Author Topic: CHEM AND BIO HELP AND TIPS HERE  (Read 357221 times)

elemis

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2445 on: June 03, 2010, 04:58:44 pm »
+rep Ivo ;)

Good Job !

Offline NidZ- Hero

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2446 on: June 03, 2010, 05:25:07 pm »
how does electrostatic dust precipitator removes charged particles from sulphur dioxide  ??? ??? ??? ???
thanz

Offline Baladya

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2447 on: June 03, 2010, 05:27:30 pm »
how does electrostatic dust precipitator removes charged particles from sulphur dioxide  ??? ??? ??? ???
thanz

wtf is that  :D ;D xD
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Offline CatAly$t

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2448 on: June 03, 2010, 05:44:46 pm »
Brine is electrolysed in a diaphragm cell.

The ions in solution are Na+, H+, Cl- and OH-.

At the anode: chloride ions lose electrons and are discharged as chlorine gas:

2Cl- -> Cl2 + 2e-

At the cathode: hydrogen ions accept electrons and hydrogen gas is discharged rather than sodium:

2H+ +2e- -> H2

The ions remaining in solution are Na+ and OH-.  So an aqueous solution of sodium hydroxide is formed.

any idea y is it called diaphragm cell?
InshaAllah 8 A* in IGCSE....Ameen.....

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Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2449 on: June 03, 2010, 05:55:24 pm »
any idea y is it called diaphragm cell?

Maybe because there is a porous membrane (a.k.a. diaphragm) separating the anode and cathode, I guess.
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Offline Helium

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2450 on: June 03, 2010, 06:06:38 pm »
Brine is electrolysed in a diaphragm cell.

The ions in solution are Na+, H+, Cl- and OH-.

At the anode: chloride ions lose electrons and are discharged as chlorine gas:

2Cl- -> Cl2 + 2e-

At the cathode: hydrogen ions accept electrons and hydrogen gas is discharged rather than sodium:

2H+ +2e- -> H2

The ions remaining in solution are Na+ and OH-.  So an aqueous solution of sodium hydroxide is formed.

Thanks Ivo u summed it up :D
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**RoRo**

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2451 on: June 03, 2010, 06:14:26 pm »
Why does the melting point, boiling point and density increase as you go down a group?
And is this statement true for all the groups [metals and non-metals]?

Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2452 on: June 03, 2010, 06:18:05 pm »
Why does the melting point, boiling point and density increase as you go down a group?
And is this statement true for all the groups [metals and non-metals]?

It really depends on which group you're talking about.

Group I metals: the melting and boling points decrease down the group.

Group VII elements: the melting and boiling points of the halogens increase down the group.  This is the opposite trend to the Group I metals.

I believe those are the 2 trends you need to know for IGCSE.
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**RoRo**

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2453 on: June 03, 2010, 06:43:36 pm »
It really depends on which group you're talking about.

Group I metals: the melting and boling points decrease down the group.

Group VII elements: the melting and boiling points of the halogens increase down the group.  This is the opposite trend to the Group I metals.

I believe those are the 2 trends you need to know for IGCSE.

OK, why is there this difference, I mean why is it that the melting and boiling point in Group I decrease as you go down the group and increase as you go up Group VII? Has this got something to do with their valence electrons? Can you please explain that a little further?

Thank you so much :)

Offline Lariemeeva

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2454 on: June 03, 2010, 06:54:06 pm »
^First you need to understand bonding in metals. Each metal atom loses its outer electrons, which are then free to move between the lattice of positively charged metal ions in the solid. The metal ions are held in a rigid formation by the force of attraction between the positive ions and the 'sea' of negative electrons surrounding them. As you go down group 1, however, the atoms become larger so that the positive nucleus gets further away from the negative sea of electrons. The force of attraction between the metal ions and the sea of electrons thus gets weaker down the group and the melting points decrease as less heat energy is needed to overcome this weakening force of attraction.

I hope this makes sense.
« Last Edit: June 03, 2010, 06:59:47 pm by Lariemeeva »

Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2455 on: June 03, 2010, 06:55:09 pm »
OK, why is there this difference, I mean why is it that the melting and boiling point in Group I decrease as you go down the group and increase as you go up Group VII? Has this got something to do with their valence electrons? Can you please explain that a little further?

Thank you so much :)

By the way, we only need to know the trends for Group I and IV for b.p.s and m.p.s.

Right.  First you need to understand bonding in metals.  Each metal atom loses its outer electrons, which are then free to move between the lattice of positively charged metal ions in the solid.  The metal ions are held in a rigid formation by the force of attraction between the positive ions and the 'sea' of negative electrons surrounding them.  As you go down the group, however, the atoms become larger so that the positive nucleus gets further away from the negative sea of electrons. The force of attraction between the metal ions and the sea of electrons thus gets weaker down the group and the melting points decrease as less heat energy is needed to overcome this weakening force of attraction.

For Group IV, as you go down the group, the molecules get bigger, so the atomic radius increases as the number of electrons increase.  Because halogens exist as diatomic molecules, they have intermolecular forces (of the Van der Waals dispersion forces).  So when the molecules are bigger, there is an increase in the chances for setting up temporary dipoles - which causes these attractive forces.  So the stronger the attractive forces, the more energy required to break these bonds, so m.p. and b.p. increases.

I have to stress however that at IGCSE you do not need to know why any of these trends happen.  You simply need to know what the trends are.  So don't worry if all this jargon doesn't make sense to you right now, more will become clear if and when you study AS Chemistry.

Anyway, hope I answered your question clearly enough.  ;D
Always willing to help!  8)
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Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2456 on: June 03, 2010, 06:56:35 pm »
^First you need to understand bonding in metals. Each metal atom loses its outer electrons, which are then free to move between the lattice of positively charged metal ions in the solid. The metal ions are held in a rigid formation by the force of attraction between the positive ions and the 'sea' of negative electrons surrounding them. As you go down group 1, however, the atoms become larger so that the positive nucleus gets further away from the negative sea of electrons. The force of attraction between the metal ions and the sea of electrons thus gets weaker down the group and the melting points decrease as less heat energy is needed to overcome this weakening force of attraction. The opposite applies to Group VII.

I hope that makes sense.

LOLS, no!  the Group IV elements aren't metals.  If you're gonna copy and paste at least think!  So halogens don't have a 'sea' of electrons or positive ions, just think for a minute!  :P
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Offline Lariemeeva

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2457 on: June 03, 2010, 06:59:32 pm »
Hm...I actually understood it that way. I totally missed the fact that were talking about non-metals. The copied part is actually right for the record, my addition was wrong. If you're going to elaborate, don't quote the whole thing since it's true.
Oh right, you got that as well, what a coincidence.
« Last Edit: June 03, 2010, 07:04:08 pm by Lariemeeva »

**RoRo**

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2458 on: June 03, 2010, 07:05:24 pm »
Thanks a lot Lariemeeva and Ivo for your help!

You've both done a great job!
THANK YOU  ;D

Offline Ivo

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Re: CHEM AND BIO HELP AND TIPS HERE
« Reply #2459 on: June 03, 2010, 07:29:50 pm »
Hm...I actually understood it that way. I totally missed the fact that were talking about non-metals. The copied part is actually right for the record, my addition was wrong. If you're going to elaborate, don't quote the whole thing since it's true.
Oh right, you got that as well, what a coincidence.

LOL, what a coincidence!  I didn't mean to criticse you in a bad way.  No hard feelings OK?  I hope I haven't caused anyone to be upset.   :-\
Always willing to help!  8)
"In helping others, we shall help ourselves, for whatever good we give out completes the circle and comes back to us."