Silicon: Silica, Silicates, Zeolites and Silicones

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Lab acids usually sit in glass bottles, but one acid is always kept in wax or plastic. Which one is it, and why? Silica holds the answer. Silicates come up often in JEE, so let's begin.

0:19Silica: one giant network

So, silica, S i O two, where the small two written low means two oxygens per silicon. Each silicon bonds to four oxygens, tetrahedrally, and each oxygen to two silicons. So why does the formula show only two oxygens? Yes, exactly! Each oxygen is shared by two silicons, so four times one half, the fraction on screen, gives two. There are no separate molecules, just one giant three D network. Clear? Its forms include quartz, cristobalite and tridymite. Silica gel is a drying agent and an adsorbent. And silica itself is very unreactive. But glass contains silica, and one acid does attack it. So, which acid is it? Yes, H F, hydrofluoric acid! S i O two plus four H F gives S i F four and two water molecules; the arrow means gives. So H F eats glass, which is why it is kept in wax or plastic, okay? Then S i F four takes two more H F, giving H two S i F six. Alkalis attack it too: with N a O H, sodium hydroxide, silica gives N a two S i O three and water. Got it?

1:42Silicates: corner-sharing tetrahedra

Now, silicates. All are built from S i O four in square brackets, with its charge, four minus, written up high. Each triangle in the figure is one tetrahedron seen from above: the dot is silicon, with the fourth oxygen above it. Okay? Tetrahedra join by sharing corners: grey circles are the shared, or bridging, oxygens, and pink ones are terminal, not shared. You see a unit, a pair, a ring and a chain. If a tetrahedron shares s of its four oxygens, how many does it own? Four minus s are its own, and each shared one counts as a half. So it owns four minus s, plus s over two, which is four minus s over two, the little expression written low. With me?

2:31The silicate types

Now the types, by s, the number of shared oxygens. s equals zero: separate four minus ions, the orthosilicates, like M g two S i O four. s equals one: two tetrahedra share an oxygen, giving S i two O seven, six minus, as in thortveitite. Okay? s equals two: rings or single chains of S i O three, with a small n outside meaning repeated n times, and charge two n minus. Beryl is a ring, and pyroxenes are chains. Right? s equals two and a half: double chains, S i four O eleven, charge six n minus. These are the amphiboles, like asbestos. Clear? s equals three: sheets, S i two O five, two n minus, as in talc and mica. And s equals four: four minus two leaves S i O two, our three D silica, quartz! Got it?

3:31Aluminosilicates and zeolites

Now a twist. Put aluminium, A l three plus, in place of some silicon, S i four plus. Each swap brings one positive charge less, so the framework turns negative. Okay? Cations sit inside to balance it: these are the aluminosilicates. In feldspar, K A l S i three O eight, potassium does the balancing. Zeolites have a honeycomb of cavities and channels. Right? Only molecules of the right size and shape fit those channels, so zeolites are shape-selective catalysts: ZSM five turns alcohols directly into petrol. Hydrated zeolites also soften water as ion exchangers. With me?

4:14Glass and Portland cement

Next, glass: an amorphous solid, often called a supercooled liquid. Soda glass is made by fusing sand, soda and lime. Okay? Now, Portland cement. Good cement needs two ratios: silica over alumina, A l two O three, the slash meaning over, from two point five to four; and C a O over silica plus alumina plus F e two O three, close to two. Right? The major constituent is tricalcium silicate, C a three S i O five, at fifty one percent. And gypsum is added to slow down the setting.

4:51Silicones: preparation, hydrolysis and condensation

Clear? Now, silicones: polymers of R two S i O units, in the heading, with R an alkyl group and the dashes showing the chain goes on. Two molecules of methyl chloride, C H three C l, are heated with silicon; over the arrow are copper, the catalyst, and five hundred and seventy kelvin. The product, C H three in brackets with a small two, then S i C l two, has two methyl groups and two chlorines on silicon. Okay? Next, hydrolysis. With two water molecules, each chlorine is replaced by O H, giving the silanol, with two O H groups on silicon, and two H C l. Right? Then the silanols condense: two O H groups lose a water molecule and leave an S i, O, S i link. Repeat this n times, and the chain in square brackets is a silicone. Clear?

5:45Chain, cross-link or chain end?

This comes up often. On hydrolysis, R two S i C l two gives two O H groups per silicon, so each silicon links on two sides: chains, or rings. So what will R S i C l three give? Yes, exactly, three O H groups, so a cross-linked three D polymer. And R three S i C l? More R groups, so a longer chain, right? That is the trap, so be careful! Only one O H means only one link. So two molecules form the dimer you see, the dashes being bonds, and stop. So R three S i C l ends chains and controls their length. Okay? Finally, the non-polar alkyl groups on the outside keep water off, so silicones are water repellent. They are also thermally stable, inert and good insulators. Got it?

6:41Example 1: check cement quality from its oxide ratios

Alright, example one. Here, a cement sample has sixty three percent C a O, twenty two percent silica, seven percent alumina and three percent F e two O three. Check its two quality ratios. Pause the video and try it first. Done? Okay, let's check. Silica over alumina is twenty two over seven, which gives three point one, inside two point five to four. Right? Careful now: the second ratio has all three oxides at the bottom, and twenty two plus seven plus three gives thirty two. Then sixty three over thirty two gives one point nine seven, close to two. So this is good cement.

7:25Example 2: is talc a double chain or a sheet silicate

Clear? Now a harder one. Talc is M g three, two O H groups, and S i four O ten. How many oxygens of each tetrahedron are shared, and what type is it? Pause the video and try it first. Done? Okay, let's check. S i four O ten is two times S i two O five: ten oxygens over four silicons gives two point five per silicon. Our table had two and a half for a double chain. So talc is a double chain, right? That is the trap, so be careful! In the table, two and a half was s, the shared corners; here it is oxygens per silicon. So four minus s over two equals two point five, and the double arrow gives s equals three: a sheet silicate. Clear? Now the charge: four silicons at plus four, and ten oxygens at minus two, give minus four. Three M g two plus and two O H minus give plus four, so it balances exactly. Got it?

8:31Quick revision: silica, silicate types, silicone chains

So, quick revision. Silica is a giant three D network, attacked by H F and alkalis. Silicates are S i O, four minus s over two, with s the shared corners, not oxygens per silicon. Silicones: R two S i C l two for chains, R S i C l three for cross-links, R three S i C l to end them. Now try the practice problems on this page. You did really well today. All the best!