Molten lead(II) bromide sim
Switch on first: the solid's ions are held in fixed positions in the lattice, so no current flows and the lamp stays off.
Molten lead(II) bromide · covers the GCSE, IGCSE, O-Level topic
What the sim shows
Switch on first: the solid's ions are held in fixed positions in the lattice, so no current flows and the lamp stays off. Light the burner. The temperature climbs, then stays at 373 °C while the solid melts (the flat part of the graph), and climbs again once it is all liquid. When the melt reaches the electrodes the ions are free to move, a current flows and the lamp lights. Pb²⁺ ions move to the cathode (−) and gain electrons: molten lead (it melts at 327 °C) collects as a silvery bead at the bottom. Br⁻ ions move to the anode (+) and lose electrons: red-brown bromine (it boils at 59 °C) is given off as a vapour. Moles of electrons = current × time ÷ 96500; lead and bromine each need 2 electrons, so they form 1 to 1 in moles. Put the burner out and it freezes at 373 °C and the lamp goes out. Bromine vapour and lead compounds are toxic, so this is done in a fume cupboard. The latent heat of fusion (about 16.4 kJ/mol) and the melting point are data-book values; the burner's heat, the losses and the cell's resistance (4 Ω when just molten, giving about 0.5 A at 6 V with an 8 Ω lamp) are this figure's estimates. The ions in the close-up are a picture, not a count. The clock runs 20 times faster than real time.
- Lamp
- Bromine vapour (red-brown)
- Molten lead (silvery)
- Pb²⁺ ion (small)
- Br⁻ ion (large)
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