas	Conc. Of H+ and SO42-increase, making electrolyte acidic
Conc. HCl acidic	carbon	Hydrogen gas 1 volume	Carbon	Chlorine gas (1 volume)	Removal of  H+ and Cl- dilutes electrolyte, making it  less .
Conc. NaCl solution  (brine)	carbon	Hydrogen gas (1 volume)	Carbon	Chlorine gas (1 volume)	Conc. Na+ and OH- increase  making electrolyte alkaline
Molten NaCl	carbon	Sodium	Carbon	Chlorine gas	Conc. Remains unchanged
Conc. Sodium hydroxide solution	platinum	hydrogen gas (2  volume)	Platinum	Oxygen gas (1 volume)	Conc. Of Na+ and OH- increase as water is removed, making electrolyte more alkaline
Conc. NaCl (brine)	mercury	Sodium amalgam	Carbon	Chlorine gas

PRESENTATION

Step I: The teacher explains electrolysis with its terminologies.

Step II: The teacher explains the factors that affect the preferential discharge of ions

Step III: The students chorus the factors that affect the preferential discharge of ions

Step IV: the teacher explains the electrolysis of some common electrolytes

EVALUATION

The teacher evaluates the lessons by asking the following questions:

i.                     Define electrolysis

ii.                   Differentiate between metallic conductor and electrolytic conductor

iii.                  State the factors that affect preferential discharge of ion

iv.                 Explain electrolysis of some electrolytes