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Author: Subject: anomalies in chemistry
Fery
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[*] posted on 7-2-2020 at 04:55
anomalies in chemistry


Please post here anomalies in chemistry.

Searching the forum, I found only this one post concering density of H2O2 + H2O here:
https://www.sciencemadness.org/whisper/viewthread.php?tid=10...

my contribution:

density of water
density of solid < density of liquid
maximal density of water at 3.98 °C
(life in water)

vanillin m.p. 81-83 °C > ethylvanillin m.p. 76 °C
(longer molecule usually higher m.p.)
(b.p. are without anomaly 285 °C < 295.1 °C)

aliphatic α-monoacids m.p.
C1 (formic) 8.4 °C
C2 (acetic) 16 - 17 °C
C3 (propionic) –21 °C
C4 (butyric) −5.1 °C
C5 (valeric) −34.5 °C
C6 (caproic) −3.4 °C
C7 (enanthic) −7.5 °C
C8 (caprylic) 16.7 °C
C9 (pelargonic) 12.5 °C
C10 (capric) 31.6 °C

solubility in water of linear α,ω-dicarboxylic acids - https://azelaic.com/how-can-we-help/process-azepur99/
C2 (oxalic) 220 g/l (25 °C)
C3 (malonic) 763 g/l (25 °C)
C4 (succinic) 83 g/l (25 °C)
C5 (glutaric) 639 g/l (20 °C)
C6 (adipic) 14 g/l (20 °C)
C7 (pimelic) 50 g/l (20 °C)
C8 (suberic) 12 g/l (20 °C)
C9 (azelaic) 2,1 g/l (20 °C)
C10 (sebacic) 0,1 g/l (20 °C)
(here a zigzag scheme visible, no anomaly if you sort them in odd/even orders: in array [C2, C4, C6, C8, C10] it is [220, 83, 14, 12, 0.1], in array [C3, C5, C7, C9] it is [763, 639, 50, 2.1]

inverted solubility curve of some sulphates in water: CaSO4, MnSO4, Ce2(SO4)3 - https://sci-hub.tw/10.1021/ed016p183




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[*] posted on 7-2-2020 at 11:17


There is a logic to every anomaly in chemistry, once you understand the logic they are no longer anonymous. But looking for them is a great way of learning.

Take the ionization of elements:

20200207_191132.png - 308kB

As the atomic number increases, the growing effective charge from the nucleus increases ionization energy. But half way through each shell the energy has a bump. Is this anonymous or predictable? Well electron pairing makes it mostly predictable.

There will always be more anomalies and more to learn the deeper you dig.
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[*] posted on 7-2-2020 at 11:33


Density
The density of gallium, bismuth, silicon, and germanium are all higher in the liquid phase than the solid - the solids float in the liquid

Melting point
You missed a more obvious one - benzene and toluene. Benzene melts at 6°C, toluene at -110°C. This is due to how the molecules pack in the solid phase - symmetrical benzene forms nice stacks that slot together easily, but toluene with its methyl group doesn't pack nearly so well.

inverted solubility curve of some sulphates in water Actually, most lanthanide sulphates have that behaviour, which makes separating Nd from the magnets comparatively simple




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[*] posted on 7-2-2020 at 12:17


Boiling points:
Benzene and cyclohexane have similar boiling points although one has nearly twice the hydrogen atoms per unit of mass to participate in Van Der Waals bonding.

I believe the boiling points of aromatics like benzene are higher than one would expect because they are such stiff molecules.

Cyclohexane can just flop around like mad once in the gas phase, but Benzene doesn't have that added degree of freedom when it vaporizes.




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[*] posted on 8-2-2020 at 06:35


The classic ones are Ammonia, Water and Hydrogen fluoride. All have much higher boiling points than expected and way higher than the next ones in the series, Phosphine, Hydrogen sulphide and hydrogen chloride. Largely explained by hydrogen bonding. Water is further anomalous in that solid water is less dense than cold liquid water.



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[*] posted on 13-2-2020 at 05:48


Electron configurations of first row transition metals...
Cr and Cu break the expected trend.

Sc — 4s2 3d1
Ti — 4s2 3d2
V — 4s2 3d3
Cr — 4s1 3d5
Mn — 4s2 3d5
Fe — 4s2 3d6
Co — 4s2 3d7
Ni — 4s2 3d8
Cu — 4s1 3d10
Zn — 4s2 3d10

Second row transitions are even more unpredictable but the third row settles down a bit.

Of course there exist logical explanations for these apparent anomalies. But these invoke higher principles than the rules that govern configurations for all lighter elements. The patterns emerging from the first 20 elements would not cause us to predict this behaviour.
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