Electrochemistry

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课程笔记

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: Positive = Anode, Negative Is Cathode.
Electrons flow from the power supply's negative terminal to the cathode; ions carry charge through the electrolyte.

Ionic conductivity: solid vs molten/dissolved

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 molten lead(II) bromide

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× 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 sodium chloride (brine)

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.

Apparatus for the electrolysis of copper(II) sulfate (graphite electrodes)

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⁻.

Reduction and Oxidation Reactions at the Electrodes

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.

Diagram showing the movement of hydrogen, oxygen and electrons in a hydrogen-oxygen fuel cell

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练习题

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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.
Easy
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.
Easy
7.Match each electrode with the process that occurs there.
Easy
- Anode
- Cathode
- Oxidation
- Reduction
8.During electrolysis of dilute sulfuric acid, the volume of hydrogen produced is twice the volume of oxygen. If 40 cm³ of oxygen is collected, what volume of hydrogen (in cm³) is produced?
Medium
- A72
- B88
- C80
- D96

闪卡

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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