2007 H2 Chemistry Paper 3 Qn 3 (b)(ii)
My question is what exactly is the information given in Data 1. Enthalphy change of formation of Ca2+ (aq)= -543 and f- (aq)= -333 ??
This data is given together with enthalphy change of atomisation of Ca(s), enthalphy change affinity of flourine atom and enthalphy change of formation of CaF2(s).
Do I have to use information from data booklet as well, as it is not stated in the question ?
2006 H2 Chemistry Paper 3Qn 5
Write a balance equation when KI(aq) is added to a solution of Pb2+(aq). A bright yellow precipitate is formed. Further addition of KI(aq) causes the ppt to redissolve.
My Answer:
2KI(s) + Pb2+(aq) ----PbI2(s)+ 2K+(aq)
Yellow ppt of PbI2 is formed.
PbI2 (s) + I- (aq)----[PbI3]- (aq)
Further addition of KI causes the formation complex ion which is soluble.
However, the formula of the complex ion given in the TYS was [PbI4]-. Does that mean my answer is wrong cause I wasn't taught about this in my school. Is there a way of coming up with the right complex ions.
Thanks In Advance :)
Originally posted by Plaxo:2007 H2 Chemistry Paper 3 Qn 3 (b)(ii)
My question is what exactly is the information given in Data 1. Enthalphy change of formation of Ca2+ (aq)= -543 and f- (aq)= -333 ??
This data is given together with enthalphy change of atomisation of Ca(s), enthalphy change affinity of flourine atom and enthalphy change of formation of CaF2(s).
Do I have to use information from data booklet as well, as it is not stated in the question ?
2006 H2 Chemistry Paper 3Qn 5
Write a balance equation when KI(aq) is added to a solution of Pb2+(aq). A bright yellow precipitate is formed. Further addition of KI(aq) causes the ppt to redissolve.
My Answer:
2KI(s) + Pb2+(aq) ----PbI2(s)+ 2K+(aq)
Yellow ppt of PbI2 is formed.
PbI2 (s) + I- (aq)----[PbI3]- (aq)
Further addition of KI causes the formation complex ion which is soluble.
However, the formula of the complex ion given in the TYS was [PbI4]-. Does that mean my answer is wrong cause I wasn't taught about this in my school. Is there a way of coming up with the right complex ions.
Thanks In Advance :)
Qn 1. The data given in the question suffices (for drawing a Hess Law enthalpy cycle), you do not need any further info from the Data Booklet. But if any exam qn appears to give inadequate data, feel free to quote values from Data Booklet to help you answer the question. It's just that in this instance, further info is not required.
Qn2. Different JCs teach different content. If you intend to get a distinction, you cannot rely on just notes provided by any JC (even so-called top JCs), but must be prepared to peruse multiple sources. Your fictitious tri-iodo complex ion will not score the mark. You must give the formula for the tetra-iodo plumbate(II) ion.
No single source (eg. any single JC's notes) will give an exhaustive list of complex ions, their formulae, their geometries and their colours. Cambridge deliberately does not release any list, to reserve the right to ask on obscure complex ions to test the candidate's ability to extrapolate and deduce probable formulae, geometries and colours.
You can find a more complete list of complex ions, their formulae, their geometries and their colours on Jim Clarke's website and Rod Beavon's website.
Originally posted by Plaxo:Thanks for reply UltimaOnline.
If you don’t mind me asking, what is the equation of the enthalpy change of formation of
the Ca2+ ion or f- ion stated in the qn. Cause I tot enthalpy change of formation is only for substance and not ions. Maybe i am mistaken it is not even enthalpy change of formation, then what is it ?
By Hess Law, formation enthalpy for Ca2+(aq) ion = atomization enthalpy + 1st ionization enthalpy* + 2nd ionization enthalpy + hydration enthalpy.
* There is a technical difference between enthalpy and energy, but for 'A' level purposes these may be used interchangeably, eg. bond enthalpy = bond energy. Enthalpy change is heat change under constant pressure, energy change is heat change under constant volume. If a reaction generates gaseous products, some of the energy is used to push away the atmosphere, and enthalpy will be slightly less exothermic than energy change.
Originally posted by Plaxo:Thank you so much. Really appreciate it.
Anyways is enthalpy change of formation of ions in our syllabus.
usually if A level has questions which are not in our syllabus, background info, like equations of enthalpy that we have not learnt, will be included.
However, this question did not include the eqn for the enthalpy change of formation of ca2+ and f-.How come?
You're expected to be able to figure it out yourself, the processes involved in forming an aqueous ion.