OCR Chemistry 2017 Practice Paper Question - HELP

Hi!

You have to treat the tests separately, as they help identify the cations and anions within the two compounds - you know you're dealing with ions as the whole question's about transition elements and their compounds.

Test 1 is an example of a precipitation reaction with transition elements (the other is with NaOH). The five transition metal ions you need to know (for OCR) that react with a normal amount (i.e. not excess) NH3 and NaOH are manganese (II), chromium (III), iron (II) and (III) and copper (II). Chromium (III) also reacts further with excesses of NH3 and NaOH, while copper (II) only reacts further with excess NH3.

Each precipitate has a distinct colour, which is used for this question.

You know that compound A reacts with excess NH3, ruling out manganese (II), iron (II) and (III) - the cation in A must therefore be Cr3+ (chromium (III)) or Cu2+ (copper (II)). Copper forms a dark blue solution with excess NH3, whereas chromium forms a purple solution - A contains chromium!

B doesn't react with excess, leaving four ions it could be. Copper forms a blue precipitate with normal amounts of NH3, iron (II) forms a pale green precipitate, iron (III)'s is yellow, so it's none of those. B must therefore contain the manganese ion, which matches the description - it forms a pink precipitate.

Test 2 is a sulphate test from AS - nitric acid to clear any contaminating ions that might give a false positive, then barium nitrate. A white precipitate only forms if the sulphate (SO4^2-) ion is present - A gives a positive result, so A must be chromium (III) sulphate - Cr2(SO4)3.

B gives a negative result, so the third test is needed - the halide test from AS is used, where nitric acid followed by silver nitrate is added. A yellow precipitate forms - this is silver iodide, so B is manganese (II) iodide - MnI2!

The ionic equations they ask for mostly use the ligand form; when writing the transition metal ions, they have 6 H2O ligands around them until they're replaced. When writing down the formulae for the metal hydroxide precipitates, you can choose not to include the extra H2Os e.g. just Cr(OH)3 instead of Cr(OH)3(H2O)3.

We know A contains chromium, so C is just the equation for the reaction between chromium ions and 3NH3; NH3 acts as a base and accepts a proton from three of the H2O ligands around chromium; the equation is therefore...

[Cr(H2O)6]3+ + 3NH3 -> Cr(OH)3(H2O)3 + 2NH4+

When excess NH3 is added, the NH3s replace all of the H2O ligands instead of removing their protons: the equation for D is...

[Cr(H2O)6]3+ + 6NH3 -> [Cr(NH3)6]3+ + 6H2O

We know B contains manganese (II), so the equation of E is very similar to that for C (but watch out for the different charge of the manganese ion):

[Mn(H2O)6]2+ + 2NH3 -> Mn(OH)2(H2O)4 + 2NH4+

F uses the normal equation for the sulphate test, where barium sulphate forms:

Ba2+ + SO42- -> BaSO4

Finally G uses the normal equation for the halide test, where a silver halide forms:

Ag+ + I- -> AgI

You don't need state symbols as they're pretty descriptive in the table.

I hope that helps, sorry it was so long but I thought I'd cover everything so you get the whole picture. Let me know if there's anything you don't get, I have a tendency to waffle on! I hope everything goes well for you on Tuesday!

Hi guys, I've been doing the practice paper set 1, but got stuck at this Transition elements question. I find it significantly more difficult than transition elements questions in F325 (old syllabus). I would be very grateful if you could help me do this question. Thank you in advance! Also, if you need practice paper I would be happy to send it to your email address.

Hi guys, I've been doing the practice paper set 1, but got stuck at this Transition elements question. I find it significantly more difficult than transition elements questions in F325 (old syllabus). I would be very grateful if you could help me do this question. Thank you in advance! Also, if you need practice paper I would be happy to send it to your email address.

Hi guys, I've been doing the practice paper set 1, but got stuck at this Transition elements question. I find it significantly more difficult than transition elements questions in F325 (old syllabus). I would be very grateful if you could help me do this question. Thank you in advance! Also, if you need practice paper I would be happy to send it to your email address.