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chemistry help A level

QUESTION: I need help with b (iii)- the last part. the answer is that KMnO4- will oxidise HCl but how do you know

Q4. A 0.263 g sample of impure iron, containing an unreactive impurity, was reacted with an
excess of hydrochloric acid. All of the iron in the sample reacted, evolving hydrogen gas and
forming a solution of iron(II) chloride. The volume of hydrogen evolved was 102 cm3
, measured at
298 K and 110 kPa.
The percentage, by mass, of iron in the sample can be determined using either the
volume of hydrogen produced or by titrating the solution of iron(II) chloride formed against
a standard solution of potassium dichromate(VI).
(a) (i) Write an equation for the reaction between iron and hydrochloric acid.
______________________________________________________________
(ii) Calculate the number of moles of hydrogen produced in the reaction.
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
(iii) Use your answers to parts (a)(i) and (ii) to determine the number of moles of
iron and the mass of iron in the original sample. (If you have been unable to
complete part (a)(ii) you should assume the answer to be 4.25 × 10–3 mol.
Moles of iron ___________________________________________________
Mass of iron ____________________________________________________
(iv) Calculate the percentage of iron in the original sample.
______________________________________________________________

Transition Metal 2 SCT Page 6 of 19
(b) (i) Write half-equations for the oxidation of Fe2+ and for the reduction of Cr2O in
acidic solution, and use these to construct an overall equation for the reaction
between these two ions.
Half-equation for the oxidation of Fe2+
______________________________________________________________
Half-equation for the reduction of Cr2O
______________________________________________________________
Overall equation
______________________________________________________________
(ii) The number of moles of iron in the sample was determined in part (a)(iii). Use
this answer to calculate the volume of a 0.0200 mol dm–3 solution of potassium
dichromate(VI) which would react exactly with the solution of iron(II) chloride
formed in the reaction.
(If you have been unable to complete part (a)(iii) you should assume the
answer to be 3.63 × 10–3 mol. This is not the correct answer.)
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________
______________________________________________________________

(iii) Explain why an incorrect value for the number of moles of iron(II) chloride
formed would have been obtained if the original solution had been titrated with
potassium manganate(VII).
______________________________________________________________
______________________________________________________________
(7)
(edited 1 year ago)
I can't see how you could work that out from the information given. However, this question is from a very old AQA paper 2006 (three specs ago!). Perhaps the oxidation of chloride ions by manganate VII was required knowledge for this spec.? It certainly used to be in the old days. In my copy of Ramsden (1990) it says: "Potassium manganate(VII) cannot be used in solutions containing chloride ions, as it oxidises them to chlorine"

I wouldn't worry about it.
Original post by tony_dolby
I can't see how you could work that out from the information given. However, this question is from a very old AQA paper 2006 (three specs ago!). Perhaps the oxidation of chloride ions by manganate VII was required knowledge for this spec.? It certainly used to be in the old days. In my copy of Ramsden (1990) it says: "Potassium manganate(VII) cannot be used in solutions containing chloride ions, as it oxidises them to chlorine"

I wouldn't worry about it.

I see. Thank you.

Also, for this ligand substitution : Co(H2O6)2+ + EDTA4- ......> (CoEDTA)2- + 6H20. What is the co-ordination number for the bonds being broken and made, is it 12 bonds broken and 12 bonds made as EDTA form 6 co-ordiante bonds.
Ahh, so there are Cl- ions present in the solution, because the HCl dissociates to produce H+ ions and Cl- ions.
The electrode potential for Cl- ions (specifically the Cl2/Cl- electrode) is lower than the electrode potential for MnO4- ions.
As you probably know already, the substance with the higher electrode potential will be the oxidising agent, while the other substance with the lower electrode potential will be the reducing agent.
So the MnO4- ions will oxidise the Cl- ions, along with the Fe2+ ions and therefore a larger volume of KMnO4 will be added to the solution compared to the actual volume needed to oxidise only the Fe2+ ions within the solution. So this means an incorrect value for the mass of iron in the impure solid will be calculated (larger than usual).

I hope I explained it well :smile:. Let me know if you still don't understand something.
Original post by bluestarr35910
I see. Thank you.

Also, for this ligand substitution : Co(H2O6)2+ + EDTA4- ......> (CoEDTA)2- + 6H20. What is the co-ordination number for the bonds being broken and made, is it 12 bonds broken and 12 bonds made as EDTA form 6 co-ordiante bonds.

It's 6 bonds broken and 6 bonds made. The original transition metal complex had a coordination number of 6 (there are 6 water molecules coordinately bonded to the central Co2+ ion.) and the new transition metal complex also has a coordination number of 6 because, (as you mentioned) EDTA4- forms 6 coordinate bonds with the central Co2+ ion.
Because of this , the enthalpy change for this reaction is essentially zero.

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