Extremely hard chemistry question!

Hi guys, how on earth are you supposed to approach this question?
Could someone let me know please? Thankss so much!Have attached ms as well

The absorbance increases up to a optimum point then it decreases after that.

going from {0,10} to {2,8} along the X axis , you see that the absorbance increases.
You're getting this reaction:
[Cu(H2O)6]2+ + 2NH3 => [Cu(OH)2(H2O)4] + 2NH4^2+
the product is a blue preciptate from a blue solution.

After that the optimum,
You are getting this reaction:
[Cu(H2O)6]2+ + 2NH3 => [Cu(OH)2(H2O)4] + 4NH3 => [Cu(H2O)2(NH3)4]2+ + 2OH- + 2H2O
Before the optimum, its the ammonia solutions thats the limiting reagent.


The absorbance decreases after the optimum because there is less copper ions hence the deep blue solution gets lighter (which makes sense if there are less complexed copper ions forming the deep blue solution. After the optimum, the copper sulphate is the limiting reagent.


The point at which two line of best fit intersect is where the initial reaction has gone to completion (i.e. no limiting reagent exists as all the copper ions and all the ammonia have have undergone the acid-base reaction).

Thanks so much for replying! Lots of things made sense to me here. However, how is ammonia solution limiting before the optimum? If you look at the graph, there are 10cm^3 of ammonia solution and 0 cm^3 of copper sulphate, then 9cm^3 of ammonia solution and 1 cm^3 of copper sulphate. Also, there is less volume of copper sulphate before the optimum than after it as outlined from the graph.

Why is the copper sulphate the limiting reagent after the optimum when the volume of copper sulphate increases as you go to the right in the graph? I thought it would increase as the volume of copper sulphate increases?

Another thing that is confusing me is the layout of the graph -the graph attached in my initial post is the mark scheme answer by they way.
So they've plotted what's happening in all 8 test tubes in the graph -I am not sure how to explain my confusion here - it's just weird that they've combined reactions from different test tubes into one graph really.

How do copper ions and ammonia undergo acid base reaction?

Please reply if possible -would be very much appreciated.

The ammonia is limiting because before the optimum, the volume of ammonia is decreasing, and the volume of Cu2+ ions increases. So the amount of ammonia decreases and the amount of copper ions increases. In this case the decrease in amount of ammonia is whats causing the any further increase in the formation of the blue preciptate [Cu(H2O)4(OH)2]. For example, if there was 2cm3 of Cu^2+ and 8cm3 of ammonia solution, then there will be X amount of the precipitate with absorbance of 0.34. Now if I had the same 2cm3 of copper(II) ions but with more ammonia eg 10 cm^3 then there will be more preciptate formed ( > X) hence the absorbance would be greater than 0.34. Because the ammonia is limiting, we aren't getting the maximum mass of blue precipate for a given volume of Cu^2+ ions.

Remember, that the copper sulphate is in solution which means that the ions will separate from each other. The Cu2+ will form a complex with the water to form hexaaquacopper(II) ions. Now ammonia is a weak base, it will deprotonate one of the water ligands in the copper complex, hence acid base reaction.

You'll know that before a ligand exchange reaction occurs (which forms the [Cu(NH3)4]2+ ) there will be an acid base reaction occuring first. What the experiment is doing is trying different amounts of copper ion and ammonia to find the exact set of volumes of each copper and ammonia are required to carry out the acid-base before the ligand exchange reaction occurs.

Before the optimum, acid base reaction only.
At the optimum, acid base reaction is carried but no ligand exchanage
After the optimum, ligand exhange occurs after the acid base.

Thank you so much again for replying.
"The ammonia is limiting because before the optimum, the volume of ammonia is decreasing, and the volume of Cu2+ ions increases. So the amount of ammonia decreases and the amount of copper ions increases. In this case the decrease in amount of ammonia is whats causing the any further increase in the formation of the blue preciptate [Cu(H2O)4(OH)2]. For example, if there was 2cm3 of Cu^2+ and 8cm3 of ammonia solution, then there will be X amount of the precipitate with absorbance of 0.34. Now if I had the same 2cm3 of copper(II) ions but with more ammonia eg 10 cm^3 then there will be more preciptate formed ( > X) hence the absorbance would be greater than 0.34. Because the ammonia is limiting, we aren't getting the maximum mass of blue precipate for a given volume of Cu^2+ ions."
Sure, the bold makes sense -however just shown below, the volume of copper increases,the volume of ammonia decreases. Why do you refer to the same volume of copper ions when the volume of copper ions is in fact increasing from tube to tube.


To confuse matters more,a precipitate doesn't form apparently. This is shown by the next part of the paper:

I think my brain cells are going to explode - this question is so weird

Ok, so refer back to what I said about the formation of the precipitate.
the ammonia would've deprotonate the the water ligands to form the hexaaquacopper(II) ions - the precipitate in question. This occurs because the N has a lone pair of electrons to form ammonium ions.

Now there was ammonium ions in the solution. Addition of ammonia (ammonium?)increases the concentration of NH3 increases, so the equilibrium will shift to decrease concentration of ammonia (ammonium?) by favouring forward reaction. Less ammonia (ammonium?) means not enough ammonia for acid base reaction. (Also,more ammonium results in more ammonia being produced because of the equilibrium shift?)

Could you please kindly check this just above?

the last sentence in your edited comment would imply a never ending equilibrum shift.

eg, when you said more ammonium results in more ammonia being produced because of equilibrium shift,
then you can also say that, more ammonia results in more ammonium
then more ammonium results in more ammonia ... and so on.

The equilibirum shift only works once. So more ammonia means reduction in ammonia and more of ammonium.

Sorry that's what I meant.
"So more ammonia means reduction in ammonia and more of ammonium."
Wait but how is this related to the question? lol sorry I am finding it so confusing to piece everything together.

"The ammonia is limiting because before the optimum, the volume of ammonia is decreasing, and the volume of Cu2+ ions increases. So the amount of ammonia decreases and the amount of copper ions increases. In this case the decrease in amount of ammonia is whats causing the any further increase in the formation of the blue preciptate [Cu(H2O)4(OH)2]. For example, if there was 2cm3 of Cu^2+ and 8cm3 of ammonia solution, then there will be X amount of the precipitate with absorbance of 0.34. Now if I had the same 2cm3 of copper(II) ions but with more ammonia eg 10 cm^3 then there will be more preciptate formed ( > X) hence the absorbance would be greater than 0.34. Because the ammonia is limiting, we aren't getting the maximum mass of blue precipate for a given volume of Cu^2+ ions."
Sure, the bold makes sense -however just shown below, the volume of copper increases,the volume of ammonia decreases. Why do you refer to the same volume of copper ions when the volume of copper ions is in fact increasing from tube to tube.

Can you please kindly let me know on this one as well?

Hi guys, how on earth are you supposed to approach this question?
Could someone let me know please? Thankss so much!Have attached ms as well

Sorry that's what I meant.
"So more ammonia means reduction in ammonia and more of ammonium."
Wait but how is this related to the question? lol sorry I am finding it so confusing to piece everything together.

"The ammonia is limiting because before the optimum, the volume of ammonia is decreasing, and the volume of Cu2+ ions increases. So the amount of ammonia decreases and the amount of copper ions increases. In this case the decrease in amount of ammonia is whats causing the any further increase in the formation of the blue preciptate [Cu(H2O)4(OH)2]. For example, if there was 2cm3 of Cu^2+ and 8cm3 of ammonia solution, then there will be X amount of the precipitate with absorbance of 0.34. Now if I had the same 2cm3 of copper(II) ions but with more ammonia eg 10 cm^3 then there will be more preciptate formed ( > X) hence the absorbance would be greater than 0.34. Because the ammonia is limiting, we aren't getting the maximum mass of blue precipate for a given volume of Cu^2+ ions."
Sure, the bold makes sense -however just shown below, the volume of copper increases,the volume of ammonia decreases. Why do you refer to the same volume of copper ions when the volume of copper ions is in fact increasing from tube to tube.

Can you please kindly let me know on this one as well?

I was talking about if we did increase the amount of ammonia in the test tube with the example of having 2cm3 of cu2+ ions.

Besides the question later mentioned that no precipitate formed so at least some of my explanation on that paragraph is wrong anyway.

Ugh
I am so confused.

I appreciate your help but I don't understand this.

You've already explained it a lot and asking you to explain it in a different way is not fair on you.

I am not really sure what to do

I’ve understood it all now,
Basically there will be an reaction with the cu2+ ions and the ammo is to directly form the copper ammonia complex.

The bit about the relative amounts of copper and ammonia mentioned earlier still hold true, because you will get the “max” yield of the deep blue solution if you combine the copper and ammonia moles as close as to the stochiometry of the reaction. This “max” of deep blue solution formed will therefore generate the peak in the absorbence curve.

If we go either left off the optimum, the copper is limiting and ammonia is in excess, if you go to the right of the optimum, the ammonia is limiting and copper is excess.

Then when you get the peak just take the ratio of the volumes as this will be equal to the stochiometry of the reaction. ( = neither reactant nor product is limiting)

Thank you so much! Just the bit in bold is confusing me -could you please kindly explain this to me? Using a balanced chemical equation to calculate amounts of reactants and products is called stoichiometry but I don't know what you mean in this context?

The volume ratio at any point along the X axis is proportional to the mole ratio of copper ion and ammonia. The closer the mole ratio of the reactant/volume ratio is to the stochiometry of the reaction, the greater the absorbance because there’s more deep blue solution formed.

Therefore to find the empirical formula of the complex, find the volume ratio of reactant at the max peak of the graph. In this case empiral formula is the molecular formula.

Stochiometry of a reaction is simplest whole number ratio of reactant and product.

Thank you so so much for your reply! I am actually getting this and I can't believe it either I genuinely am getting this

I will definitely update you soon about my full understanding of this -it's just that my computer's extra slow today