OCR B Salters - F335 Exam - 15 June 2011

Does anyone know why ions dissolve in water or polar solvents? is it to do with ionic lattice breaking up and ion-dipoles are forming? if this is the case, how do i expand on it?

also do we need to know about colourfastness?

because the delta plus charge on the hydrogen in a water molecule attracts the negative ions and the oxygen that has a delta plus charge in the H2O molecule attracts the positive ions. so the ions separate from the ionic lattice and become surrounded by the water molecules and this particular process of the water molecules surrounding the ions is called hydration.

and yea i think we do need to know colourfastness

Reply 181

joestevens2092

3

Original post by pav

Does anyone know why ions dissolve in water or polar solvents? is it to do with ionic lattice breaking up and ion-dipoles are forming? if this is the case, how do i expand on it?

also do we need to know about colourfastness?

Colour fastness is briefly covered in story lines, so i suppose it could come up. What they could ask would be limited though :s-smilie:

Reply 182

mooniibuggy

14

Original post by Jamesrb

When you say finishing off, do you mean reading? From CI?

Doing the problems

Original post by limetang

I know I know.

I only became paranoid today after my biology exam xD

Reply 183

zahre

Original post by pearlover

because the delta plus charge on the hydrogen in a water molecule attracts the negative ions and the oxygen that has a delta plus charge in the H2O molecule attracts the positive ions. so the ions separate from the ionic lattice and become surrounded by the water molecules and this particular process of the water molecules surrounding the ions is called hydration.

and yea i think we do need to know colourfastness

Can you please explain colourfastness to me. What do we need to know regarding it?

Reply 184

pearlover

Original post by zahre

Can you please explain colourfastness to me. What do we need to know regarding it?

sure thing... a dye needs to have a good colourfast-this means that it won't wash out too easily or fade in the light.. and colourfastness depends on the strenght of the bonding between the dye and the fibre..so the bonding needs to be strong in order for the dye to be colourfast. An example of a good colourfast dye are ones with carboxylic acid groups or sulfonic acid groups in the dye molecule and these can form ionic interactions with fabrics that contain alkaline groups -NH- group and so as the bond between the fibre and dye is ionic interactions its strong and so is a good colourfast dye..

Reply 185

zahre

Original post by pearlover

sure thing... a dye needs to have a good colourfast-this means that it won't wash out too easily or fade in the light.. and colourfastness depends on the strenght of the bonding between the dye and the fibre..so the bonding needs to be strong in order for the dye to be colourfast. An example of a good colourfast dye are ones with carboxylic acid groups or sulfonic acid groups in the dye molecule and these can form ionic interactions with fabrics that contain alkaline groups -NH- group and so as the bond between the fibre and dye is ionic interactions its strong and so is a good colourfast dye..

Thanks for explaining. Very helpful. :smile:

Reply 186

Conor 419

The awkward moment when you realise 10% of the content will appear in the paper.

Reply 187

Conor 419

Why exactly do ligands cause orbitals to split?

Reply 188

bzzz

Original post by Conor 419

Why exactly do ligands cause orbitals to split?

do we need to know about ligands for unit 5?

Reply 189

Rei5

someone provide me with a model answer to why ionic compounds dissolve whereas others ie co2 do not!!!

Reply 190

bzzz

Original post by Rei5

someone provide me with a model answer to why ionic compounds dissolve whereas others ie co2 do not!!!

was doing the same question on a paper today. from the mark scheme:

CO2
A CO2 has instantaneous dipole-induced dipole between molecules;
BBCO2 forms hydrogen bonds with water (2);
C detail of imf, eg diagram;

ions
D electrostatic forces between ions;
E ions form ion-dipole bonds with water (or description);
F hydration description;
G detail (eg diagram);

reasons
H water forms hydrogen bonds with itself;
J more hydrogen bonds broken than made for CO2/ imf in water stronger than in
CO2;
K imf between ions and water stronger than hydrogen bonds/ stronger than imf
in CO2;

Reply 191

ABCDemily

2

Can somebody please confirm this:

Ka -> pKa = -log10 x ka and

pKa -> ka = 10^-pka

Teacher had neglected to teach us this. And I only just realised, sooo, is this right?

Reply 192

apo1324

11

Original post by ABCDemily

Can somebody please confirm this:

Ka -> pKa = -log10 x ka and

pKa -> ka = 10^-pka

Teacher had neglected to teach us this. And I only just realised, sooo, is this right?

That's right. :smile:

Reply 193

Jamesrb

Original post by ABCDemily

Can somebody please confirm this:

Ka -> pKa = -log10 x ka and

pKa -> ka = 10^-pka

Teacher had neglected to teach us this. And I only just realised, sooo, is this right?

Yeah that looks right, although I didn't think we'd ever need to go from pKa to Ka?

where log is to the base 10:

Ka = [H+(aq)]^2
---------
[HA]

pH = -log([H+(aq)])

pKa = -log(Ka)

Reply 194

pav

Original post by pearlover

sure thing... a dye needs to have a good colourfast-this means that it won't wash out too easily or fade in the light.. and colourfastness depends on the strenght of the bonding between the dye and the fibre..so the bonding needs to be strong in order for the dye to be colourfast. An example of a good colourfast dye are ones with carboxylic acid groups or sulfonic acid groups in the dye molecule and these can form ionic interactions with fabrics that contain alkaline groups -NH- group and so as the bond between the fibre and dye is ionic interactions its strong and so is a good colourfast dye..

won't a good colourfast dye be the one that forms covalent bonds? ( since that is the strongest bond )

Reply 195

ABCDemily

2

Original post by pav

Does anyone know why ions dissolve in water or polar solvents? is it to do with ionic lattice breaking up and ion-dipoles are forming? if this is the case, how do i expand on it?

also do we need to know about colourfastness?

That H- bonded dyes, like those used on cotton and other cellulose based fibres aren't very colourfast as the bonds are very easily broken when washed/exposed to heat. Then, you get the ionic interactions, between an acid and an alkali group, ie -COOH in the dye and -NH3 in the fibre. Slightly more colourfast as the bonds are stronger, meaning that they will withstand washing and heat.

The strongest dyes are fibre reactive dyes which covalently bond to the fibres.

Reply 196

pav

Original post by ABCDemily

That H- bonded dyes, like those used on cotton and other cellulose based fibres aren't very colourfast as the bonds are very easily broken when washed/exposed to heat. Then, you get the ionic interactions, between an acid and an alkali group, ie -COOH in the dye and -NH3 in the fibre. Slightly more colourfast as the bonds are stronger, meaning that they will withstand washing and heat.

The strongest dyes are fibre reactive dyes which covalently bond to the fibres.

so does that mean that dyes that covalent bond with fibres will most colourfast?..also do you know what mordant means?

Reply 197

ABCDemily

2

Original post by pav

so does that mean that dyes that covalent bond with fibres will most colourfast?..also do you know what mordant means?

Yesss. Fibre reactive dyes are the most colourfast.

Mordant is 'serving to fix colours when dying', basically, it's a fixing agent within dyes, and increases colourfastness?

Reply 198

Kayak

1

Ahh, this time tomorrow I will be petrified! And wondering how to get to my exam.... Great! I could walk, but it's a long ish way (45 mins) and I don't want to waste that time walking and stressing and then get there all hot and sweaty! Or, If it happens to be raining, soaked!

But anyway, azo dyes are killing me at the moment! Can read through it all and nod along, yes I understand this, but reading through it with the intention of remembering any of that... Not wanting to happen!

Think a day of past papers is in order. :frown: