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OCR AS Salters Chemistry F332 - Wednesday 23rd May 2012 1:30pm

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I'm so not sure how to draw out the NaCl molecules, do I put the sodium ion in the centre and surround it with small chloride ions ?
Reply 181
There are a few things i don't understand about this exam coming up on wednesday:

How is ozone depletion caused?
What are the possible questions we could be asked about the advance notice article?
Hydrogen bonding in water and ice?

I would be really grateful if someone could answer these questions :smile:

Thanks
Original post by Roshniroxy
I'm so not sure how to draw out the NaCl molecules, do I put the sodium ion in the centre and surround it with small chloride ions ?



You are almost right however Cl- is bigger than Na+ (just look at their relative masses on the periodic table). Also remember that each Na+ (you must indicate it has a positive charge) is surrounded by 4 Cl- in a large ionic lattice structure.

Therefore it looks like this:
NaCl_crystal_structure.png


Original post by totw
There are a few things i don't understand about this exam coming up on wednesday:

How is ozone depletion caused?
What are the possible questions we could be asked about the advance notice article?
Hydrogen bonding in water and ice?

I would be really grateful if someone could answer these questions :smile:

Thanks


As far as hydrogen bonding goes..

Original post by Salmonidae
Hydrogen Bonds are a specific type of permanent dipole-permanent dipole bond where a delta-negatively charged Oxygen atom is attracted to a delta-positively charged Hydrogen atom (delta meaning small). Hydrogen is such a small atom that when these attractions form the distance between the hydrogen and the oxygen/nitrogen/fluorine atom (has to be one of those due to electronegative) it is covalently bonded to is almost the same as the distance between the hydrogen and the oxygen atom it is hydrogen bonded to. Therefore this type of intermolecular bond is exceptionally strong and we call it a hydrogen bond.

This diagram shows hydrogen bonding between water molecules
water.gif


As far as hydrogen bonding in water and ice is concerned, all extra you will need to know is that it is hydrogen bonding that causes ice to be less dense than water. This is because the hydrogen bonds keep the water molecules further away from each other than molecules normally would be in a solid.

The break down of ozone happens naturally due to high energy UV radiation, but we make it break down faster by adding radicals which catalyse the breakdown of ozone.

This diagram explains it very well but by chlorine atom it means chlorine radical

clcycle.jpg
(edited 11 years ago)
Original post by totw
There are a few things i don't understand about this exam coming up on wednesday:

How is ozone depletion caused?
What are the possible questions we could be asked about the advance notice article?
Hydrogen bonding in water and ice?

I would be really grateful if someone could answer these questions :smile:

Thanks


I will try to help.. not sure hwo useful it will be though.

One way ozone can be delpleted is first photodissciation of an O2 molecule by uv radiation of a specific frequency, this then creates two oxygen radicals (O2 ---> O + O). If one of these oxygen radicals then collides with an ozone molecule it create two oxygen molecules (O + O3 ---> O2 + O2).
There are also catalysts for depleting ozone, one of these being Cl, from CFCs. CFCs can't react in the troposphere because the C-Cl bond is too stable, so they move to the stratosphere, here UV radioation causes homolytic fission of the
C-Cl bond, producing a Cl radical. This then acts as a catalyst for the destruction of ozone. (1. Cl + O3 --> ClO + O2, then 2. ClO + O ---> Cl + O2)

Advanced notice possible questions:
What is a radical?
Structure, properties and bonding in LDPE and HDPE
Initiation, propergation, termination
Id-id, pd-pd, hygrogen bonds - how they arise
Copolymerisation
Polymerisation reactions

And I'm not too sure about the water and ice thing, I'd quite like to know myself really :-)
Original post by Salmonidae
You are almost right however Cl- is bigger than Na+ (just look at their relative masses on the periodic table). Also remember that each Na+ (you must indicate it has a positive charge) is surrounded by 4 Cl- in a large ionic lattice structure.

Therefore it looks like this:
NaCl_crystal_structure.png




As far as hydrogen bonding goes..



As far as hydrogen bonding in water and ice is concerned, all extra you will need to know is that it is hydrogen bonding that causes ice to be less dense than water. This is because the hydrogen bonds keep the water molecules further away from each other than molecules normally would be in a solid.

The break down of ozone happens naturally due to high energy UV radiation, but we make it break down faster by adding radicals which catalyse the breakdown of ozone.

This diagram explains it very well but by chlorine atom it means chlorine radical

clcycle.jpg


thanks but i thought each chloride ion was surrounded by 6 chloride ions ?
Reply 185
Just done a few past papers and i have noticed a pattern in them, usually the same questions come up but just wordly differently so if you do some past papers it should help you alot on what you need to know :smile:
Original post by Roshniroxy
thanks but i thought each chloride ion was surrounded by 6 chloride ions ?


It is surrounded by 6 if you look at it in a 3D structure:

naclcell.jpg
Reply 187
Revision starts today!
Original post by Salmonidae
It is surrounded by 6 if you look at it in a 3D structure:

naclcell.jpg


but when we draw it in the exam it's 4 ?
Reply 189
Original post by kathryn123
I will try to help.. not sure hwo useful it will be though.

One way ozone can be delpleted is first photodissciation of an O2 molecule by uv radiation of a specific frequency, this then creates two oxygen radicals (O2 ---> O + O). If one of these oxygen radicals then collides with an ozone molecule it create two oxygen molecules (O + O3 ---> O2 + O2).
There are also catalysts for depleting ozone, one of these being Cl, from CFCs. CFCs can't react in the troposphere because the C-Cl bond is too stable, so they move to the stratosphere, here UV radioation causes homolytic fission of the
C-Cl bond, producing a Cl radical. This then acts as a catalyst for the destruction of ozone. (1. Cl + O3 --> ClO + O2, then 2. ClO + O ---> Cl + O2)

Advanced notice possible questions:
What is a radical?
Structure, properties and bonding in LDPE and HDPE
Initiation, propergation, termination
Id-id, pd-pd, hygrogen bonds - how they arise
Copolymerisation
Polymerisation reactions

And I'm not too sure about the water and ice thing, I'd quite like to know myself really :-)


Thanks :smile:
Reply 190
Original post by Salmonidae


As far as hydrogen bonding in water and ice is concerned, all extra you will need to know is that it is hydrogen bonding that causes ice to be less dense than water. This is because the hydrogen bonds keep the water molecules further away from each other than molecules normally would be in a solid.

The break down of ozone happens naturally due to high energy UV radiation, but we make it break down faster by adding radicals which catalyse the breakdown of ozone.

This diagram explains it very well but by chlorine atom it means chlorine radical

clcycle.jpg


Thanks :smile:
Original post by Roshniroxy
but when we draw it in the exam it's 4 ?


It will specify in the question whether it wants a 3D structure or just one layer.
Original post by Salmonidae
It will specify in the question whether it wants a 3D structure or just one layer.


thanks :smile:
Can someone answer these questions please?

What are the Structure, properties and bonding in LDPE and HDPE?
Is there any pages in CI or story lines that help and what other addition info is needed about this?

Id-id, pd-pd, - how they arise
(A concise step by step guide of what happens)

Copolymerisation
Is there anything else we need to know, other than they are formed from different monomers by addition polymerization?

Polymerisation reactions
??? examples please?

thanks in advance
Original post by Jammy4410
Can someone answer these questions please?

What are the Structure, properties and bonding in LDPE and HDPE?
Is there any pages in CI or story lines that help and what other addition info is needed about this?

Id-id, pd-pd, - how they arise
(A concise step by step guide of what happens)

Copolymerisation
Is there anything else we need to know, other than they are formed from different monomers by addition polymerization?

Polymerisation reactions
??? examples please?

thanks in advance


RE: intermolecular forces:

Original post by Salmonidae
Hydrogen Bonds are a specific type of permanent dipole-permanent dipole bond where a delta-negatively charged Oxygen atom is attracted to a delta-positively charged Hydrogen atom (delta meaning small). Hydrogen is such a small atom that when these attractions form the distance between the hydrogen and the oxygen/nitrogen/fluorine atom (has to be one of those due to electronegative) it is covalently bonded to is almost the same as the distance between the hydrogen and the oxygen atom it is hydrogen bonded to. Therefore this type of intermolecular bond is exceptionally strong and we call it a hydrogen bond.

This diagram shows hydrogen bonding between water molecules
water.gif

A permanent dipole-permanent dipole is formed exactly the same way as a hydrogen bond (i.e is the result of 2 polarised bonds) but can be with other atoms and is weaker due to bigger atoms involved and longer distances.

Here is a diagram showing permanent dipole-permanent dipole bonding in hydrogen chloride
Hydrochloric+Acid+Hydrogen+Bonding.png

Permanent dipole - induced dipole is where a polarised bond (lets stick with H-Cl from the diagram above) induces (i.e creates) a dipole on another molecule and is therefore attracted to it. The works because if a negatively charged atom, i.e Cl in H-Cl, approaches a neutral atom the negative charge will distort the electron cloud on the neutral atom and push away the electrons creating a positive charge there. Therefore there will now be a permanent dipole-induced dipole attraction.

Here is a diagram showing water (a permanent dipole) inducing a dipole in an O2 molecule
induced dipole vs permanent dipole.jpg

Finally an instantaneous dipole - induced dipole works in exactly the same way as a permanent dipole - induced dipole except that instead of a permanent dipole there is an instantaneous dipole. These arise randomly in any molecule and are caused by the random movement of electrons creating an imbalance of charge which then induces a dipole in the molecule next door and forming an intermolecular bond. This is the weakest intermolecular force, and is also known as Van der Waals forces.

Final diagram showing instantaneous dipole - induced dipole
instantaneous-dipole--instantaneous-induced-dipole-illustration.gif



All we need to know about copolymers is that they are made out of 2 or more different type of monomer and they join up in random order.

HDPE and LDPE main differences are to do with LDPE being flexible (e.g in plastic bags) and HDPE being rigid and hard for example in a plastic box. It is worth noting that a high density polymer will have extensive crystalline regions (where side groups are organised in a regular way) allowing the chains to fit very closely to each other. There is more about crystalline polymers on page 113 of CI.

Hope this helps!!
S
Original post by Roshniroxy
thanks but i thought each chloride ion was surrounded by 6 chloride ions ?


That is correct but when drawing it 2 dimensional one of the Cl- ions is coming out of the page and one of them is going into the page.
Original post by 4 Mathlete the win
That is correct but when drawing it 2 dimensional one of the Cl- ions is coming out of the page and one of them is going into the page.


cheers !

Also for alcohol I thought you write them as CxHyOH, for example in this past paper i wrote the molecular formula of this skeletal structure was drawn that had a functional group of an alcohol at the end of the chain as C16H29OH but the mark scheme had C16H30O :s-smilie:
Reply 197
so scared about this exam ;(
Original post by Roshniroxy
cheers !

Also for alcohol I thought you write them as CxHyOH, for example in this past paper i wrote the molecular formula of this skeletal structure was drawn that had a functional group of an alcohol at the end of the chain as C16H29OH but the mark scheme had C16H30O :s-smilie:


I would think they will accept both as correct. That's what our teacher tells us anyway. I prefer to go down the C16H30O route as if there's a calculation after it helps you work out the Mr marginally quicker.
O and the mark scheme is just a guide, it is not designed for students. It is simply an outline for the markers to follow. The exam markers are supposed to us their discretion when marking.
(edited 11 years ago)
Reply 199
Anyone got any ideas what sort of questions will come up for the pre release article? :smile:

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