VCE Chemistry — Unit 3 AOS 2
Primary Cells — Flashcards & Quiz
Primary cells are non-rechargeable batteries that convert chemical energy into electrical energy via a spontaneous redox reaction. VCE Chemistry Unit 3 AOS 2 contrasts them with secondary (rechargeable) cells and asks you to write half-equations and calculate standard cell EMF. Common examples include zinc-carbon (dry cell) and alkaline cells.
Key Points
- Primary cells are non-rechargeable: once the reactants are consumed, the cell is discarded.
- Galvanic cell structure: anode (oxidation, negative terminal), cathode (reduction, positive terminal), electrolyte, salt bridge or separator.
- Dry cell (zinc-carbon): Zn anode, MnO₂/C cathode, NH₄Cl paste electrolyte. EMF ~1.5 V.
- Alkaline cell: Zn anode, MnO₂ cathode, KOH electrolyte. Longer life than dry cell, steadier voltage.
- Calculating EMF: E°_cell = E°(cathode) – E°(anode) using standard reduction potentials.
- Limitations: cost per energy (high for primary cells), environmental waste, and the voltage drops as reactants deplete.
Common Mistakes to Avoid
- Confusing primary (non-rechargeable) with secondary (rechargeable) cells.
- Mixing up polarity: in a galvanic cell, anode is NEGATIVE and cathode is POSITIVE.
- Forgetting the salt bridge or separator — without it, charge accumulates and the cell stops.
- Applying cell EMF formulas incorrectly — it's cathode minus anode, not the other way.
- Calling reactions "reversible" for primary cells — they're thermodynamically reversible in principle but kinetically locked.
Exam Strategy
VCAA Unit 3 AOS 2 primary cell questions ask you to write half-equations, calculate EMF, or compare primary and secondary cells. Method: (1) identify anode and cathode, (2) write half-equations with states, (3) calculate E°_cell using the standard reduction potential table, (4) discuss advantages and disadvantages vs secondary cells.
Sample Flashcards
Q1: What are the common design features of a primary galvanic cell?
A primary (non-rechargeable) galvanic cell converts chemical energy into electrical energy using a spontaneous redox reaction. The oxidising agent and reducing agent are kept apart in two half-cells, so electrons must travel through an external circuit. The anode is where oxidation occurs and is the negative electrode; the cathode is where reduction occurs and is the positive electrode. Electrons flow from anode to cathode through the external circuit. A salt bridge (or porous barrier) containing an inert electrolyte allows ions to move, completing the circuit and keeping each half-cell electrically neutral.
Q2: What roles do electrodes and electrolytes play in a primary galvanic cell?
Electrodes conduct electrons to and from the external circuit. A reactive electrode takes part in the reaction (e.g. a zinc anode that is oxidised and dissolves). An inert electrode (e.g. platinum or graphite) does not react and is used when the oxidising or reducing agent is a gas or dissolved ion. Electrolyte solutions supply ions that react at the electrodes and conduct charge within the cell; the salt bridge electrolyte (e.g. KNO₃) is chosen so it will not react or form precipitates. A primary cell stops producing current when a reactant is used up and cannot be practically recharged.
Revision Tip
Half-equation writing is procedural — drill a Revizi deck with common primary cell reactions until you can write them from memory.
Related Concepts
Last updated: 9 October 2026 · 2 sample flashcards