Cells

A simple chemical cell needs three things:

• Two electrodes made of different metals.
• An electrolyte (a solution that conducts ions — e.g. salt water, dilute acid, or a fruit's juice).
• A wire connecting the electrodes externally.

What happens:

• The MORE REACTIVE metal LOSES electrons → forms positive ions → dissolves. It becomes the NEGATIVE electrode.
• The LESS REACTIVE metal GAINS electrons from the external circuit → it becomes the POSITIVE electrode.
• Electrons flow through the WIRE from the negative to the positive electrode — that's the current.
• Inside the electrolyte, ions flow to balance the charge.

The voltage of the cell depends on the DIFFERENCE in reactivity between the two metals:

• Magnesium and copper: ~2.7 V (Mg far above Cu).
• Zinc and copper: ~1.1 V.
• Copper and silver: ~0.5 V (close together).

A 'lemon battery' works because the lemon juice is acidic enough to be an electrolyte; two different metal nails (e.g. zinc-coated and copper) act as the electrodes.

Primary cells (non-rechargeable) — the chemical reaction is NOT reversible. Once the reactants are used up, the cell is dead and must be discarded. Examples:

• Alkaline AA/AAA (Zn + MnO₂ in KOH paste): common in remotes, torches.
• Zinc-carbon: older, cheaper, but lower capacity.
• Lithium primary cells (button batteries): used in watches, hearing aids.

Secondary cells (rechargeable) — the chemical reaction can be REVERSED by passing electricity through it in the opposite direction. Examples:

• Lead-acid (Pb + PbO₂ in H₂SO₄): car batteries. Heavy but cheap, robust.
• Nickel-metal hydride (NiMH): older rechargeable AAs.
• Lithium-ion: phones, laptops, electric cars. High energy density per kg.

The choice between primary and secondary depends on the use case:

• Smoke alarms, watches, hearing aids — long shelf life, low current, infrequent change → primary.
• Phones, laptops, EVs — high power, frequent use → secondary.

Fuel cells (covered in the previous subtopic) are sometimes called 'cells with external fuel supply' — they don't run down like primary cells, and don't need recharging like secondary ones. They just need fuel.

Modern life depends on batteries, but they come with environmental concerns:

• Mining: extracting lithium, cobalt and rare earth metals damages landscapes and uses water. Cobalt mining in the Democratic Republic of the Congo raises serious human-rights concerns.
• Waste: spent batteries thrown in landfill leak heavy metals (Pb, Cd, Hg) and lithium salts into soil and groundwater.
• Recycling: batteries CAN be recycled — lead-acid car batteries are ~95% recycled in many countries; lithium-ion recycling rates are still low (~5% globally) but rising.
• Energy in production: making a battery uses energy. For EVs, manufacturing emits more CO₂ than making a petrol car — but lifetime emissions are still lower, depending on the electricity used to charge.

Designing for sustainability:

• Use less rare/toxic material (e.g. lithium-iron-phosphate instead of cobalt-based chemistries).
• Build batteries that last longer (more charge cycles per battery).
• Design for easy disassembly (so recovery is cost-effective).
• Second-life uses: ex-EV batteries used for grid storage when no longer car-grade.

MYP criterion D big-picture question: compare the lifetime environmental impact of (a) a petrol car, (b) a lithium-ion EV charged by gas-powered grid, (c) a Li-ion EV charged by renewables. Strong answers note that even (b) usually beats (a) overall, and (c) is much lower than both — provided we manage the end-of-life batteries responsibly.