Electrochemistry

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Notes

Electrolysis Principles

**Electrolysis** is the breakdown of a molten or aqueous ionic compound by an **electric current**.
Ionic compounds conduct electricity only when **molten** or **in solution** because ions are free to move.
**Electrolyte**: the ionic compound (molten or dissolved) that conducts electricity.
**Electrodes**: rods (usually graphite or platinum) that conduct current into/out of the electrolyte.
**Anode**: positive electrode; attracts **anions** (negative ions).
**Cathode**: negative electrode; attracts **cations** (positive ions).
**PANIC** mnemonic: **P**ositive = **A**node, **N**egative **I**s **C**athode.
Electrons flow from the power supply's negative terminal to the cathode; ions carry charge through the electrolyte.

Electrolysis of Molten Compounds

Molten binary ionic compounds (e.g., PbBr₂) decompose into their elements.
Example: molten **lead(II) bromide** → at cathode: grey **lead** metal forms; at anode: brown **bromine** gas bubbles.
**Cathode product** = metal (reduction: Mⁿ⁺ + n e⁻ → M).
**Anode product** = non‑metal (oxidation: 2X⁻ → X₂ + 2 e⁻).
Electrodes must be **inert** (graphite or platinum) to avoid side reactions.

Electrolysis of Aqueous Sodium Chloride & Dilute Sulfuric Acid

Aqueous solutions contain ions from the compound **and** from water (H⁺ and OH⁻).
**Brine** (conc. NaCl): at cathode, H⁺ is discharged → **hydrogen** gas; at anode, Cl⁻ is discharged → **chlorine** gas; NaOH remains in solution.
Industrial uses: chlorine → bleach; hydrogen → margarine; NaOH → soap/detergents.
**Dilute H₂SO₄**: at cathode, H⁺ → **hydrogen**; at anode, OH⁻ → **oxygen** $(2\times$ volume of H₂ compared to O₂).
Gas tests: H₂ – ‘squeaky pop’ with lit splint; O₂ – relights glowing splint; Cl₂ – bleaches damp litmus.

Electrolysis of Aqueous Solutions (Extended Tier)

Water dissociates: H₂O ⇌ H⁺ + OH⁻; these ions compete with ions from the solute.
**At cathode**: if metal is **less reactive** than H, metal is deposited; if **more reactive**, H₂ gas forms.
**At anode**: if **halide ions** are present (conc. solution), halogen forms; otherwise, OH⁻ is discharged → **oxygen**.
Concentration matters: conc. halide → halogen; dilute halide → oxygen.
Example: **CuSO₄(aq)** with graphite electrodes → cathode: pink‑brown **copper**; anode: **oxygen**; blue colour fades.
With **copper electrodes**: cathode gains mass (Cu²⁺ + 2e⁻ → Cu); anode loses mass (Cu → Cu²⁺ + 2e⁻); Cu²⁺ concentration stays constant.

Ionic Half Equations (Extended Tier)

**Oxidation** = loss of electrons; **Reduction** = gain of electrons (OIL RIG).
**RED CAT**: Reduction at the **Cathode**; **AN OX**: Oxidation at the **Anode**.
Half‑equations must balance atoms and charge.
Metal formation: Mⁿ⁺ + n e⁻ → M (e.g., Cu²⁺ + 2e⁻ → Cu).
Hydrogen formation: 2H⁺ + 2e⁻ → H₂.
Halogen formation: 2X⁻ → X₂ + 2e⁻ (e.g., 2Cl⁻ → Cl₂ + 2e⁻).
Oxygen formation: 4OH⁻ → O₂ + 2H₂O + 4e⁻.

Hydrogen‑Oxygen Fuel Cells

A **fuel cell** converts chemical energy of a fuel (H₂) and oxidant (O₂) directly into electricity.
At anode: H₂ → 2H⁺ + 2e⁻ (oxidation).
At cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O (reduction).
Overall reaction: 2H₂ + O₂ → 2H₂O; only product is **water**.
**Advantages**: no pollutants, high efficiency, renewable (H₂ from electrolysis).
**Disadvantages**: H₂ production often uses fossil fuels; storage is difficult (flammable, high‑pressure); expensive materials; limited refuelling stations.

Bohr model of sodium atom showing electron configuration (2,8,1). Used to illustrate ion formation in electrolysis.

Practice questions

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1.Which statement about electrolysis is correct?
Easy
- ACovalent compounds can undergo electrolysis when molten.
- BSolid ionic compounds conduct electricity because ions are free to move.
- CIonic compounds conduct electricity when molten or in aqueous solution.
- DElectrolysis breaks down covalent compounds by an electric current.
2.In electrolysis, the anode is the negative electrode.
Easy
True or false?
3.Complete the sentence about the PANIC mnemonic.
Easy
In the PANIC mnemonic, Positive $is ____ and$ Negative $is ____$ .
4.State the product formed at the cathode during the electrolysis of molten lead(II) bromide.
Medium
5.During the electrolysis of concentrated aqueous sodium chloride (brine) using inert electrodes, which gas is produced at the anode?
Medium
- AHydrogen
- BOxygen
- CChlorine
- DSodium
6.Give the test for hydrogen gas.
Medium
7.Match each electrode with the process that occurs there.
Medium
- Anode
- Cathode
- Oxidation
- Reduction
8.During electrolysis of dilute sulfuric acid, the volume of hydrogen produced is twice the volume of oxygen. If $40 cm^{3}$ of oxygen is collected, what volume of hydrogen $(in cm^{3})$ is produced?
Hard
- A72
- B88
- C80
- D96

Slides

What is Electrolysis?

Electrolysis is the breakdown of a molten or aqueous ionic compound by an electric current.
Ionic compounds in the solid state cannot conduct electricity because the ions are fixed in place.
When molten or dissolved in water, the ions are free to move and carry charge.
Covalent compounds do not conduct electricity, so they cannot undergo electrolysis.

Which of the following is required for electrolysis to occur?

A solid ionic compound
A molten or aqueous ionic compound
A covalent compound
A metal

Key Terms in Electrolysis

Electrode: a rod (usually metal or graphite) that conducts electricity into or out of the electrolyte.
Electrolyte: the ionic compound in molten or dissolved form that conducts electricity.
Anode: the positive electrode; anions (negative ions) are attracted to it.
Cathode: the negative electrode; cations (positive ions) are attracted to it.
Use PANIC: Positive Anode, Negative Is Cathode.

In electrolysis, the positive electrode is called $the ____ and$ the negative electrode is called $the ____$ .

Charge Transfer in Electrolysis

Electrons flow from the negative terminal of the power supply to the cathode.
At the cathode, cations gain electrons (reduction).
Anions move to the anode and lose electrons (oxidation).
Electrons then flow back to the positive terminal of the power supply.
In the external circuit, electrons are the charge carriers; in the electrolyte, ions are the charge carriers.

What happens at the cathode during electrolysis?

Anions lose electrons
Cations gain electrons
Anions gain electrons
Cations lose electrons

Electrolysis of Molten Lead(II) Bromide

Heat lead(II) bromide until it melts, freeing the ions.
Use inert graphite electrodes.
At the anode: bromide ions lose electrons to form brown bromine gas.
At the cathode: lead ions gain electrons to form grey lead metal.
Binary ionic compounds always produce their corresponding elements.

Which of these are true about the electrolysis of molten lead(II) bromide?

Bromine gas is produced at the anode.
Lead metal is produced at the cathode.
The electrolyte must be solid.
Graphite electrodes are used because they are reactive.

Electrolysis of Aqueous Sodium Chloride (Brine)

Brine is concentrated aqueous NaCl. Ions present: Na⁺, Cl⁻, H⁺, OH⁻.
At the cathode: H⁺ ions are discharged (less reactive than Na⁺) to form hydrogen gas.
At the anode: Cl⁻ ions are discharged to form chlorine gas.
Na⁺ and OH⁻ remain in solution, forming sodium hydroxide.
Products: chlorine (for bleach), hydrogen (for margarine), sodium hydroxide (for soap).

In the electrolysis of concentrated aqueous sodium chloride, what is produced at the anode?

Oxygen
Hydrogen
Chlorine
Sodium

Electrolysis of Dilute Sulfuric Acid

Ions present: H⁺, SO₄²⁻, H⁺, OH⁻ (from water).
At the cathode: H⁺ ions gain electrons to form hydrogen gas.
At the anode: OH⁻ ions lose electrons to form oxygen gas and water.
Twice as much hydrogen is produced compared to oxygen (by volume).
Gas tests: hydrogen gives a squeaky pop with a lit splint; oxygen relights a glowing splint.

In the electrolysis of dilute sulfuric acid, the gas produced at the cathode $is ____ and$ at the anode $is ____$ .

Electrolysis of Aqueous Solutions: Extended

Water dissociates: H₂O ⇌ H⁺ + OH⁻, so both compound ions and water ions are present.
At the cathode: if the metal is less reactive than hydrogen, the metal is deposited; if more reactive, hydrogen is produced.
At the anode: if halide ions are present (and concentrated), the halogen is produced; otherwise, oxygen is produced.
Concentration matters: concentrated halide solutions give halogen; dilute give oxygen.

During electrolysis of aqueous copper(II) sulfate with graphite electrodes, what is observed at the cathode?

Bubbles of oxygen gas
A pink-brown layer of copper
Bubbles of hydrogen gas
The blue colour fades

Ionic Half-Equations

Oxidation is loss of electrons; reduction is gain of electrons (OIL RIG).
At the anode (oxidation): anions lose electrons.
At the cathode (reduction): cations gain electrons.
Half-equations must balance atoms and charges.
Examples: Cu²⁺ + 2e⁻ → Cu; 2Cl⁻ → Cl₂ + 2e⁻; 4OH⁻ → O₂ + 2H₂O + 4e⁻.

The half-equation for the reduction of copper(II) ions is Cu²⁺ + ____ → Cu.

Hydrogen-Oxygen Fuel Cells

A fuel cell produces electricity by combining hydrogen and oxygen, forming only water.
At the anode: H₂ → 2H⁺ + 2e⁻ (oxidation).
At the cathode: O₂ + 4H⁺ + 4e⁻ → 2H₂O (reduction).
Overall reaction: 2H₂ + O₂ → 2H₂O.
Advantages: no pollutants, high efficiency, renewable fuel. Disadvantages: hydrogen production often uses fossil fuels, storage is difficult, expensive.

Which of these are advantages of hydrogen fuel cells over petrol engines?

The only product is water.
They produce carbon dioxide.
They are more efficient per kilogram of fuel.
Hydrogen is easy to store safely.

Flashcards

What is electrolysis?

Electrolysis is the process in which a molten ionic compound is broken down by an electric current. It also occurs for aqueous solutions of ionic compounds.

Why can't solid ionic compounds conduct electricity?

In solids, ions are fixed in place and cannot move. When molten or in solution, they are free to move and carry charge.

Define: electrode, electrolyte, anode, cathode, anion, cation.

Electrode: rod (metal/graphite) conducting electricity into/out of electrolyte. Electrolyte: ionic compound in molten/dissolved solution conducting electricity. Anode: positive electrode. Cathode: negative electrode. Anion: negative ion attracted to anode. Cation: positive ion attracted to cathode.

What mnemonic helps remember that the anode is positive and cathode is negative?

PANIC: Positive (is) Anode, Negative Is Cathode.

What is produced at the cathode when a metal is less reactive than hydrogen?

The metal is produced. For example, copper forms at the cathode.

What is produced at the cathode when a metal is more reactive than hydrogen?

Hydrogen gas is produced instead of the metal.

What is produced at the anode during electrolysis?

Non-metals (except hydrogen) are formed at the anode. For example, chlorine or oxygen gas.

How does charge flow in an electrolytic cell?

Electrons flow from the negative terminal to the cathode. Ions move in the electrolyte: cations to cathode, anions to anode. Electrons are charge carriers in the external circuit; ions are charge carriers in the electrolyte.

What are the products of electrolysis of molten lead(II) bromide?

At the anode: brown bromine gas. At the cathode: grey lead metal.

What are the products of electrolysis of concentrated aqueous sodium chloride (brine)?

At the anode: chlorine gas. At the cathode: hydrogen gas. Sodium hydroxide solution remains.

What are the products of electrolysis of dilute sulfuric acid?

At the anode: oxygen gas. At the cathode: hydrogen gas. Twice as much hydrogen is produced compared to oxygen.

How do you test for oxygen, hydrogen, and chlorine gases?

Oxygen: glowing splint relights. Hydrogen: lit splint gives a squeaky pop. Chlorine: damp litmus paper turns red then bleaches white.

In electrolysis of aqueous solutions, what determines which ion is discharged at the cathode?

If the metal is above hydrogen in the reactivity series, hydrogen is produced. If below, the metal is produced.

What are the products of electrolysis of aqueous copper(II) sulfate using graphite electrodes?

At the cathode: copper metal (pink-brown layer). At the anode: oxygen gas. The blue colour fades.

Write the half-equation for the reduction of copper(II) ions to copper metal.

Cu²⁺ + 2e⁻ → Cu

Write the half-equation for the oxidation of hydroxide ions to oxygen.

4OH⁻ → O₂ + 2H₂O + 4e⁻

What does OIL RIG stand for?

Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).

What does RED CAT and AN OX stand for?

REDuction at the CAThode, ANode for OXidation.

What is a hydrogen-oxygen fuel cell?

An electrochemical cell where hydrogen donates electrons at one electrode and oxygen gains electrons at the other, producing electricity and water.

Write the overall equation for the reaction in a hydrogen-oxygen fuel cell.

2H₂ + O₂ → 2H₂O

State two advantages of hydrogen fuel cells over petrol engines.

No pollutants produced (only water); high efficiency (more energy per kilogram).

State two disadvantages of hydrogen fuel cells.

Hydrogen is flammable and difficult to store safely; most hydrogen is currently produced from fossil fuels, releasing CO₂.

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