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# AQA CHEM5 A2 Chemistry - 19th June 2013 watch

1. (Original post by vgautam)
1. Copper(ii) ions oxidise iodide ions to iodine. The iodine produced can be titrated with standard thiosulfate solution, and, from the amount of iodine produced, the concentration of the copper(ii) ions in the solution can be calculated. The relevant equations for this process are given below:

2Cu2+(aq) + 4I-(aq) → 2CuI(s) + I2(aq)
2S2O32-(aq) + I2(aq) → S4O62-(aq) + 2I-(aq)
A sample of 4.256 g of CuSO45H2O is dissolved and made up to 250 cm3 in a volumetric flask. A 25 cm3 portion is added to an excess of potassium iodide. The iodine formed required 18.00 cm3 of a 0.0950 mol dm-3 solution of sodium thiosulfate for reduction. Calculate the percentage of copper in the crystals.

Someone help I calculated the answer as 50%. I don't have the answer but could some check to see if they get the same answer as me.
I am so bad at these questions. Why do they have to ask Amount of Substance stuff in our final exam? Makes me so sad!
2. (Original post by vgautam)
1. Copper(ii) ions oxidise iodide ions to iodine. The iodine produced can be titrated with standard thiosulfate solution, and, from the amount of iodine produced, the concentration of the copper(ii) ions in the solution can be calculated. The relevant equations for this process are given below:

2Cu2+(aq) + 4I-(aq) → 2CuI(s) + I2(aq)
2S2O32-(aq) + I2(aq) → S4O62-(aq) + 2I-(aq)
A sample of 4.256 g of CuSO45H2O is dissolved and made up to 250 cm3 in a volumetric flask. A 25 cm3 portion is added to an excess of potassium iodide. The iodine formed required 18.00 cm3 of a 0.0950 mol dm-3 solution of sodium thiosulfate for reduction. Calculate the percentage of copper in the crystals.

Someone help I calculated the answer as 50%. I don't have the answer but could some check to see if they get the same answer as me.
Hmm I'm not sure about that 50%, I got a value of about 25.5% :L what steps did you use to get to the 50?
3. Can someone help me with the following question please! I am not too sure why each diaminoethane molecule has a 2+ charge or the whole equation to be honest. Can some1 please gently guide me through the whole equation please!

Question: Diaminoethane (H2NCH2CH2NH2), like ammonia, can react as a base and as a ligand.8 (e) (i) Write an equation for the reaction that occurs between an aqueous solution of
aluminium chloride and an excess of aqueous diaminoethane.
Describe the appearance of the aluminium-containing reaction product.

4. Write an equation for the reaction that occurs between an aqueous solution of
aluminium chloride and an excess of aqueous diaminoethane.
Describe the appearance of the aluminium-containing reaction product

I don't get why the markscheme starts with [Al(H20)3]3+ when it says aluminium chloride ??
5. (Original post by flyylikejetz)
Write an equation for the reaction that occurs between an aqueous solution of
aluminium chloride and an excess of aqueous diaminoethane.
Describe the appearance of the aluminium-containing reaction product

I don't get why the markscheme starts with [Al(H20)3]3+ when it says aluminium chloride ??
That's because the aluminium ions and chloride ions are separated in an aqueous solution and the aluminium ions gain water ligands.
6. (Original post by Tayeb)
I actually cant wait for a level chemistry! A FEW MONTHS TO GO!!
How is it like????
P.S. im in year 11
HELL. I'm sorry but thats hard truth, loved it in year11 cos it was so easy got an A but I really dislike it now
7. (Original post by Gazfink)
Yeah that's what my teacher said. But then he also did it and got 0.1. He thought it was just an unfair question really haha :-P
It was an awful question, we got the right value in the end though :L oh well, that paper was just awful :/
8. (Original post by Sherlockedd)
It was an awful question, we got the right value in the end though :L oh well, that paper was just awful :/
hello which paper ...
9. Do you know when they ask which is the strongest oxidising/reducing.. How do you know when to state product or reactant?
10. (Original post by laurawoods)
hello which paper ...
Jun 2012. I think
11. (Original post by JSN)
Do you know when they ask which is the strongest oxidising/reducing.. How do you know when to state product or reactant?
Strongest oxidising agent is the highest E value and the reactant of the reduction. (The equations are in terms reduction...so the higher it's E value the more it want to be reduced, which makes it a good oxidising agent)

Strongest reducing agent is lowest E value and the product. For the opposite reasons.
12. (Original post by flyylikejetz)
Write an equation for the reaction that occurs between an aqueous solution of
aluminium chloride and an excess of aqueous diaminoethane.
Describe the appearance of the aluminium-containing reaction product

I don't get why the markscheme starts with [Al(H20)3]3+ when it says aluminium chloride ??
AlCl3 dissociates into its aqueous ions.
Its a white percipitate
13. (Original post by Gazfink)
Strongest oxidising agent is the highest E value and the reactant of the reduction. (The equations are in terms reduction...so the higher it's E value the more it want to be reduced, which makes it a good oxidising agent)

Strongest reducing agent is lowest E value and the product. For the opposite reasons.
So more +ve is a stronger oxidising agent..

M2+ + 2e- ---> M

so M2+ is the oxidising agent right?
14. (Original post by JSN)
So more +ve is a stronger oxidising agent..

M2+ + 2e- ---> M

so M2+ is the oxidising agent right?
Yep.
Oxidising agents get reduced.
Reducing agents get oxidised.

Sorry if I made it even more confusing for you
15. (Original post by Gazfink)
i wrote them out on a previous reply. But I'll paste them here

Sodium...yellow flame...white powder
Magnesium...white flame...white powder
Alluminum...white flame...white powder
silicon...white flame...white powder
Phosphorous...white flame...white powder
Sulphur...blue flame...choking fumes produced.
thank you!
16. can someone please explain to me what the function of the electrolyte is in electrochemics cells? why is it in some cells and not others? and why is it needed in the hydrogen oxygen fuel cells ??
17. (Original post by ynm1)
Can someone help me with the following question please! I am not too sure why each diaminoethane molecule has a 2+ charge or the whole equation to be honest. Can some1 please gently guide me through the whole equation please!

Question: Diaminoethane (H2NCH2CH2NH2), like ammonia, can react as a base and as a ligand.8 (e) (i) Write an equation for the reaction that occurs between an aqueous solution of
aluminium chloride and an excess of aqueous diaminoethane.
Describe the appearance of the aluminium-containing reaction product.

I didn't get it at first either but then I realised (Al(H20)6)3+ doesn't react with excess NH3 so
(H2NCH2CH2NH2) must act as a base and so produce Al(H20)3(OH)3. (H2NCH2CH2NH2) has two nitrogen so they both accept a proton to become positive. Ammonia has one nitrogen and goes from NH3 --> NH4+ so two nitrogens mean a 2+ charge.
Hope that helps!
18. Can someone pleeeaaaassse help me with chapter 14 about the emfs I'm not exactly sure about what is going on???
19. (Original post by laurawoods)
hello which paper ...
June 2012
20. (Original post by ChemBoss)
Can someone pleeeaaaassse help me with chapter 14 about the emfs I'm not exactly sure about what is going on???
Its not a hard chapter. What do you need to know?

If an entity has a higher E standard value, it gets reduced.
If another entity has a lower e standard value, it gets oxidised.

This now apples to all questions to do with emf.

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