1.9.1Principles of Chemistry
Explaining Conductivity — Edexcel Linear - OL (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 22 of 25 Unit quiz
Section 1 of 5What Carries the Current
Conductivity depends on whether mobile charged particles are present, not simply on the type of bonding.
Electric current
- An electric current is a flow of charged particles, which may be electrons in a metal or ions in a liquid.
Electrons
- In metals and graphite, delocalised electrons are free to move through the structure and carry charge.
Ions
- In molten or dissolved ionic compounds, positive and negative ions move towards oppositely charged electrodes.
Condition
- A substance conducts only if it contains charged particles that are free to move.
Section 2 of 5Ionic and Covalent Compounds
Ionic and Covalent Compounds
Solid ionic
- Ions are locked in fixed positions in the lattice by strong attractions, so a solid ionic compound cannot conduct.
Molten ionic
- Heating beyond the melting point frees the ions, so the liquid conducts and is split up by electrolysis.
Aqueous ionic
- When an ionic compound dissolves in water its ions separate and move freely, so the solution conducts.
Covalent compounds
- Most simple molecular substances have no ions or free electrons, so they are insulators in every state.
Uses of insulators
- Covalent plastics coat electrical wires, and sulfur hexafluoride gas insulates equipment such as transformers.
| Substance | Conducts? | Reason |
|---|---|---|
| Solid sodium chloride | No | Ions fixed in lattice |
| Molten sodium chloride | Yes | Ions free to move |
| Sodium chloride solution | Yes | Ions free to move |
| Pure water or sugar solution | No (practically) | No free ions |
| Copper metal | Yes | Delocalised electrons |
Section 3 of 5Electrolysis Vocabulary
Electrolysis Vocabulary
Electrolysis
- The decomposition of an ionic compound, molten or dissolved, by passing an electric current through it.
Electrolyte
- The molten or aqueous ionic compound that conducts and is decomposed during electrolysis.
Electrodes
- Solid conductors, often inert graphite or platinum, that carry current into and out of the electrolyte.
Anode and cathode
- The anode is the positive electrode attracting anions;
- the cathode is the negative electrode attracting cations.
Circuit
- Electrons flow through the wires from anode to cathode while ions carry the charge through the electrolyte.
Section 4 of 5From Compound to Products
1
The ionic compound is melted or dissolved so that its ions become free to move.
2
The power supply makes one electrode negative and the other positive.
3
Positive ions migrate to the cathode and negative ions migrate to the anode.
4
At each electrode ions gain or lose electrons and are discharged as neutral elements.
5
The compound is therefore broken down into its elements, which is why electrolysis is a decomposition.
Section 5 of 5Exam Checklist & Tips
1.9.1 – Key Points for the Exam
- Conductor A substance that allows an electric current to pass through it.
- Insulator A substance with no mobile charged particles, so no current flows.
- Electrolyte An ionic liquid or solution that conducts and is decomposed by current.
- Anion A negatively charged ion, attracted to the anode.
- Cation A positively charged ion, attracted to the cathode.
Common Mistakes
- Saying that solid ionic compounds do not conduct because they have no ions;
- the ions are present but cannot move.
- Claiming electrons travel through the electrolyte;
- ions carry charge in the liquid and electrons in the wires.
- Thinking all covalent substances are insulators; graphite conducts because of delocalised electrons.
- Mixing up the electrodes; the anode is positive and attracts negative ions.
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