Edexcel A2 Chemistry Unit 5 (CH05) - 24 June 2011

Cr3+ to Cr2+ needs Zinc and HCl (green to blue)
Cr3+ to Cr2O7^2- needs Hydrogen peroxide and an alkali (Green to orange)
To turn it into a complex it needs to react with a ligand, such as H2O or NH3!

Chromium(III) to Chromium(VI) is an oxidation reaction so you would need OH^- /H20. Chromium(III) to Chromium (II) is reduction so you need Zn (in an inert atmosphere). Chromium (III) can form a complex for e.g. when you add excess NaOH ligand exchange occurs or you could also add excess ammonia instead, this is an example of ligand exchange too.

( i think this is right, sorry if it didn't help)

is the cgp revision guide enough for an A-grade in unit 5?

Yeah they're nucleophiles, I have no idea why I said electrophile O_o

its a book. don't think it exists online man....


BTW PEOPLE just a quick question : when adding small drops of NH3 to a water complex .... does it simply deprotonate the complex meaning OH ligands are formed or do they actually swap and form NH3 ligands - or does the ligand exhange only occur with excess ammonia.

and when few drop of ammonia are added can someone give an example of the equation.... MERCI

whats the need for a reference electrode?

can anyone link me to jan 2011 paper please?

Deprotonation is on page 29 of the Nelson Advanced Science book titled "Transition Metals, Quantitative Kinetics and Applied Organic Chemistry", under the subheading "The Effect of Alkali on Aqua Complexes"

I will do my best to sum it up for you.

Firstly, it's important to remember that an alkali is a water soluble base, and a base being an electron pair donor.

An aqua complex is a metal ion surrounded by molecules called ligands (e.g. water molecules), which have lone pairs that form dative covalent bonds to the ion. Because the metal ion is small and highly charged (i.e. it has a high charge density), the ligands are highly polarised.
Because of this the ligand will undergo nucleophilic attack from the basic molecule (which could be a hydroxide ion, ammonia molecule, even a water molcule... they all have lone electron pairs).
The nucleophile (the species in the base with the lone pair on) will donate its electron pair one of the hydrogen atoms on the ligand. This hydrogen atom is effectively a proton due to the polarisation of the ligand. The bond between the hydrogen atom (proton) and its parent ligand is broken with the bonding electrons remaining on the ligand. A new bond has been formed with the basic molecule and the proton.

Since the aqua complex has lost a proton it is said to have been deprotonated.

I will attempt to show this in a diagram for you (adapted from the above textbook).

(See attatched image)

This happens further until you end up with Fe(OH)3. In some cases (depending on the complex) there are further reactions with excess base. See page 30 for that.

Hope this helps,

Steve.

what are the steps for recrystallisation?

(like if it were a 7 marker tomorrow)

solid compound or organic?

what are the steps for recrystallisation?

(like if it were a 7 marker tomorrow)

-add hot solvent so impure product just dissolves
-should now have a saturated solution of impure product
-allow the solution to cool slowly - crystals of the product will form as it cools
-ensure products are soluble at high temp, insoluble at low temp
-impurities will stay in the solution
-filter off crystals
-wash crystals with ice cold solvent and leave to dry

Does anyone know the grade boundries for jun 10 and jan11??

For June 10 it was

120-90
108-73
96-66
84-59
72-53
60-47
58-41
46-35

and Jan11 was similar