List compounds in order of increasing melting point

[toothpaste666]

Homework Statement

The Attempt at a Solution

My answer was C but the correct answer is A. My reasoning is that they are all bonded to chlorine, so the further away the element is from chlorine on the periodic table, the more ionic the compound is and the higher the melting point is. The correct answer was the opposite ordering of the answer i picked and i can't figure out why

 

[Bystander]

Ah, nothing like the trends of the periodic table --- what can you tell me about the charges on the cations?

 

[toothpaste666]

Al is +3 Mg is +2 and Na is +1, so since the cation of Al has a higher charge, it is more ionic and therefore has the highest melting point?

 

[Bystander]

Let's not get too far ahead --- you are on the right track, but let's take things a step at a time. All these are in the same row/period of the table, and what effect does increasing nuclear charge have on atomic radius as you move across the table? Step 2, what effect on radius of cations as ionic charge increases? Three, what effect of that change in radius on lattice energy? Four, what effect of lattice energy on melting point?

 

[toothpaste666]

increasing nuclear charge reduces the atomic radius. as for the effect of cations , the more positive the charge the smaller the radius. As for the lattice energy I am not sure if it is the same as ionization energy, but i know ionization energy is higher if the radius is smaller. is this right so far?

 

[Bystander]

You are cooking.

 

[toothpaste666]

so if this is right then i would say that the higher the ionization energy the higher the melting point. if it takes more energy to break the compound apart it makes sense that it would break apart at a higher temperature right?

 

[Bystander]

"Lattice energy." The more highly charged the cation, the larger the energy released by anions falling into its electric field, and that is the energy that must be overcome to melt the compound.

 

[toothpaste666]

I see. Time to do some more reading on lattice energy. thank you!

 

[Borek]

Time to stick to facts ;) According to wikipedia:

AlCl₃ 192.4°C
MgCl₂ 714°C
NaCl 801°C

 

[Bystander]

Time to stick to facts ;)

Another beautiful periodic trend ruined by ugly facts --- not only 192.4 C, but at 2.5 atm. AgCl melted on an anodized Al surface (Al₂O₃) on a hotplate, under a hood, yields AgO + AlCl₃, the AgO decomposes to form a nice silver film bonded to metallic Al, the chloride having sublimed, and you're all set to attach thermocouples, other electrical leads, to insulated (heavily anodized) Al heat sinks, or other Al parts. Been doing that for years, and couldn't remember it in this context? Gotta stop taking it for granted that teaching exercises and answer keys are prepared or proof-read.

 

[toothpaste666]

Wait now i am confused lol

 

[Bystander]

Was the question at the end of a discussion of periodic trends? Asking you, "If this, then what would you expect the order to be?" I walked right into that trap as Borek has pointed out --- didn't even bother to think about what's in the handbooks.

 

[Borek]

TBH I was ready to fell into that trap as well, I just remembered that AlCl₃ is surprisingly covalent - so I decided to check what the real numbers are before commenting.

Toothpaste: chances are whoever asked the question and listed a "correct" answer did it without checking facts and following one the simplified models. As you see, that's often dangerous.

 

[toothpaste666]

yeah this was after learning about periodic trends as well as learning about different types of bonding. so according to periodic trends it would be A but really its C?

 

[Bystander]

You are catching on --- sorry I didn't catch it earlier. Other effects at work that don't really count as "periodic trends" include "covalent" nature of some ionic bonds, and ratios of ionic radii and how they pack into crystalline lattices. Highly charged ions, like Al³⁺, will distort the electron distribution around the chloride ions, leading to a molten state in which molecular species like AlCl₃ and Al₂Cl₆ are dominant rather than ionic species like Na⁺ and Cl^- for the other end of the T range. MgCl₂ is going to be splitting the difference.