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

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


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


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


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


Hope this helps, ask if you're still unsure!

S

Does anyone have a link to the JAN 2012 past paper and mark scheme please? x

Sorry, concerned parent muscling in here, but hope you can help a bit please guys.

My son has literally just finished the course (last Friday) so not much real revision done as yet (Chemistry wasn't the only syllabus not finished! And he had three exams this week) He doesn't get study leave and has 4 other exams next week too, so time is veeeery short. Plus he has learning issues that slow him up!

He's hoping to do this to degree level so it's pretty important to him.

I've read the thread, but are there any other tips you can suggest to make sure he spends his little time wisely please?

Zhy's comments on mark scheme answers sound like a great idea - J's pretty good at Chemistry, but lacks precision in his written answers so tends to drop marks on longer answers. I don't suppose anyone has summarised these and could let me have a copy please?

Any other thoughts gratefully received.

Thanks

Hey!

OK so the way displacement reactions work when it comes to the halogens is that the higher up elements displace the lower down ones. This is because the higher up elements are better oxidising agents, therefore they oxidise the other halogen and essentially 'steal' their electrons, becoming reduced themselves.

So in the case of your reactions:

Cl₂ + 2KBr ---> Br₂ + 2KCl This is because Cl is a better oxidising agent that Br so Br is oxidised and Cl is reduced, taking its place in the compound.

Cl₂ + 2KI ----> I₂ + 2KCL
Br₂ + 2KI ----> I₂ + 2KBr

Both these reactions work on the same principle.

Now as for colours in cyclohexane you just have to see which halogen is oxidised for it is this one which will dissolve in the cyclohexane.

Chlorine in cyclohexane is virtually colourless
Bromine in cyclohexane is an orange/red colour.
Iodine in cyclohexane is a pink/violet colour.


From this information you should be able to figure out that the first equation Bromine is oxidised, so the cyclohexane would be orange/red, and the 2nd and 3rd equations Iodine is oxidised so the cyclohexane would be pink/violet.


Hope this helps
S

Does anyone have a link to the JAN 2012 past paper and mark scheme please? x

what is the best advices to tackle this paper???

Does anyone have a link to the JAN 2012 past paper and mark scheme please? x

can someone explain to me how i would work out the formula of calcium chlorate(I)?

The formula is Ca(ClO₃)₂ and the best way to remember it would just to remember that the chlorate ion (ClO₃^-1) has a negative 1 charge. However this is hardly a common ion and I doubt it would come up.

mark scheme says Ca(ClO)2 and i'm not entirely sure how they got there

mark scheme says Ca(ClO)2 and i'm not entirely sure how they got there

It is Ca(ClO)2. Since the oxidation state of Cl is +1, the chlorate ion is ClO-. The Ca ion is 2+ so the formula must be Ca(ClO)2.

How could that possibly right? The oxygen would have to form some sort of dative double bond with the chloride....

Well either way the internet disagrees. Which paper and question is this?

It is Ca(ClO)2. Since the oxidation state of Cl is +1, the chlorate ion is ClO-. The Ca ion is 2+ so the formula must be Ca(ClO)2.

Ah right I see, just interesting that if you google calcium chlorate you get a completely different compound.

for these types of questions where it asks you to show the overall equation for the reaction, what do you do?
equation 2.1: NO(g) + ½ O2(g) --> NO2(g)
equation 2.2: NO2(g) --> NO(g) + O(g)
equation 2.3: O2(g) + O(g) --> O3(g)

i don't understand how the overall equation ends up being 1½ O2 --> O3, since you start with just ½ O2

Thats because the chlorine exists in many oxidation states.