U5-Jan-2011-NS
Posted by: intechemistry on: September 25, 2010
Here we will discuss the Unit 5, Jan 2011, ‘New Syllabus’ past years papers (PYP’s):
Question paper:
Mark scheme:
4 Responses to "U5-Jan-2011-NS"
April 28, 2012 at 8:08 pm
Salam Shahrizan.
Yes, it is a ligand exchange, although it’s a reaction we’ve not really considered before.
The 4Cl-’s are replaced, and it’s three OH- ligands that replace them. (the OH- comes from the reaction of NH3 with H2O as follows:
NH3 + H2O NH4+ + OH-)
Hence Cr(OH)3(s) which has no charge forms.
OH- is a stronger ligand than Cl- hence will displace it.
April 28, 2012 at 8:21 pm
Thanks sir for clearing my mind..the conclusion is stronger ligand can displace weaker ligand in almost all reaction is it, but the coordination number may not be the same after ligand exchange reaction right? correct me if I’m wrong
April 28, 2012 at 9:36 pm
Yes, but weaker ligands can also replace stronger ligands too – in certain circumstances – e.g. due to equilibrium factors, which in turn all depends on the energy / entropy of the process.
It’s much more common to come across cases where the stronger ligand simply replaces the weaker ligand., and yes, changes in coordination number can occur depending on several factors.
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April 28, 2012 at 7:31 pm
Salam sir,
Regarding the reaction scheme in question 17, how can reaction 3 happen? it is a ligand substitution reaction or deprotonation? how does [CrCl4]- change to green precipitate, [Cr(H20)3 (0H)3] when aqueous ammonia is added..I don’t understand the reaction..Thanks in advance..