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April 16

Heat of fusion: Cl > Na

I'm trying to understand why the heat of fusion of chlorine (3.2 kJ/mol) is higher than that of sodium (2.6 kJ/mol).

Chlorine has a melting point of 172 K and I gather this means that the intermolecular forces holding the lattice of chlorine molecules together are relatively weak.

Sodium has a melting point of 372 K so the interatomic forces holding a lattice of sodium atoms together are stronger.

Now, at each element's melting point, why does it take more energy to disrupt the solid chlorine lattice and complete the transformation to its liquid structure than is the case for the sodium lattice? (In both cases I understand that some short term structural order is preserved.) Sandbh (talk) 01:34, 16 April 2016 (UTC)[reply]

You will have to look at the liquids that are produced. Presumably liquid sodium is still held together by metallic bonds, whereas liquid chlorine only has van der Waals forces holding it together. You could look at the heat of vaporization (which for sodium is much greater than for chlorine) and specific heat between these points as well to see the whole story. Chlorine molecules have a more awkward shape and so could be more difficult to solidify than the spherical sodium atoms, and so that will affect the melting point too. Graeme Bartlett (talk) 05:46, 16 April 2016 (UTC)[reply]

Heat of fusion only takes into account enthalpy portions of the transitions. Melting point is based on the ratio of enthalpy and entropy. In this case, while the heat of fusion for chlorine is higher, the discrepancy of melting points can ONLY be explained by entropy. Wikipedia's article on entropy of fusion covers how melting point works as a relationship between entropy and enthalpy. --Jayron 32 23:30, 16 April 2016 (UTC)[reply]

Tx for the responses. I see that the heat of vaporisation figures are 99 for sodium, and 10.2 for chlorine. So the heat of fusion for Na is merely 2.5% of its heat of vaporisation whereas for chlorine it's a whopping 30%. So when sodium melts there seems to be hardly any bond disruption whereas there must be more of this action when solid chlorine melts. To eliminate any complications associated with comparing monatomic sodium with diatomic chlorine, I see that the heat of fusion of xenon (2.3) and radon (2.9) are comparable or greater than that of sodium (2.6). For Xe and Rn the heat of fusion values (12.6; 16) are about 18% of the heat of vaporisation figures. The only explanation that comes to mind is that, for some reason, the clusters of xenon and radon atoms in their liquid forms are smaller than the clusters of sodium atoms, which may be about 100 atoms a piece, in liquid sodium. Smaller clusters would then presumably require more interatomic bonds to be broken. Pure guess work on my part. Sandbh (talk) 23:52, 17 April 2016 (UTC)[reply]

"Magnetic in spring temper"

This site[1] says that:

   Stainless Steel Wire AISI 302/304... Magnetic in spring temper.

What does "magnetic in spring temper" mean? Does it mean that 302/304 stainless steel is not normally attracted by magnets, but in its spring temper condition it is? Or that it actually emits a magnetic field and attracts other materials in its spring temper condition?Johnson&Johnson&Son (talk) 12:11, 16 April 2016 (UTC)[reply]

The way I read that line: this alloy is manufactured and tempered specifically for use as a spring; and after it is tempered, the resulting product is ferromagnetic. Other (non-spring) products manufactured from the same alloy or same stock material might not include the tempering process and therefore might not be magnetized. Tempering involves application of heat, and that's a fantastic way to activate or inactivate ferromagnetism, depending on the ambient conditions during the heat treatment procedure.

Most variations of stainless steel are non-magnetic - which is a little unintuitive, considering that steel is mostly made of iron... so the fact that these stainless steel spring wires are tempered in some fashion that magnetizes them is worth calling out.

Nimur (talk) 13:50, 16 April 2016 (UTC)[reply]

Yeah our article on SS also says that work hardening of austenitic SS (normally non magnetic) can make it slightly magnetic.--178.99.232.11 (talk) 17:22, 16 April 2016 (UTC)[reply]

Thanks, guys. I'm asking because I brought some 304 spring temper steel wires, and some of them are attracted by magnets and some are not (more accurately it's not binary, there's various levels of how strong the attractions is). Could this be that some of the wires are actually not in spring temper but just regular 304 stainless steel wires? Johnson&Johnson&Son (talk) 04:01, 17 April 2016 (UTC)[reply]

Anything's possible - I certainly don't know what you've actually got in your inventory or where it came from. I think it's more probable that the material is all the same alloy; but the magnetism is an unintentional side-effect. The manufacturer is not trying to magnetize the metal during heat treatment, but it can occur, largely depending on ambient magnetic conditions during the heating. The manufacturer probably doesn't want or need to control those conditions, so the amount of magnetization may vary between batches. Even if the springs all started out with identical magnetization, it's ferromagnetism, which can be easily lost during normal handling just by, say, banging the material against the packaging during shipment, or being in close proximity to a differently-aligned magnetic field, or being stored at elevated temperatures, and so on. Nimur (talk) 05:18, 17 April 2016 (UTC)[reply]