Question bank — Silicon and silicates; silicones; zeolites

False. $C{O}_2$ is a discrete $O=C=O$ molecule (strong pπ–pπ double bonds); $Si{O}_2$ is a giant covalent solid of Si–O–Si single bridges because large Si has poor sideways 3p overlap and cannot form good double bonds. See Allotropy and giant covalent solids.

False. Si–Si single bonds are weak ($\approx 222$ kJ/mol) and easily attacked by water/oxygen, so Si catenates far less than carbon. Silicon prefers strong Si–O chains instead.

False, and it also isn't the element. A silicone is the synthetic polymer $[R_{2}SiO{]}_n$ — it has an Si–O backbone; the word just distinguishes it from silicon (element), silica ($Si{O}_2$) and silicate ($Si{O}_4^{4-}$).

False — it lowers it. Each shared (bridging) O is split between two tetrahedra, so it contributes less negative charge; sharing all four corners gives neutral $Si{O}_2$.

False. Al carries one less positive charge than Si, so each substitution makes the framework $-1$; that is exactly why cations sit in the cages. See Ion exchange and water softening.

False. The action is mostly shape-selectivity (pore size lets only certain molecules in/out) plus acidic framework sites — the loose cations are chiefly there for charge balance and exchange.

True. Beyond the ordinary σ bond, O lone-pair density donates into Si's empty 3d orbitals (pπ–dπ), reinforcing it — see Lewis acids and empty d-orbitals. Si–O ($\approx 452$) beats Si–Si and Si–C.

False. Quartz is a covalent giant network — melting means breaking countless strong Si–O bonds, so it melts very high; ionic melting is a different (and here irrelevant) argument.

They reused the isolated-unit formula. A chain shares 2 corners, so per Si there are 2 terminal + 2 bridging(½) O = 3 O, giving $Si{O}_3^{2-}$. Always re-derive from the tetrahedron.

Backwards. $R_{3}SiCl$ has only one Si–Cl (one reactive site), so it can only cap an end and stop growth. Linear chains come from the dichloride $R_{2}SiC{l}_2$ (two reactive sites).

Hydrolysis only makes the silanol $R_{2}Si(OH{)}_2$ plus HCl. The polymer forms in the next step: condensation, where two Si–OH lose water to make Si–O–Si. See Condensation polymerisation.

A double chain shares more corners on average (2.5), so its repeat unit is $S{i}_4{O}_{11}^{6-}$, not the single-chain $Si{O}_3^{2-}$. Sharing count sets the formula.

Only true for a $+1$ metal. A metal of charge $n$ balances $n$ negative sites, so you need $x/n$ of them — e.g. $C{a}^{2+}$ balances two $Al{O}_2^{-}$.

Being giant/covalent is unrelated to conduction. All valence electrons are locked in localised Si–O σ bonds (no free/delocalised electrons), so silica is an insulator — unlike graphite.

Its large 3p orbitals overlap sideways poorly, so pπ–pπ double bonds are weak; Si satisfies its bonding with strong single Si–O–Si bridges into giant solids instead. Contrast Carbon and its allotropes.

Si–O ($\approx 452$ kJ/mol) is far stronger than Si–Si ($\approx 222$), and Si–Si is easily oxidised/hydrolysed, so thermodynamics and kinetics both drive silicon into oxygen-bonded networks.

$RSiC{l}_3$ has three reactive Si–Cl bonds, so it creates cross-links between chains; more cross-links means a more rigid 3D network — thin oil becomes rubber/resin.

Each $A{l}^{3+}$-for-$S{i}^{4+}$ swap gives the framework $-1$ charge; the cages must hold exchangeable $+$ cations to keep the whole solid neutral.

Its cages and channels have a fixed molecular-scale size, so small molecules pass through while larger ones are excluded — geometry, not chemistry, does the selecting.

A bridging O is shared by two Si and satisfies both its bonds internally; a terminal corner O bonds to only one Si and keeps a negative charge to complete its octet. Trace this in Group 14 trends.

The isolated $Si{O}_4^{4-}$ tetrahedron: Si $(+4)$ + 4 O $(-2)$ = $-4$. This is the limiting "0 sharing" case (e.g. olivine).

Every O is then split between two Si (counts ½), giving 2 O per Si; $+4+2(-2)=0$, so the framework $Si{O}_2$ is neutral — the other limiting extreme.

All four sites react and cross-link in every direction, producing a dense rigid 3D network — no flexible chains, so it can't behave as a pourable oil or bendy rubber.

Long linear chains capped short by the occasional end-blocker — a thin, flowing silicone oil; the $R_{3}SiCl$ trace tunes (shortens) the chain length.

You break Löwenstein's rule (Al–O–Al bridges become forced), which is energetically disfavoured — real zeolites keep Si/Al $\ge 1$, so pure "all-Al" frameworks don't form.