C4 Electrochemistry

Electrochemistry

Mr. Jake P. Villagonzalo

CORE
1. Define electrolysis as the decomposition of an ionic compound, when molten or in aqueous solution, by the passage of an electric current

2. Identify in simple electrolytic cells:
(a) the anode as the positive electrode
(b) the cathode as the negative electrode
(c) the electrolyte as the molten or aqueous substance that undergoes electrolysis

3. Identify the products formed at the electrodes and describe the observations made during the electrolysis of:
(a) molten lead(II) bromide
(b) concentrated aqueous sodium chloride
(c) dilute sulfuric acid
using inert electrodes made of platinum or carbon/graphite

Learning Objectives

SUPPLEMENTARY
4. Describe the transfer of charge during electrolysis:
(a) the movement of electrons in the external circuit
(b) the loss or gain of electrons at the electrodes
(c) the movement of ions in the electrolyte

5. Identify the products formed at the electrodes and describe the observations made during the electrolysis of aqueous copper(II) sulfate using carbon/graphite electrodes and when using copper electrodes

6. State that metals or hydrogen are formed at the cathode and that non-metals (other than hydrogen) are formed at the anode

7. Predict the identity of the products at each electrode for the electrolysis of a binary compound in the molten state

8. Construct ionic half-equations for reactions at the cathode (showing gain of electrons as a reduction reaction)

​All metals conduct electricity, but carbon in the form of graphite is the only non-metallic element that conducts electricity. Metals and graphite conduct electricity because they have mobile free electrons (e^-) in their structure.

In figure C4.02, the battery 'pumps' free electrons in one direction around the circuit, If the bulb lights up, then the material is an electrical conductor.

No chemical change when an electric current passed through a metal or graphite.

​

​

Some liquids conduct electricity. The process in which an electric current flows through a liquid compound or solution is called electrolysis.

​Electrolysis in liquid compounds or solutions produces chemical reaction, causing the substance to change.

To test the conductivity, graphite rods are dipped into the test liquid.

Liquid compounds, solutions and molten materials can be tested this way.

-If liquid compounds or solutions are tested using the apparatus in C4.03, the result will depend on the type of bonding in the compound.
-Electrons are still able to move through the liquid metal to carry the charge.
-In solid metals, no chemical change takes place when liquid metals conduct electricity
-If compound is bonded covalently, then it will not conduct electricty as a liquid or as a solution. Examples of of such liquids are ethanol, petrol (gasoline), pure water and sugar solution.
-Ionic compounds conduct electrcity if they are either molten or dissolved in water. Examples: molten lead bromide, sodium chloride solution and copper (II) sulfate solution

​Take note:

• When an electric current is passed through a molten ionic compound the compound decomposes or breaks down

• The process also occurs for aqueous solutions of ionic compounds

• Covalent compounds cannot conduct electricity hence they do not undergo electrolysis

• Ionic compounds in the solid state cannot conduct electricity either since they have no free ions that can move and carry the charge

Electrolysis General Principles:

Key terms used in a simple electrolytic cell

• Electrode is a rod of metal or graphite through which an electric current flows into or out of an electrolyte

• Electrolyte is the ionic compound in a molten or dissolved solution that conducts the electricity

• Anode is the positive electrode of an electrolysis cell

• Anion is a negatively charged ion which is attracted to the anode

• Cathode is the negative electrode of an electrolysis cell

• Cation is a positively charged ion which is attracted to the cathode

Electrolysis General Principles:

Electrolysis General Principles:

​The basic set-up of an electrolytic cell:

• Metals and hydrogen form positively charged ions and so either a metal or hydrogen gas is formed at the cathode 

• Non-metals form negatively charged ions and so non-metals (except hydrogen) are formed at the anode 

Electrolysis General Principles:

Electrolysis:Charge Transfer

• During electrolysis, current needs to flow around the circuit

• In order for this to occur, charge must be transferred around the circuit (current is a measure of the rate of flow of charge) by charge carriers

• The power supply provides the cathode with a supply of electrons, causing it to become negatively charged

• Positive ions (cations) in the electrolyte move towards the cathode where they gain electrons

• Negative ions (anions) in the electrolyte move towards the anode

  where they lose electrons

• The electrons move from the anode back towards the power supply

• So, in a complete circuit:

  • Electrons are the charge carriers in the external circuit

  • Ions are the charge carriers in the electrolyte

During electrolysis:

​It is important to demonstrate a deep understanding of this concept.


Be reminded of the following points.

Product of Electrolysis

Electrolysis of Molten Compounds-Challenge

Instruction: Watch the video in the next slide that presents the summary of the idea or principles of electrolysis of molten compounds. Take note some of important key words as it may be confusing to understand the concept.




To help you understand, in your note construct or use mind map/concept map to summarize the idea about electrolysis.

Electrolysis of Molten Compounds.mp4 - Google Drive

You can open this webpage in a new tab.

Copy link

Open in new tab

Electrolysis of Molten Compounds

• ​A binary ionic compound is one consisting of just two elements joined together by ionic bonding

• When these compounds undergo electrolysis they always produce their corresponding elements 

• To predict the products made at each electrode, first identify the ions 

• The positive ion will migrate towards the cathode and the negative ion will migrate towards the anode 

• Therefore, the cathode product will always be the metal, and the product formed at the anode will always be the non-metal

Electrolysis of Molten Compounds

Method:

• Add lead(II) bromide into a beaker and heat it so it will turn molten, allowing ions to be free to move and conduct an electric charge

• Add two graphite rods as the electrodes and connect this to a power pack or battery

• Turn on the power pack or battery and allow electrolysis to take place

• Negative bromide ions move to the positive electrode (anode) and each loses one electron to form bromine molecules. There is bubbling at the anode as brown bromine gas is given off

• Positive lead ions move to the negative electrode (cathode) and gain electrons to form a grey lead metal which deposits on the surface of the electrode

​The electrons released at the anode flow through the circuit towards the cathode.

During the electrolysis of molten salts, the metal ions, which are always positive (cations), move to the cathode and are discharged.

Non-metal ions (except Hydrogen) are always negative (anions) and move to the anode to be discharged.

​The movement of ions when molten ionic compound is electrolysed:

1. the metal is always formed at the cathode (the negative electrode)

2. the non-metal is always formed at the anode (the positive electrode)

The movement of ions to the different electrodes during electrolysis results in the breakdown of the ionic compound.

​During electrolysis,

Bromide ions (Br^-) move to the anode. Bromide ion gives up one electron to become a Bromine atom:

Br^- --> Br + e^-

Two of these bromine atoms together bond together to make a bromine molecule:

Br + Br --> Br₂

The Lead ions (Pb²⁺) move to the cathode. Each lead ion gains (accepts) two electrons and becomes a lead atom:

Pb²⁺ + 2e- --> Pb

These electrode reactions are represented by half equations.

If the two half equations are added together, they give the overall equation for the reaction taking place in the electrolytic cell.

Electrolysis of Aqueous Solution-Challenge

Instruction: Watch the video in the next slide that presents the summary of the idea or principles of electrolysis of aqueous solution. Take note some of important key words as it may be confusing to understand the concept.

RPReplay_Final1708851824.mp4 - Google Drive

You can open this webpage in a new tab.

Copy link

Open in new tab

Electrolysis of Aqueous Solution

• Aqueous solutions will always have water present (H₂O)

• In the electrolysis of aqueous solutions, the water molecules dissociate producing H⁺ and OH^– ions:

H₂O ⇌ H⁺ + OH^–

• These ions are also involved in the process and their chemistry must be considered

• We now have an electrolyte that contains ions from the compound plus ions from the water

• Which ions get discharged and at which electrode depends on the relative reactivity of the elements involved

• Concentrated and dilute solutions of the same compound give different products

• For anions, the more concentrated ion will tend to get discharged over a more dilute ion

Electrolysis of Aqueous Solution

Electrolysis of Aqueous Solution

Electrolysis of Aqueous Sodium Chloride

Electrolysis of Aqueous Dilute Sulfuric Acid

Electrolysis of Aqueous Copper Sulfate

ionic half equations

Ionif Half Equations - Google Drive

You can open this webpage in a new tab.

Copy link

Open in new tab

Multiple Choice

What is the correct reason for electrolysis not working?

• Solid zinc chloride has been used

• The electrodes don't conduct electricity

• The power supply is not connected properly

• The crucible is too small to allow ions to flow

Multiple Choice

Solid lead (II) bromide is heated until it is molten. The molten lead (II) bromide then undergoes electrolysis. Which substances are formed at each electrode?

• A

• B

• C

• D

Multiple Choice

Concentrated aqueous sodium chloride can be electrolysed.

Which statement is correct?

• Hydrogen gas is formed at the anode, and chlorine gas is formed at the cathode

• Hydrogen gas is formed at the cathode, and chlorine gas is formed at the anode.

• Sodium metal is formed at the anode, and chlorine gas is formed at the cathode.

• Sodium metal is formed at the cathode, and chlorine gas is formed at the anode

Multiple Choice

Which statement describes what happens to the electrons during the electrolysis?

• They are added to chloride ions.

• They are added to hydrogen ions.

• They move through the circuit from negative to positive.

• They move through the solution from negative to positive.

Multiple Choice

The diagram shows the electrolysis of concentrated hydrochloric acid and concentrated aqueous sodium chloride using carbon electrodes.

At which electrode(s) is hydrogen produced?

• Electrode 1 only

• Electrodes 1 and3

• Electrode 2 only

• Electrodes 2 and 4

Multiple Choice

Which statement about electrolysis is correct?

• Electrons move through the electrolyte from the cathode to the anode.

• Electrons move towards the cathode in the external circuit

• Negative ions move towards the anode in the external circuit.

• Positive ions move through the electrolyte towards the anode during electrolysis.

Multiple Choice

Dilute sulfuric acid is electrolysed using inert electrodes.

What are the products of this electrolysis?

• A

• B

• C

• D

Multiple Choice

Electricity is passed through concentrated aqueous sodium chloride.

Inert electrodes are used.

Which products are formed at the electrodes?

• A

• B

• C

• D

Multiple Choice

The diagram shows the electrolysis of aqueous copper (II) sulfate.

Which statement is correct?

• Copper metal is deposited at the positive electrode.

• In the external circuit the electrons move from negative to positive.

• In the solution the electrons move from positive to negative.

• Oxygen gas is produced at the positive electrode.

Multiple Choice

Concentrated aqueous potassium bromide solution is electrolysed using inert electrodes. The ions present in the solution are K ,Br , H and OH . To which electrodes are the ions attracted during this electrolysis?

• A

• B

• C

• D

Multiple Choice

Which is the correct equation for the formation of chlorine gas at the anode during the electrolysis of concentrated sodium chloride solution?

• Cl + e^- → Cl^-

• 2Cl^- + 2e^- →Cl₂

• 2Cl^- → Cl₂+ 2e^-

• Cl^- → Cl +e

Open Ended

Why is the ionic compound heated during electrolysis?

Multiple Select

What is electrolysis?

Select all that applies

• breaking down of a compound using a current

• making a compound using a current

• decomposition of a compound using a current

• conduction of electricity by ionic compound in molten and aqueous state

Multiple Choice

Which statement correctly describes the 2 electrodes?

• the anode is negative and the cathode is positive

• the anode is positive and the cathode is negative

• the anode and cathode are both positive

• 
  The anode and cathode are both negative.

Multiple Choice

What is the name given to the solution that is being electrolysed?

• Salt solution

• 
  Mineral solution

• Electric solution

• Electrolyte

Multiple Choice

Why is the electrolyte has to be a liquid?

• 
  So the ions can move

• So that it doesn't get too hot

• So the electrons can move

• 
  So the fish are ok

Multiple Choice

What happens at the positive electrode?

• Positive non-metal ions are attracted

• Negative non-metal ions are attracted

• Positive metal ions are attracted

• 
  Negative metal ions are attracted

Multiple Choice

Positive ions (cations) will move towards the cathode (-) where they will discharge by....

• 
  breaking apart

• clumping together.

• losing electrons

• 
  gaining electrons

Multiple Choice

When NaCl solution is electrolysed what has forms at the anode (+)?

• Hydrogen

• Chlorine

• Oxygen

• Sodium

Multiple Choice

What does the cathode (-) do to ions?

• give electrons to the positive ions to turn them back into atoms

• 
  turn ions back into atoms by removing electrons

• take electrons from the positive ions?

• 
  turn atoms into ions by adding electrons

Multiple Choice

What is the equation to show what happens to Cl ions at the anode.

• Cl^- --> Cl + e-   

• Cl₂ + 2e-   2Cl^-

• 
  2Cl^- --> Cl₂ + 2e-   

• Cl₂ + 2e-  _-->  2Cl^-

Multiple Choice

The positively charged ions migrate toward the ________.

• anode

• cathode

• electrode

• electrolyte

Multiple Choice

What is the product formed at the anode (+) during the electrolysis of concentrated copper(II) chloride?

• 
  copper

• 
  hydrogen

• 
  chlorine

• 
  oxygen

Multiple Choice

What element is used to make up the electrodes?

• oxygen

• carbon

• iron

• magnesium

Multiple Choice

What is the name of a positively charged ion?

• 
  positron

• cation

• proton

• anion

Multiple Choice

What is the equation to show what happens to Cl ions at the anode.

• 
  Cl^- --> Cl + e-   

• Cl₂ + 2e-   2Cl^-

• 2Cl^- --> Cl₂ + 2e-   

• Cl₂ + 2e-  _-->  2Cl^-

Multiple Choice

The diagram shows the electrolysis of lead(II) bromide using inert electrodes. Why does the bulb only light up  when the lead(II) bromide is melted?

• Bromine atoms in lead(II) bromide are converted to ions when it is melted

• The ions in lead(II) bromide are acting as the mobile charge carriers when it is melted

• Electrons flow through the lead(II) bromide when it is melted

• There are no ions in solid lead(II) bromide

Multiple Choice

Complete the half equation Al³⁺ --> 

• 
  Al³⁺ - 3e^- --> Al

• Al³⁺ + 3e^- --> Al

• Al²⁺ + 3e^- --> Al

• 
  Al³⁺ --> Al + 3e^-