Bonding and melting point?

Question

Describe the bonding in each oxide, and how it relates to its melting point.?

(i) CO2
(ii) SiO2
(iii) SnO2

Answer 1

hi

i will explain u the bonding in these 3 compounds ...
make sure u have ure basic concepts clear...

Firstly.......carbon dioxide.......

CO2 has a linear shape (sp hybridization)
Carbon dioxide could be drawn (without making any assumptions about the shape) as

in link...http://i180.photobucket.com/albums/x281/rohan_aurora/shapeco2.gif


The carbon originally had 4 electrons in its outer level (group 14). Each oxygen contributes 2 electrons - 1 for each bond. That means there are a total of 8 electrons around the carbon, in 4 pairs. Because there are 4 bonds, these are all bond pairs.

Each double bond uses 2 bond pairs - which are then thought of as a single unit. Those two double bond units will try to get as far apart as possible, and so the molecule is linear. The structure we've drawn above does in fact represent the shape of the molecule.

Boiling point −78 °C (195 K), sublimes
Melting point −57 °C (216 K), pressurised

Due to samll size and high electonegativity carbon forms p pi - d pi multiple bonds .In CO2 the molecules are held together by weak Vander Waals forces of attration......so it exist as a gas in room temperature...

its structure predicts that bothe carbon oxygen bond lengths in CO2 should be equal to CO2 double bond ..122pm

make its resonating structures.....
due to resonance carbon-oxygen bond acquires some triple bond character thus its bond length reduces from 122pm to 115 pm...!!!..This way its melting point is also found to be very less..................−57 °C


FOR RESONANCE GO,..
http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Resonance-CO2-Ans.html

Now u must have understood the basic concept in detail...this way i will tell u in bried the bondiinf in SiO2 and SnO2




Si02

Melting point (°C) 1830 C

it basically has complex structure....

Silicon dioxide is formed when silicon is exposed to oxygen (or air). A very thin layer (approximately 1 nm or 10 Å) of so-called 'native oxide' is formed on the surface when silicon is exposed to air under ambient conditions..

Silicon dioxide has covalent bonding and forms a network structure (also known as lattice or continuous)....thus it has very high melting point...............




SnO2
it is ,theoretically ionic but largely covalent,
it has very high melting point
Melting Point (°C) 1630

HOPE THIS IS THE BEST ANSWER UVE GOT..
all the best..........i had explained u everything properly...

(never read incomplete answers..the above guy's answer)

Answer 2

There are corrrect statements in both previous answers but they do not tell the whole story. The melting point of a substance is related to the amount of energy needed to separate particles of the substance so that they are able to move freely relative to one another in the liquid state.

CO2 forms small covalent molecules with covalent bonding (O=C=O). The individual atoms do not need to be separated from one another on melting - only each molecule from its neighbours. This only requires the breaking of the intermolecular forces known as van der Waals' (or dispersion) forces which are weak and hence require a low temperature. It is irrelevant whether the covalent bonds are single, double or whatever - what matters is that the intermolecular forces are weak.

SiO2 foms a giant covalent lattice in which each Si is singly covalently bonded to four Os and each O is bonded to two Sis. To melt SiO2, it is necessary to break all the covalent bonds in the lattice which requires a large amount of energy and hence a high melting point.

SnO2 is often considered to be an ionic compound but the Sn4+ ion would be very small and hence highly polarising. The O2- ion is polarised enough that the bonding between Sn and O can be considered to be significantly covalent. Thus, to melt SnO2, a whole network of strong bonds has to be broken and the melting point is high (generally accepted as 1127C) [ http://physchem.ox.ac.uk/MSDS/TI/tin_IV_oxide.html ]

Answer 3

I'm not 100% sure, but I would say that diamond does, because it's in a much more ordered crystal (aka in a lower energy form, so to speak) and would require more energy to break the crystalline lattice, whereas graphite is carbon in sheets, not as ordered, and therefore not as low energy of a form.

Answer 4

CO2 -covalent, simple molecular, vdW attractive forces only, v low MP.
SiO2, covalent, giant molecular, all covalent bonds need to be broken on melting/ boiling, v high melting point.
SnO2, theoretically ionic but largely covalent, low melting point.