Electrolysis Concepts and Applications

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Questions and Answers

What happens to the anode during the electrolysis process of copper refining?

It increases in mass.

It transforms into a different metal.

It remains unchanged in mass.

It decreases in mass. (correct)

What is the role of the cathode in the purification of impure copper?

It absorbs impurities from the anode.

It collects copper ions from the solution. (correct)

It actively releases electrons into the solution.

It serves as a source of copper ions.

In electroplating, what type of solution is used as the electrolyte?

An aqueous solution of a salt of the plating metal. (correct)

A solution containing only copper ions.

A mixture of water and metal powder.

A concentrated acid solution.

Where do the Cu2+ ions discharged at the cathode primarily come from during copper electrolysis?

The copper anode. Signup and view all the answers

What occurs to the Cu2+ ions in the electrolyte during the copper refining process?

They remain constant in concentration. Signup and view all the answers

What occurs at the cathode during electrolysis?

Reduction takes place Signup and view all the answers

Which statement best describes the movement of ions during electrolysis?

Cations move towards the cathode Signup and view all the answers

What is the role of electrons at the cathode during electrolysis?

They are gained by cations Signup and view all the answers

During electrolysis, what constitutes the flow of electric current through the electrolyte?

Flow of ions towards electrodes Signup and view all the answers

Which statement is true regarding the electrodes during electrolysis?

The anode is positive and attracts cations Signup and view all the answers

What is the primary process occurring at the cathode in an electrolytic cell?

Reduction of cations Signup and view all the answers

What characterizes the flow of electrons during electrolysis?

Electrons flow from anode to cathode Signup and view all the answers

Which ions are attracted to the cathode during the electrolysis process?

Positively charged cations Signup and view all the answers

Which ion is discharged at the anode during the electrolysis of dilute NaCl solution?

OH- Signup and view all the answers

What product is formed at the cathode during the electrolysis of dilute NaCl solution?

Hydrogen gas Signup and view all the answers

Which statement correctly describes the ions present during electrolysis of dilute NaCl solution?

Na+, Cl-, H+, and OH- ions are all present. Signup and view all the answers

What happens to OH- ions during electrolysis in a dilute NaCl solution?

They are preferentially discharged to form water and oxygen. Signup and view all the answers

What determines the cation that is discharged at the cathode during electrolysis?

The metal reactivity series Signup and view all the answers

After electrolysis, which ions remain in the solution when dilute NaCl is electrolysis is performed?

Na+ ions, Cl- ions, H+ ions, and OH- ions Signup and view all the answers

What will happen to the products of electrolysis as a result of combining OH- and H+ ions?

They will form water. Signup and view all the answers

If a stronger anion like Cl- is present in higher concentration, what happens at the anode?

Cl- ions will be discharged. Signup and view all the answers

What is the name of the electrode connected to the positive terminal of the battery in an electrolytic cell?

Anode Signup and view all the answers

Which of the following best describes the function of the electrolyte in an electrolytic cell?

Conducts electricity through free-moving ions Signup and view all the answers

What occurs at the anode during the process of electrolysis?

Oxidation of anions Signup and view all the answers

Which materials are typically used for electrodes in electrolytic cells?

Carbon rods or metal plates Signup and view all the answers

How do the free-moving ions in the electrolyte contribute to electrolysis?

They allow electricity to flow through the solution. Signup and view all the answers

What is produced at the cathode during electrolysis?

Electrons Signup and view all the answers

What type of compound can the electrolyte be in an electrolytic cell?

A molten ionic compound or aqueous solution Signup and view all the answers

During electrolysis, which ions move toward the anode?

Anions Signup and view all the answers

What is the ionic half equation at the cathode during the electrolysis of copper(II) sulfate using copper electrodes?

Cu^{2+}(aq) + 2e^- → Cu(s) Signup and view all the answers

Which ion is discharged at the anode during the electrolysis of copper(II) sulfate using copper electrodes?

Cu^{2+} Signup and view all the answers

What would be observed during the electrolysis of a concentrated solution of potassium iodide using platinum electrodes?

Gas bubbles at the cathode and a brown solution colored by iodine at the anode Signup and view all the answers

What is the overall reaction during the electrolysis of CuSO4 solution using copper electrodes?

Cu^{2+}(aq) + 2e^- + SO_4^{2-}(aq) → Cu(s) Signup and view all the answers

In electrolysis, which ion is likely to be preferentially discharged at the cathode when both Cu^{2+} and H^+ are present?

Cu^{2+} Signup and view all the answers

What type of electrodes are used in the electrolysis of a concentrated potassium iodide solution?

Platinum electrodes Signup and view all the answers

What role does electrolysis play in metal purification?

It separates and reduces metal ions to pure metal Signup and view all the answers

How are copper electrodes described during the electrolysis of CuSO4 solution?

Reactive electrodes Signup and view all the answers

What gas is produced at the anode during the electrolysis of CuSO4 solution?

Oxygen gas Signup and view all the answers

Why are SO42- ions not discharged at the anode during the electrolysis of CuSO4?

They remain in solution. Signup and view all the answers

Which half-reaction represents the process occurring at the anode during the electrolysis?

4OH- → 2H2O + O2 + 4e- Signup and view all the answers

Which of the following correctly summarizes the overall reaction at the cathode during electrolysis of CuSO4?

Cu2+ + 2e- → Cu(s) Signup and view all the answers

What is the role of inert electrodes in electrolysis?

To provide a surface for the reactions without participating Signup and view all the answers

Which ions remain in solution after the electrolysis of CuSO4?

Cu2+ and SO42- Signup and view all the answers

What is the result of the electrolysis of copper(II) sulfate solution using inert electrodes?

Copper metal is deposited at the cathode and oxygen gas is given off at the anode. Signup and view all the answers

Study Notes

Electrolysis

Electrolysis is the process of using electricity to break down or decompose a compound (usually an ionic compound in the molten state or in aqueous solution).
Electrolysis takes place in an electrolytic cell.

Learning Objectives

Define electrolysis
Describe electrolysis as evidence of the structure of ionic compounds in the solid, molten, and aqueous states.
Predict the products of the electrolysis of a molten ionic compound.
Predict the products of the electrolysis of an aqueous electrolyte using the idea of selective discharge of ions.
Construct ionic equations for the reactions occurring at the electrodes during the electrolysis, given relevant information.

Parts of an Electrolytic Cell

Battery: Acts as an electron pump, drawing electrons away from the anode and supplying them to the cathode. Anode becomes positively charged and cathode becomes negatively charged.
Electrodes:
- Conduct electricity
- Usually carbon rods or metal plates
- Electrode connected to the positive terminal of the battery is the anode
- Electrode connected to the negative terminal of the battery is the cathode
Electrolyte:
- Conducts electricity
- Contains free-moving ions to allow electricity to flow
- Is a molten ionic compound or an aqueous solution
- Is decomposed to form positive ions (cations) and negative ions (anions)

How Does Electrolysis Work?

At the anode:
- Anions move to the anode, giving up electrons.
- Oxidation occurs at the anode
At the cathode:
- Cations move to the cathode, receiving electrons.
- Reduction occurs at the cathode
Within the electrolyte:
- The flow of ions towards the electrodes is the flow of electric current through the electrolyte
In the external circuit:
- During electrolysis, electrons flow from the negative terminal to the positive terminal of the battery.
Examples of anion: Cl⁻, OH⁻
Examples of cation: Na⁺, H⁺

What happens to ions at the electrodes?

Cations receive electrons at the negatively-charged cathode
Anions give up electrons at the positively-charged anode.
When cations/anions gain/lose electrons at the electrodes, they form atoms or molecules. This is called discharging.

Electrolysis of molten Ionic Compounds

When a molten binary compound (containing only 2 elements) undergoes electrolysis, a metal and a non-metal are formed as products.
The molten compound does not contain water molecules

Electrolysis of molten sodium chloride

At the anode:
- Negatively-charged Cl⁻ ions are attracted to the anode.
- Cl⁻ ions lose electrons to form chlorine gas.
- Oxidation occurs
2Cl⁻(l) → Cl₂(g) + 2e⁻
The Cl ions are discharged.
At the cathode:
- Positively-charged Na⁺ ions are attracted to the cathode.
- Each Na⁺ ion gains one electron to form a sodium atom, which is reduced.
- Na⁺(l) + e⁻ → Na(l)
- The Na⁺ ions are discharged.
The overall reaction: 2NaCl(l) → 2Na(l) + Cl₂(g)

Inert Electrodes

Inert carbon electrodes are used to prevent chlorine from reacting with the electrode.
Inert electrodes do not react with the products of electrolysis or with the electrolyte.
Carbon (graphite) and platinum electrodes are considered inert because they are rarely involved in electrolytic reactions.

Electrolysis of dilute NaCl Solution

At cathode:
- H⁺ and Na⁺ ions are attracted to cathode.
- Sodium is higher in the reactivity series. Therefore, H⁺ ions are preferentially discharged as hydrogen gas.
2H⁺ + 2e⁻ → H₂(g)
Na+ ions remain in solution.
At anode:
- OH⁻ and Cl⁻ ions are attracted to the anode.
- OH ions give up electrons more readily than Cl⁻ions.
- 4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻
- Cl⁻ ions remain in solution
Overall reaction: 2H₂O(l) → 2H₂(g) + O₂(g)

Electrolysis of concentrated NaCl Solution

At cathode:
- H+ and Na+ ions are attracted to the cathode.
- Sodium is higher in the reactivity series. Therefore, H+ ions are preferentially discharged as hydrogen gas.
- 2H⁺ + 2e⁻ → H₂(g)
- Na⁺ ions remain in solution.
At anode:
- OH⁻ and Cl⁻ ions are attracted to the anode.
- Since Cl⁻ ions are more numerous than OH⁻ ions, Cl⁻ ions are discharged to form chlorine gas.
2Cl⁻(aq) → Cl₂(g) + 2e⁻
OH⁻ ions remain in solution

Electrolysis of Aqueous Solutions

An aqueous solution of a compound is a mixture of the compound with water.
In an aqueous solution, more than one type of cation or anion is present in the electrolyte.
E.g., aqueous sodium chloride contains Na⁺, H⁺, Cl⁻, and OH⁻ ions.
Only one cation and one anion are preferentially discharged during electrolysis. This is known as selective discharge of ions.

Selective Discharge of Ions

The ions discharged during electrolysis depend on three factors:
- Selective discharge of cations
- Selective discharge of anions
- Effect of concentration on the selective discharge of anions
The more reactive the metal, the greater its tendency to form ions.
Therefore, in electrolysis, the ease of discharge of cations depends on the position of the metal in the reactivity series.
- H⁺ ions are discharged in preference to ions of metals above hydrogen in the reactivity series.
The OH⁻ ions will give up electrons preferentially to form water and oxygen gas during electrolysis.
Sulfate (SO₄²⁻) and nitrate (NO₃⁻) ions normally remain in solution during electrolysis.

Predicting the Products of Electrolysis

Identify the cations and anions in the electrolyte
Determine the anion that is discharged at the anode (OH⁻ is often easier if not a high concentration of Cl⁻, Br⁻ or I⁻ anions)
Determine the cation that is discharged at the cathode (using activity series)
The cations and anions remaining in the solution after electrolysis form the product remaining in the solution.

Electrolysis of CuSO₄ Solution (inert electrodes)

At anode:
- OH⁻ and SO₄²⁻ ions are attracted to the anode.
- SO₄²⁻ ions are not discharged.
- OH⁻ ions are preferentially discharged to form water (H₂O) and oxygen (O₂) gas.
- 4OH⁻(aq) → 2H₂O(l) + O₂(g) + 4e⁻
- SO₄²⁻ ions remain in solution
At cathode:
- H⁺ and Cu²⁺ ions are attracted to the cathode.
- Copper is lower than hydrogen in the reactivity series. Therefore, Cu²⁺ ions are preferentially discharged as copper metal.
- Cu²⁺(aq) + 2e⁻ → Cu(s)
- H⁺ ions remain in solution.
Summary:
- Copper metal is deposited at the cathode, and oxygen gas is given off at the anode.

Purification of Impure Copper

To refine impure copper, the impure copper is used as the anode, while a thin sheet of pure copper is used as the cathode.
The electrolyte must be an aqueous solution of copper (II) sulfate (CuSO₄).

Electroplating

The object to be electroplated is made the cathode, and the anode is the source of the plating metal.
The electrolyte is an aqueous solution of a salt of the plating metal (e.g., CuSO₄ for copper plating.).

Industrial Applications of Electrolysis

Purify metals (e.g. refining copper)
Deposit a layer of metal on another substance (electroplating) for decorative purposes or preventing corrosion (e.g., silver or gold plating).

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Description

This quiz covers the fundamental concepts of electrolysis, including the definition, processes involved, and the structure of ionic compounds. It also addresses the predictions regarding the products of electrolysis in various states and the ionic equations relevant to these reactions. Test your understanding of electrolytic cells and their components.