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Y9 Chemistry: Help!

(6 Posts)
Tansie Tue 02-Oct-12 15:57:40

2 identical test tubes,
1- potassium chloride solution
2-platinum chloride solution.

A zinc strip is dropped into both. The platinum TT's zinc strip gets slowly covered in grey deposit; nothing happens in the potassium TT.

Qs: What was the deposit in the platinum TT (on the zinc)?
Why was it formed?
Why did no reaction happen in the potassium TT?

I got a 'U' for Chem 'O' level in 1978...

TIA!

cleanandclothed Tue 02-Oct-12 16:04:54

This is about the relative reactivity of zinc, potassium and platinum. The deposit is, I think, zinc chloride as the zinc ions have displaced the platinum ions from the platinum chloride. They don't displace the potassium ions because potassium is very reactive. For metals, especially group 1 and 2, reactivity increases with size ( or going down the periodic table) as it is easier to lose an electron the further out the electrons are from the nucleus. Hence potassium loses two electrons easily. Look up the electron configurations for the metal ions and it should make sense.

radicalsubstitution Tue 02-Oct-12 16:09:11

The deposit is platinum. Zinc is more reactive than platinum, so displaces platinum from platinum chloride leaving zinc chloride (soluble) and platinum metal.

Potassium is more reactive than zinc, so there is no reaction. Zinc will not displace potassium from a compound.

radicalsubstitution Tue 02-Oct-12 16:11:23

Potassium is a group 1 metal, and loses only one electron when forming an ion.

In Year 9, students are expected to know that a more reactive metal will displace a less reactive metal from a compound, and use this to decide which is the more reactive metal.

They do not need to know how electron configuration/atomic size affects this.

radicalsubstitution Tue 02-Oct-12 16:12:13

I am a chemistry teacher BTW.

Tansie Tue 02-Oct-12 18:06:41

Thanks, that was very helpful!

I had to find a reactivity table in his course book that gave some clues!

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